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'[EE] Building a Direct Conversion AM Radio Receive'
2016\10\15@192352 by Jason White

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part 1 1363 bytes content-type:text/plain; charset="utf-8" (decoded base64)

Hello, as a personal project to become more familiar with RF circuits I
would like to build an AM radio receiver. Online, I found a schematic which
uses the NE612 mixer IC to create a Direct Conversion ("Syncrodyne")
receiver. I have modified the design so that the local oscillator does not
need a custom wound transformer.

In my case, I don't require the receiver to have very good performance. I'm
okay with the oscillator drifting and the audio "warbling" (due to
frequency mismatch) under normal operation.

I don't have any sort of matching network or filtering on the antenna
connection; but I am hoping a 10m wire antenna will produce a sufficiently
strong signal for the mixer. I am unsure how strongly signals and noise
outside of the AM band are going to affect the receiver. Ideally, I'd have
a buffer amplifier on the antenna input and a bandpass filter that covered
the AM band, but I want to avoid making this too complex.

I have attached a picture of the original schematic, a picture of my
ltspice schematic, and zip containing my ltspice simulation.

I would like to know if my design (ltspice simulation) seems reasonable
before I try to build it. I welcome advice on improving the design, but at
the same time I do wish to keep things a simple as possible.

Thanks,
Jason White

part 2 11114 bytes content-type:image/gif; name="synchrodyne.gif" (decode)


part 3 3914 bytes content-type:application/zip; name="direct_conversion_ltspice.zip" (decode)

part 4 8144 bytes content-type:image/png; name="direct_conversion2.png" (decode)


part 5 197 bytes content-type:text/plain; name="ATT00001.txt" (decode)

2016\10\15@200401 by Sean Breheny

face picon face
Hi Jason,

I'm not sure about some of the detailed points of your schematic (such as
whether your oscillator tuning circuit will result in continuous
oscillation), but it looks to me like it will probably work to some degree
but I'd expect it to have very poor selectivity when faced with strong
signals. This is because the mixer will have some tendency to rectify
strong input signals, which for AM signals, will result in their being
present on the IF output even if the LO is not tuned close to the frequency
of the strong signal. I would expect that there would be several strong
stations which you will hear all the time, regardless of the tuning, and
then weaker stations that would be tuned in and out by the LO.

A preselector circuit would help greatly with this.

Another pathology which DC receivers have is feedback where they receive
their own LO AND strong audio signals can modulate their LO through
unintended power supply coupling. This can result in motorboating or other
oscillations in the audio. It is also very difficult to both have enough
gain for weak signals AND avoid picking up AC hum and even feedback from
the audio output back in to the audio input. This is a fundamental problem
when all of your gain is concentrated at one frequency rather than being
spread across RF, IF, and AF stages.

I would suggest you add a simple LC preselector and then try your design.
There will likely be other issues as I have indicated above but they can be
more or less fixed later on.

Sean


On Sat, Oct 15, 2016 at 7:23 PM, Jason White <
spam_OUTwhitewaterssoftwareinfoTakeThisOuTspamgmail.com> wrote:

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2016\10\15@202917 by Jason White

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If I were to physically construct the LO tuning circuit using a 2N3904 on a
breadboard with the same biasing configuration as given in the datasheet
(from the internal schematic for the NE612), and probe it with my
oscilloscope to see if it runs, would that be a reasonable test?

Per your advice: I shall add a preselector consisting of a buffer amplifier
(to get a known impedance from the antenna) followed by a ~300kHz bandwidth
LC filter.

My intention is to build this, have it "mostly" work for one or two
stations, and then move on to a conventional (more complex) superheterodyne
AM receiver.

On Sat, Oct 15, 2016 at 8:03 PM, Sean Breheny <.....shb7KILLspamspam@spam@cornell.edu> wrote:

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2016\10\15@212144 by Jason White

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part 1 5059 bytes content-type:text/plain; charset="utf-8" (decoded base64)

Here is the final schematic, 250kHz bandwidth centered around 1100kHz.
There are 5 local stations within the tuning range.

WJTB-1040kHz:   7.61mV/m
WILB-1060kHz:   5.81mV/m
WTAM-1100kHz:   40.28mV/m
WHKW-1220kHz :  2.91mV/m
WCCR-1260kHz:   4.97mV/m

WTAM is by far the strongest at 40mV per meter. Does that mean with an
ideal 10 meter antenna, that the WTAM would produce a signal of 400mV?
(across/through the antennas characteristic >5k impedance). If that
were the case, I would need a potentiometer to attenuate the antenna's
signal.



