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'R/C RF scanner design. Suggestions?'
1997\07\30@054457 by Oyvind Kaurstad

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I'm planning on designing a "channels in use" indicator
for R/C purposes. I know there are commercial units for
sale, but they are expensive.

I have some ideas, but I'd like to hear other suggestions
on how to implement this.

I'm going to use an LCD module and a 16C84 for simplicity.

I have interfaced this module to the '84 earlier, so that part will be
easy.

The hard part is obviously the RF design, and I wonder if anyone
has any cheap and clever ideas on this.

I live in Norway, and the RF frequency in question is 35.xxx MHz.

The channel spacing is 10 kHz.

Any suggestions will be appreciated.

-Oyvind

spam_OUToyvind.kaurstadTakeThisOuTspamnofac.abb.no

1997\07\30@113910 by Jerry Meng

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Hi Oyvind,

At 11:52 AM 7/30/97 +0100, you wrote:
>I'm planning on designing a "channels in use" indicator
>for R/C purposes. I know there are commercial units for
>sale, but they are expensive.
>
R/C=???

>I have some ideas, but I'd like to hear other suggestions
>on how to implement this.
>I'm going to use an LCD module and a 16C84 for simplicity.
>I have interfaced this module to the '84 earlier, so that part will be
>easy.
>The hard part is obviously the RF design, and I wonder if anyone
>has any cheap and clever ideas on this.
>I live in Norway, and the RF frequency in question is 35.xxx MHz.
>The channel spacing is 10 kHz.
I made a 10Meter FM transceiver years ago, It is not cheap and clever
design but do works, I used Two Motorola ICs, one is for LO OSC PLL
a MC145162, it works fine unfer 60MHz,Easy to realize 10kHz step
around 35MHz . The other IC is for Receiving a MC13135 complete
dual conversion FM receiver with a RSSI can be used for carrier detect.
Datasheet upon these two ICs can be found on Motorola web.
More you can also try to find some local Radio Amateurs to help you,
I worked a lot of Norway Hams on HF band. :)



Jerry Meng, BA1FB

.....ba1fbKILLspamspam@spam@qsl.net
http://www.qsl.net/ba1fb

1997\07\30@121337 by Martin McCormick

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Oyvind Kaurstad writes:
>I'm planning on designing a "channels in use" indicator
>for R/C purposes. I know there are commercial units for
>sale, but they are expensive.=20
>
>I have some ideas, but I'd like to hear other suggestions
>on how to implement this.


>The hard part is obviously the RF design, and I wonder if anyone
>has any cheap and clever ideas on this.
>
>I live in Norway, and the RF frequency in question is 35.xxx MHz.
>
>The channel spacing is 10 kHz.

       This may be a pretty sophisticated project.  Your receiver will
need to be selective enough to not receive signals on adjacent channels
for starters.  It will also need to have a selective enough front end,
(the antenna input and first mixer), to not respond to what are called image
frequencies.  Super heterodyne receivers do their thing by mixing a local
oscillator signal with a signal coming in on the antenna to form an
intermediate frequency or IF.  Unfortunately, it is a law of physics that
one gets a sum and a difference frequency when mixing two carriers in this
way.  Cheap receivers don't have much filtering in the first mixer stage
and they can receive signals that are derived from both the sum of the local
oscillator and the incoming signal and the difference between the LO and
a signal.  Here is a brief example:

       In North America, FM radio can be found between 88 and 108 MHZ.
Our television channels 5 and 6 occupy 76 to 88 MHZ.  The aviation band
occupies 108-136 MHZ.  The designers of most FM broadcast receivers use a
scheme in which the local oscillator tunes from 98.7 to 118.7 MHZ.  The IF
is 10.7 MHZ so you hear music when the oscillator is exactly 10.7 MHZ above
the frequency of the station you want to hear.  If the radio is a poorly
designed one, you will hear the music and also something else thrown in for
free (GRRRR).  Here's what happens.  Let's say that the local airport tower
is on 122.7 MHZ.  Your favorite music station is on 101.3.  The local
oscillator must be tuned to 10.7 MHZ above 101.3 or exactly 112 MHZ.  You
now hear the music.  The problem is that 10.7 MHZ above the oscillator
happens to be 122.7.  The Q of the first mixer is bad because it is cheaper
to only put one stage of filtering at the antenna