On Sat, Oct 15, 2016 at 8:29 PM, Jason White
<.....whitewaterssoftwareinfoKILLspamspam.....gmail.com> wrote:
> If I were to physically construct the LO tuning circuit using a 2N3904 on a
> breadboard with the same biasing configuration as given in the datasheet
> (from the internal schematic for the NE612), and probe it with my
> oscilloscope to see if it runs, would that be a reasonable test?
>
> Per your advice: I shall add a preselector consisting of a buffer amplifier
> (to get a known impedance from the antenna) followed by a ~300kHz bandwidth
> LC filter.
>
> My intention is to build this, have it "mostly" work for one or two
> stations, and then move on to a conventional (more complex) superheterodyne
> AM receiver.
>
> On Sat, Oct 15, 2016 at 8:03 PM, Sean Breheny <EraseMEshb7spam_OUTspamTakeThisOuTcornell.edu> wrote:
>>
>> Hi Jason,
>>
>> I'm not sure about some of the detailed points of your schematic (such as
>> whether your oscillator tuning circuit will result in continuous
>> oscillation), but it looks to me like it will probably work to some degree
>> but I'd expect it to have very poor selectivity when faced with strong
>> signals. This is because the mixer will have some tendency to rectify
>> strong input signals, which for AM signals, will result in their being
>> present on the IF output even if the LO is not tuned close to the
>> frequency
>> of the strong signal. I would expect that there would be several strong
>> stations which you will hear all the time, regardless of the tuning, and
>> then weaker stations that would be tuned in and out by the LO.
>>
>> A preselector circuit would help greatly with this.
>>
>> Another pathology which DC receivers have is feedback where they receive
>> their own LO AND strong audio signals can modulate their LO through
>> unintended power supply coupling. This can result in motorboating or other
>> oscillations in the audio. It is also very difficult to both have enough
>> gain for weak signals AND avoid picking up AC hum and even feedback from
>> the audio output back in to the audio input. This is a fundamental problem
>> when all of your gain is concentrated at one frequency rather than being
>> spread across RF, IF, and AF stages.
>>
>> I would suggest you add a simple LC preselector and then try your design.
>> There will likely be other issues as I have indicated above but they can
>> be
>> more or less fixed later on.
>>
>> Sean
>>
>>
>> On Sat, Oct 15, 2016 at 7:23 PM, Jason White <
>> whitewaterssoftwareinfospamspam_OUTgmail.com> wrote:
>>
>> > Hello, as a personal project to become more familiar with RF circuits I
>> > would like to build an AM radio receiver. Online, I found a schematic
>> > which
>> > uses the NE612 mixer IC to create a Direct Conversion ("Syncrodyne")
>> > receiver. I have modified the design so that the local oscillator does
>> > not
>> > need a custom wound transformer.
>> >
>> > In my case, I don't require the receiver to have very good performance.
>> > I'm
>> > okay with the oscillator drifting and the audio "warbling" (due to
>> > frequency mismatch) under normal operation.
>> >
>> > I don't have any sort of matching network or filtering on the antenna
>> > connection; but I am hoping a 10m wire antenna will produce a
>> > sufficiently
>> > strong signal for the mixer. I am unsure how strongly signals and noise
>> > outside of the AM band are going to affect the receiver. Ideally, I'd
>> > have
>> > a buffer amplifier on the antenna input and a bandpass filter that
>> > covered
>> > the AM band, but I want to avoid making this too complex.
>> >
>> > I have attached a picture of the original schematic, a picture of my
>> > ltspice schematic, and zip containing my ltspice simulation.
>> >
>> > I would like to know if my design (ltspice simulation) seems reasonable
>> > before I try to build it. I welcome advice on improving the design, but
>> > at
>> > the same time I do wish to keep things a simple as possible.
>> >
>> > Thanks,
>> > Jason White
>> >
>> > --
>> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
>> > View/change your membership options at
>> > mailman.mit.edu/mailman/listinfo/piclist
>> >
>> >
>> --
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>> mailman.mit.edu/mailman/listinfo/piclist
>
>
>
>
> --
> Jason White



--
Jason White

part 2 10925 bytes content-type:image/png; name="direct_conversion3.png" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
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2016\10\16@061142 by RussellMc

face picon face
More later maybe.
The then Philips TDA7000 may be of interest if you can find any.
Single IC RF/audio.
DC IF :-).

!!!
https://www.google.co.nz/search?q=tda7000&num=100&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjA09rOid_PAhUM0mMKHXZFAmgQsAQIIg&biw=1527&bih=836

Lonnnnng ago I used them in receivers for a data link for America's Cup
yachts.
TX x n  in pockets on sails.
Cluster of RXs.
Strain gauges -->  horsepower .

Neither a NZ nor a US nor Australian challenger .


 R


On 16 October 2016 at 11:23, Jason White <@spam@whitewaterssoftwareinfoKILLspamspamgmail.com>
wrote:

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2016\10\16@071042 by Rodolfo

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face
www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=TDA7000&_sacat=0

TDA7000

En Sun, 16 Oct 2016 07:11:00 -0300, RussellMc <KILLspamapptechnzKILLspamspamgmail.com>  escribió:

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2016\10\16@155826 by Jason White

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I created my schematic into KiCad, and intend on doing a PCB layout
soon. PDF of the schematic is available here [1] for scrutiny.

The goal is for this to be (9V) battery powered, and to it put into a
metal box. I chose a low frequency switching power supply to get my 5V
rail. Happily, the application circuit for the MC34036 uses the same
value of inductor as my turning and LO circuits; I have copied it
verbatim.

Remarks:
(1) I am aware that a superheterodyne would be better, but my design
skills are limited and ceramic 455kHz filters appear to becoming
obsolete. Mouser has a few filters for which the specifications are
available, but I am not confident enough try one (yet).
(2) Hopefully the power supply (Fswitching >75kHz) does not feed into
the the input.
(3) I am vaguely aware of the MC34036 's bad reputation. However, I
have it in stock. Worst case scenario: I throw in a linear regulator
and power it from a wall wart transformer.
(4) The abundance of trimmers is to deal with the fact that I really
don't know what my signal levels are going to be. I shall find out
soon enough :) ...

Schematic Download (Google Drive Shared Link)
[1] https://drive.google.com/file/d/0BwP0qhqyaTIIOHZDVWhFZF9EOUk/view?usp=sharing

On Sun, Oct 16, 2016 at 7:10 AM, Rodolfo <spamBeGoneseoane_respamBeGonespamsinectis.com.ar> wrote:
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2016\10\16@162912 by Jason White

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RE: Russle and Rodolfo on the TDA7000

I looked at single chip receiver ICs, they look really nice. I am
confident that I could put together a "proper" radio with very little
effort that way. In particular what I looked at was even more
integrated: the Si4734 AM/FM/SW (short wave) digital receiver IC.
Literally all I would have to do is hook up an antenna, a speaker
driver, and a microcontroller (to send tuning commands) and I would
have a completely functional radio.

But, with this design I am looking to "get my hands dirty" with the
NE612 mixer IC. I realize that what I've thrown together probably
won't work the first time; I'm okay with that. If I can get my
receiver (somewhat) working I'll be quite happy since it will be "my
own" rather than a ready bought solution. (Although, the more I think
about it, the more attractive the single chip solution seems to be.
But, I still think I will save the something like the Si4734/TDA7000
for another time)

On Sun, Oct 16, 2016 at 6:11 AM, RussellMc <apptechnzEraseMEspam.....gmail.com> wrote:
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2016\10\16@173358 by Harold Hallikainen

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The July 2016 issue of QST (
http://www.nxtbook.com/nxtbooks/arrl/qst_201607/index.php#/36 ) has a
direct conversion 40 meter (7 MHz) receiver built around the NE602.
There's an LC tuned circuit at the input. The local oscillator can be
either LC or crystal. It appears that in both cases, it's a Colpitts
oscillator with the crystal or the remainder of the LC acting as an
inductor. Non the NE602, the local oscillator is an emitter follower with
the base on pin 6 and the emitter on pin 7. Output from pin 6 is
multiplied by the LC to provide feedback to the base. Back when I was
doing vacuum tube stuff, I called this a cathode follower Colpitts. I've
always thought of the Colpitts oscillator as being similar to the Hartley
oscillator except that the tapped inductor is replaced with a tapped
capacitor. Of course the inductor has mutual inductance between sections
while the capacitor does not have "mutual capacitance" between sections,
but the circuitry is very similar except for where you drive put the
signal into the feedback circuit.

Anyway, in the article, they allow for crystal or LC. In both cases, they
use a 1N4004 as a varactor to provide a fine tune control based on reverse
DC bias. The output of the NE602 is AC coupled into an LM386 which drives
a headphone output.

Pretty simple circuit!

Now, for something really impressive, have a look at
http://websdr.ewi.utwente.nl:8901/ . This is a software defined radio that
tunes from DC to 29.16MHz. A complete description of the hardware is at
http://wwwhome.ewi.utwente.nl/~ptdeboer/ham/sdr/#nov2008 . The antenna
goes through a balun to convert to balanced, then into an LTC2216 16 bit
running at 77.76 M samples per second. The resulting ADC goes to an FPGA
which converts the data to be suitable for transmission over Ethernet. I
don't know what the data format over Ethernet is, but it MIGHT be raw
samples over UDP. Anyway, the Ethernet goes to a standard Linux server.
Multiple users can access the stream of ADC samples with each using their
desired software demodulator at their desired frequency. This is the
ultimate "hook an antenna to an ADC and do arithemetic!" There are a bunch
of web SDRs around the world. They use a variety of hardware. You can find
them at http://websdr.org/ .

Carrying wide band SDR further, the Reverse Beacon Network uses wideband
SDRs and "skimmer" software to decode Morse and data (RTTY) traffic
looking for the station ID. The results are reported to
http://www.reversebeacon.net/dxsd1/dxsd1.php?f=0&c=w6iwi&t=dx# . This URL
actually does a search for anything heard from me (W6IWI) throughout the
world. The location and SNR are reported.

People are doing some interesting stuff on the radio!

Harold

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2016\10\16@184826 by Jason White

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part 1 4124 bytes content-type:text/plain; charset="utf-8" (decoded base64)

Thanks Harold, I'm looking at it now. I am also impressed by the
circuit's simplicity, I'm quite tempted to gut the op-amps from my
circuit and hook the output of the mixer directly to my speaker driver
like they did in the kit.

For those who do not have a subscription to QST magazine,the receiver
Harold mentioned is the ZZRQX-40 kit direct conversion receiver. I
have attached the schematic. Also a complete manual (with schematics)
is available from [1].

I'm quite curious about R4, which appears to be increasing the gain of
the LO transistor. That's odd because (at least in simulations) I have
had the exact opposite problem, too much gain, which yields a LO
amplitude of greater than the specified 300mV maximum. Maybe harmonics
aren't a problem in a direct conversion receiver?

As for SDR, I have a RTL2832 dongle and an upconverter. It is quite
fun to snoop on local signals, but of course being an EE student what
gets me excited is designing, building, and learning new stuff, so my
SDR mostly sits mostly unused.

[1] www.4sqrp.com/kits/zzrx40/zzrx40manualv7.2_160214.pdf

-Jason White

On Sun, Oct 16, 2016 at 5:33 PM, Harold Hallikainen
<RemoveMEharoldEraseMEspamEraseMEmai.hallikainen.org> wrote:
> The July 2016 issue of QST (
> http://www.nxtbook.com/nxtbooks/arrl/qst_201607/index.php#/36 ) has a
> direct conversion 40 meter (7 MHz) receiver built around the NE602.
> There's an LC tuned circuit at the input. The local oscillator can be
> either LC or crystal. It appears that in both cases, it's a Colpitts
> oscillator with the crystal or the remainder of the LC acting as an
> inductor. Non the NE602, the local oscillator is an emitter follower with
> the base on pin 6 and the emitter on pin 7. Output from pin 6 is
> multiplied by the LC to provide feedback to the base. Back when I was
> doing vacuum tube stuff, I called this a cathode follower Colpitts. I've
> always thought of the Colpitts oscillator as being similar to the Hartley
> oscillator except that the tapped inductor is replaced with a tapped
> capacitor. Of course the inductor has mutual inductance between sections
> while the capacitor does not have "mutual capacitance" between sections,
> but the circuitry is very similar except for where you drive put the
> signal into the feedback circuit.
>
> Anyway, in the article, they allow for crystal or LC. In both cases, they
> use a 1N4004 as a varactor to provide a fine tune control based on reverse
> DC bias. The output of the NE602 is AC coupled into an LM386 which drives
> a headphone output.
>
> Pretty simple circuit!
>
> Now, for something really impressive, have a look at
> http://websdr.ewi.utwente.nl:8901/ . This is a software defined radio that
> tunes from DC to 29.16MHz. A complete description of the hardware is at
> http://wwwhome.ewi.utwente.nl/~ptdeboer/ham/sdr/#nov2008 . The antenna
> goes through a balun to convert to balanced, then into an LTC2216 16 bit
> running at 77.76 M samples per second. The resulting ADC goes to an FPGA
> which converts the data to be suitable for transmission over Ethernet. I
> don't know what the data format over Ethernet is, but it MIGHT be raw
> samples over UDP. Anyway, the Ethernet goes to a standard Linux server.
> Multiple users can access the stream of ADC samples with each using their
> desired software demodulator at their desired frequency. This is the
> ultimate "hook an antenna to an ADC and do arithemetic!" There are a bunch
> of web SDRs around the world. They use a variety of hardware. You can find
> them at http://websdr.org/ .
>
> Carrying wide band SDR further, the Reverse Beacon Network uses wideband
> SDRs and "skimmer" software to decode Morse and data (RTTY) traffic
> looking for the station ID. The results are reported to
> http://www.reversebeacon.net/dxsd1/dxsd1.php?f=0&c=w6iwi&t=dx# . This URL
> actually does a search for anything heard from me (W6IWI) throughout the
> world. The location and SNR are reported.
>
> People are doing some interesting stuff on the radio!
>
> Harold

part 2 8794 bytes content-type:image/png; name="zzrx-40-schematic.png" (decode)


part 3 197 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2016\10\16@200313 by Jean-Paul Louis

picon face

Jason,
A DC/DC converter right near the receiver is a NOGO!.
the 75kHz harmonics are going to ruin your reception of small signals.
If you are concerned about power consumption, then adjust your DC/DC switcher voltage to 5.5V and add a low noise LDO regulator to feed the receiver. the layout will be critical to avoid any spurious from the switcher(shield might be needed.


Just my $0.02,

Jean-Paul
N1JPL


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2016\10\16@204619 by Harold Hallikainen

flavicon
face

> I'm quite curious about R4, which appears to be increasing the gain of
> the LO transistor. That's odd because (at least in simulations) I have
> had the exact opposite problem, too much gain, which yields a LO
> amplitude of greater than the specified 300mV maximum. Maybe harmonics
> aren't a problem in a direct conversion receiver?

Yes, R4 is interesting. It's a 22k that's in parallel with the internal
25k. I don't think it increases the gain since this is an emitter
follower, so the gain is limited to a maximum of 1. I'd be concerned that
this would mess up the DC bias of the circuit. The theory of operation in
the schematic does not explain why they deviate from the chip datasheet.

So far, my SDR experience is limited to listening to websdr. But, I'm very
impressed with what they've done.

Somewhat like the NE602, I did a receiver using the SA639. This is an
infrared data receiver (
www.uslinc.com/index.php?option=com_virtuemart&page=shop.product_details&flypage=flypage.tpl&category_id=23&product_id=67&Itemid=65&vmcchk=1&Itemid=65
). It has a bunch of IR photodiodes in parallel. They are reverse biased
through a tunable inductor becoming resonant at 1.8MHz. The data is FSK at
10kbps at 1.8MHz. The photodiodes drive an op amp-based preamp, then into
the SA639. A crystal based local oscillator converts the signal up to
10.7MHz where it goes through IF amplifiers with ceramic filters. The
ceramic based quadrature detector from the datasheet is used to demodulate
the FSK. The resulting signal drives a comparator in a PIC, then the UART
in the PIC.

That's my recent "RF" design experience.

Harold



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2016\10\16@214738 by Jason White

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Jean, I will add a 5V LDO to be design and make it so that I can
bypass and disable the 5.5V switching supply with jumpers.

The PCB is going to be approximately 4"x4" (100mm square). The
Switching power supply and the antenna input are going to be on
opposite corners. I intend to place the LDO physically near the
switching supply, and then to run the regulated 5V all the way to the
other corner. This way the trace length and, hopefully, radiation from
the switching supply is isolated to the far corner of the PCB.

On Sun, Oct 16, 2016 at 8:03 PM, Jean-Paul Louis <louijpSTOPspamspamspam_OUTgmail.com> wrote:
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2016\10\16@221502 by Sean Breheny

face picon face

First, a comment about the SDR Harold posted a link to. I would think that
unit would have serious dynamic range problems. A decent antenna is going
to present about 0.1V (100mV) into a 50 ohm load if you take the entire
shortwave 2MHz to 30MHz range. You want to be able to see signals as low as
about 100nV. This is a 1 million to 1 voltage ratio. A 16 bit ADC has a
dynamic range of no better than 65535:1. There are tricks you can do in DSP
post-processing which will get you a bit more than this but I am surprised
if it has good performance on a full size antenna. SDR is awesome but there
is still room for good RF front-end design.

Now, back to Jason's design. Looks like you are making good progress. A few
points:

1) It looks like the preselector is not tunable - is that intentional?
2) Is the input buffer amp intended to isolate the preselector from the
variable antenna impedance? I do think that is a good idea, although
another approach (which can yield a big improvement in signal strength) is
to use a tunable preselector with only moderate to light coupling to the
antenna and enough tuning range to tune-out some of the antenna reactance.
This gets you an antenna tuner/matching network AND preselector in one and
gets rid of the need for a buffer amp.
3) It looks to me like your strongest station is going to saturate that
gain stage after the preselector.
4) You mentioned a method to compute the approximate antenna terminal
voltage from RF E-field strength. I don't think the simple method of
multiplying the E-field strength by the antenna length will be very good.
The real way to do this is to convert from E field strength to power
density (E^2/eta_o), multiply by the effective area of your antenna (for
monopoles and dipoles it is almost independent of the antenna length
(roughly lambda^2/(4*pi))). This gives you the total maximum power
received. Now estimate the radiation resistance of the antenna as well as
the reactance. Model the antenna as a voltage source with a series
resistance of the radiation resistance and compute the voltage magnitude
which would give you the max power received into a matched load. Then,
augment your model by adding the reactance to your series resistance. If
you add your load, then you can determine the terminal voltage into that
load.
5) I would ditch the switching regulator completely for now. It will be
nothing but trouble as a noise generator. I designed a switcher to use in
an RF receiver environment once and it required massive shielding and
multiply layers of filtering on the output lines. You won't fix the problem
by post-regulating it with an LDO. The devil is conducted noise through the
ground and also near-field magnetic interference.
6) The NE602's mixer might expect to see a particular impedance at its
output. Typically mixers perform best if they see a fixed impedance across
the entire output frequency range, including the image frequency range.
This is why the filtering on the output of a mixer is typically either
preceded by an attenuator (the easy way when signal to noise ratio is not
critical) or it is a diplexer (a filter which has a constant input
impedance and filters by dumping stopband energy into a separate load). If
you don't have the proper load on the mixer, it can degrade the dynamic
range of the mixer.

Sean


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2016\10\17@085731 by Van Horn, David

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I agree 100% on not using an SMPS in the system at least at the outset.  Get it working first, then add that complication if you must.



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2016\10\17@095345 by Jason White

picon face

RE: Sean

(1) I had originally intended for the design to be limited to a single
~300kHz Band. But, I see from the discussion that a narrow-band
(~50kHz) tunable preselector would do much better; since it would
allow for WTAM-1100 to be tuned out.
(2) I will take a look at that approach. Honestly, I am quite worried
about the physical proximity of objects/people/etc. detuning the
preselector.
(3) I will include a attenuator potentiometer before the preselector.
(4) Will investigate proper modeling of received antenna power.
(5) I will make the switching supply bypassable so that I can turn it
off if it is generating too much noise.
(6) Not entirely sure how to do that, will also investigate.

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2016\10\17@095533 by Jason White

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Okay David,

The switching supply will be designed such that is can be disabled and
bypassed with a switch/jumpers during normal operation.

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2016\10\17@101852 by Harold Hallikainen

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> Okay David,
>
> The switching supply will be designed such that is can be disabled and
> bypassed with a switch/jumpers during normal operation.

I agree that it'd be great to get it working without the SMPS first. Then
see how much worse it gets with the SMPS. In the IR receiver I linked to
before, I used an LTC dual boost converter to get 2-3V battery up to 3.3V
for the PIC and 7V for the LCD. The power supply is on the same PCB as the
PIC. The receiver is on a separate board with heavy RF filtering on the
lines between the two.

More on SDR, the Kiwi SDR at http://kiwisdr.com/KiwiSDR/index.html looks
interesting. It's only a 14 bit ADC, but listening to some of them around
the world, it seems to work pretty well. I imagine a tuned front end would
make it work better, but then you could not have multiple users listening
to multiple frequencies.

Harold





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2016\10\17@125056 by alan.b.pearce

face picon face
> More on SDR, the Kiwi SDR at http://kiwisdr.com/KiwiSDR/index.html looks
> interesting. It's only a 14 bit ADC, but listening to some of them around the
> world, it seems to work pretty well. I imagine a tuned front end would make
> it work better, but then you could not have multiple users listening to
> multiple frequencies.

Well, that gave me a happy hour or so listening to home radio. !!! Certainly seems to work well enough, makes me very tempted to get one.



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2016\10\17@184908 by RussellMc

face picon face

Jason - did you glance at the TDA7000 capabilities.
May be more interesting than you'd expect. Or not.
A look at the images page gives many instant lads to DIY uses.

 â€‹
https://www.google.co.nz/search?q=tda7000&num=100&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjA09rOid_PAhUM0mMKHXZFAmgQsAQIIg&biw=1527&bih=836


​Normalish​
<https://www.google.co.nz/imgres?imgurl=http%3A%2F%2Felectronics-diy.com%2Fschematics%2F1079%2FTDA7000-FM-Radio.jpg&imgrefurl=http%3A%2F%2Felectronics-diy.com%2Ffm-radio-with-tda7000.php&docid=vC5asYRt_bieSM&tbnid=IMOWQ390QpyKaM%3A&w=566&h=418&bih=836&biw=1527&ved=0ahUKEwjamZTs9OLPAhUL02MKHZsoDS0QMwhKKCMwIw&iact=mrc&uact=8>

Also
<https://www.google.co.nz/imgres?imgurl=https%3A%2F%2Fi.ytimg.com%2Fvi%2FPT_EmkEXJiM%2Fmaxresdefault.jpg&imgrefurl=https%3A%2F%2Fhttp://www.youtube.com%2Fwatch%3Fv%3DPT_EmkEXJiM&docid=WKxBavWevHhRIM&tbnid=VantSu56nvDJaM%3A&w=1920&h=1080&bih=836&biw=1527&ved=0ahUKEwjamZTs9OLPAhUL02MKHZsoDS0QMwhLKCQwJA&iact=mrc&uact=8>

! <http://www.cqham.ru/projects/tda7000_20rx/t7000-2.jpeg>

:-) <http://jcoppens.com/sat/rx/img/tda_osc.jpeg>


:-) !
<http://2.bp.blogspot.com/-no_oBVJwZcU/Ui7TQ3wSh-I/AAAAAAAAABQ/RVTXYyb-zU4/s1600/201201265466.jpg>

! :-)
<http://hardwaresl.0fees.net/wp-content/uploads/2009/04/photo4.jpg?i=1>

!!! :-)  <http://ejenn.free.fr/images/Emetteur_2606_recepteur_tda7000.jpg>





​Russell

​

On 16 October 2016 at 22:11, RussellMc <@spam@apptechnzspam_OUTspam.....gmail.com> wrote:

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2016\10\17@194000 by Jason White

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Hello Russell,

Yes, I did look at the TDA7000. Long story short, I think it would be
almost too easy, I am hoping to gain some basic RF design experience
which I don't think a dedicated receiver IC would provide. Also, the
TDA7000 is FM only (not a big problem) and seems to be getting a big
long in the tooth (is not stocked by Mouser or Digikey). If I were to
go that route, I think I would use something that I could readily buy
since I'm not a big fan of buying chips from Ebay (or other less
reputable electronics distributors).

Thanks,
-Jason White

On Mon, Oct 17, 2016 at 6:48 PM, RussellMc <spamBeGoneapptechnzEraseMEspamgmail.com> wrote:
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2016\10\17@194234 by Jason White

picon face
Hello Harold,

Thanks for sharing your design, quite interesting! Both the
application and that style of data transmission is completely new to
me. For some reason, the idea that it is possible to do RF style
transmissions over infrared is mind-boggling to me.

-Jason White


On Sun, Oct 16, 2016 at 8:45 PM, Harold Hallikainen
<RemoveMEharold@spam@spamspamBeGonemai.hallikainen.org> wrote:
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2016\10\17@195250 by James Cameron

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face
Well, IR is RF in the terahertz range, so what was described was
amplitude modulation of a signal using a frequency modulated signal as
input?  ;-)

On Mon, Oct 17, 2016 at 07:42:30PM -0400, Jason White wrote:
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2016\10\17@201629 by Tony Miller

picon face
Jason,

If you looking for basic RF design, you might find useful the Amateur Radio
Relay League (ARRL) book, "Experimental Methods in RF Design" by Wes
Hayward, et al.  I   It's a  getting a bit dated but It will take you from
direct conversion to advanced super heterodyne receivers.

Tony

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2016\10\17@212000 by Randy Dawson

picon face
Jesus - $194 on abebooks!


________________________________
From: .....piclist-bouncesRemoveMEspammit.edu <.....piclist-bouncesSTOPspamspam@spam@mit.edu> on behalf of Tony Miller <tmillazEraseMEspam@spam@gmail.com>
Sent: Monday, October 17, 2016 5:16 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Building a Direct Conversion AM Radio Receiver?

Jason,

If you looking for basic RF design, you might find useful the Amateur Radio
Relay League (ARRL) book, "Experimental Methods in RF Design" by Wes
Hayward, et al.  I   It's a  getting a bit dated but It will take you from
direct conversion to advanced super heterodyne receivers.

Tony

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>
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2016\10\17@215449 by Kerry

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Yeah, that's what I found when I wanted a copy.  Eventually, I found a copy for $7.


On 10/17/2016 8:19 PM, Randy Dawson wrote:
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2016\10\17@223826 by Harold Hallikainen

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> Well, IR is RF in the terahertz range, so what was described was
> amplitude modulation of a signal using a frequency modulated signal as
> input?  ;-)

Yes, it's FM subcarriers on the terahertz "RF." This is the similar to the
DSB-SC subcarrier on FM used to carry the stereo difference signal. The
original definition of FM stereo used this frequency division multiplex
description. But, you get the same thing if you use time division
multiplex spending a little time on the left channel and a little on the
right. It's amazing how you can look at the same signal in different ways.

Thinking about the direct conversion AM receiver, I'm thinking there will
be problems due to the presence of both sidebands unless the local
oscillator is phase locked to the received carrier. If the local
oscillator is phase locked at 0 degrees, you will get DC plus the audio
(plus two times carrier that is taken out by the low pass filter). If the
local oscillator is 90 degrees out of phase with the received carrier, you
will get nothing. If the local oscillator is not the same frequency as the
received carrier, you'll get a tone corresponding to the difference in the
frequencies plus a mix of the audio shifted up a bit in frequency and the
audio shifted down a bit in frequency. If the frequencies are very close
(say 0.1 Hz apart),  I suspect you will hear the audio disappear and
reappear over a 10 second period as the phase between the two rotates.

The null at 90 degrees is an interesting situation. You can transmit two
signals on the same frequency without interference by setting the carriers
90 degrees apart. This is the basis of QAM or Quadrature Amplitude
Modulation. This is a common digital modulation system where, for example,
8 levels are transmitted on the I (in phase) carrier and 8 are transmitted
on the Q (quadrature) carrier. For each transition of the carrier (a
baud), 3 bits are transmitted on I and 3 bits are transmitted on Q for a
total of 6 bits per baud. It turns out that when you add sine waves that
are 90 degrees apart and vary the amplitude of each, the result is a sine
wave whose amplitude is the square root of the sum of the squares of the
amplitudes and the resulting phase angle is the arcsine of the Q amplitude
over the I amplitude. You can plot the resulting signal on a
"vectorscope." With the 8x8 QAM mentioned before, you get dots in 64
different positions (an 8x8 array). This is "64QAM."

But, the QAM signal could carry analog signals. In NTSC color television,
the "baseband" carries a linear combination of red, green, and blue that
produces a good picture on a monochrome television. Another linear
combination of red, green, and blue modulates the I subcarrier at 3.58MHz.
Another linear combination of red, green, and blue modulates the Q
subcarrier at 3.58MHz. Through the baseband, I, and Q signals, we are
transmitting three signals that are "linearly independent." In the
receiver, we can multiply each of the signals by different constants, then
add them to yield the original red, green, and blue. Very clever!

But then, a monochrome signal does not modulate the I and Q subcarriers at
all. As such, we can adjust the gain of the I and Q signal to vary the
color saturation (no gain gives monochrome). Further, adjusting the phase
of the local I and Q signals relative to that at the transmit end (and
transmitted in the color burst for reference) varies the "hue" of the
resulting color. So here we have a QAM signal, but the amplitude and phase
have their own meaning. When you put up color bars, you should see dots in
specific locations on a vectorscope.

Modulation... It's fascinating!

Harold






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2016\10\19@193134 by Jason White

picon face
The revision B schematic is available for viewing at [1]. I'm not
super happy about needing 20 varactor diodes to get a turning range of
700kHz to 1600kHz (both preselector and LO get their own bank of 10
diodes). I could avoid the need for so many diode by using a 10x
larger inductor, but then the Q of the preselector and the LO drop
significantly.

Based off of the fact that ZZRX-40 works and that my design is
essentially a copy of that, I am optimistic that I will only need to
alter a few component values (like the LO feedback capacitors) to have
the design receiving audio.

Work has begun towards the PCB (enclosure selection/footprint creation/etc....)

[1] https://drive.google.com/file/d/0BwP0qhqyaTIIcEdFMTkwQXlwWFk/view?usp=sharing

On Mon, Oct 17, 2016 at 10:38 PM, Harold Hallikainen
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2016\10\19@202222 by Harold Hallikainen

flavicon
face
Do you think you need to use an LC oscillator for the LO? I used an
LTC6907CS6 as an FM oscillator at 1.8MHz, 2.3MHz, and 2.8MHz in the IR
transmitter I talked about earlier.

Another trick is to use binary weighted capacitors and analog switches to
to tuning. I recall a chip that has binary weighted capacitors on the chip
and an SPI interface.

The antenna tuner I use on amateur radio has binary weighted inductors and
binary weighted capacitors and then relays that switch them in and out to
adjust the tuning as required.

Harold





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2016\10\19@210430 by RussellMc

face picon face
Without digging up specs - my long ago memory is that you can get larger
delta-C from larger voltages. If so, might a HV supply for tuning (agh!)
assist. Presumably o\power consumption is minimal.

Russell
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2016\10\20@022510 by Richard Prosser

picon face
Rather than using banks of varactor diodes, I'm wondering if there's
some way of using a capacitance multiplier circuit and just the one
diode? I haven't seen it done anywhere but your frequency is low so
opamps and/or "standard" transistors etc should be up to the task.
Maybe something to look at after the first version is working OK?

RP

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2016\10\20@121646 by Van Horn, David

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You could use mosfets to switch in capacitors in binary values, then maybe use a varactor or two for fine tuning.
They DO still make the old 365pF variable caps, BTW.


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2016\10\20@135420 by Sean Breheny

face picon face
There are also older varactor diodes with a much larger capacitance, such
as the MVAM109 or MVAM125. There are a few sellers on eBay who have these.
I don't know if they are made anymore but there are still sources. I used
some in a receive-only remote antenna tuner I made. They have about 10x the
capacitance of the ones you are using now, and almost as large a cap ratio.

On Thu, Oct 20, 2016 at 12:16 PM, Van Horn, David <
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2016\10\20@135609 by Sean Breheny

face picon face
Another thought - the 1N400x series, especially the 1N4002 and up, can make
decent varactors. I've used them for FM modulating oscillators but I don't
know what the actual performance is numerically, but I bet they have a
bigger capacitance than the ones you are using.

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2016\10\20@164713 by Van Horn, David

flavicon
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>>I recall a chip that has binary weighted capacitors on the chip and an SPI interface.

Harald: Any chance you can fire those neurons?  This is one I've not seen.


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2016\10\20@172424 by Ariel Rocholl

picon face
There are a few out there, typically listed as "programmable capacitor
array" or "digital capacitor", etc.

Check NCD2100 http://www.ixysic.com/Products/ProgCap.htm or RFMD rfha3942

Best Regards,
----
Ariel Rocholl
http://www.rf-explorer.com

{Original Message removed}

2016\10\20@172935 by Jason White

picon face

There are a handful of digital capacitors out there, for example the
MAX1474 "Miniature Electronically Trimmable Capacitor". This is not
the one that Harold mentioned, but it probably is quite similar.

Excerpt:
"The MAX1474 is a fine-line (geometry) electronically trimmable
capacitor (FLECAP) programmable through a simple digital interface.
There are 32 programmable capacitance values ranging from 6.4pF to
13.3pF in 0.22pF increments (see Table 1). The quartz dielectric
capacitance is highly stable and exhibits a very low voltage
coefficient. It has virtually no dielectric absorption and has a very
low temperature drift coefficient (<33ppm/°C). The MAX1474 is
programmed through two digital interface pins, which have Schmidt
triggers and pulldown resistors to secure capacitance programming. "

-Jason White

On Thu, Oct 20, 2016 at 4:47 PM, Van Horn, David
<spamBeGonedavid.vanhornspam@spam@backcountryaccess.com> wrote:
>
>
>>>I recall a chip that has binary weighted capacitors on the chip and an SPI interface.
>
> Harald: Any chance you can fire those neurons?  This is one I've not seen.

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2016\10\20@174330 by Jason White

picon face
Sean, something like the MVAM109/MVAM125 wold be great, just one of
those diodes could replace the entire bank. Sadly, they appear to be
(effectively) obsolete/EOL. Since I'm confining myself what is readily
available from the catalogs of the major electronics distributors (ie.
Digiket, Mouser, Farnell, etc..), I won't be able to use it. However,
it is quite handy to know that those really big varactor diodes exist,
even if they are hard to get ahold of.

As for the 1N400X diodes, I looked and their overall capacitance
(~20pF) is not quite enough for my application.

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2016\10\23@233929 by RussellMc

face picon face
You could use a few taps of binary selected caps and then a smaller number
of the higher ratio variable caps.
Every stage of dinary selection allows a halving of varicaps needed.

Or a mechanically variable unit :-).
Piezo, memory wire, linear actuator, motor driving fine thread, stepper,
... .  :-)
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2016\10\23@234048 by RussellMc

face picon face
I now see this almost exactly parallels on both main points a suggestion of
Dave's from 3 days ago :-).


On 24 October 2016 at 15:38, RussellMc <spam_OUTapptechnzspam_OUTspamspam_OUTgmail.com> wrote:

> You could use a few taps of binary selected caps and then a smaller number
> of the higher ratio variable caps.
> Every stage of dinary selection allows a halving of varicaps needed.
>
> Or a mechanically variable unit :-).
> Piezo, memory wire, linear actuator, motor driving fine thread, stepper,
>  ... .  :-)
>
>
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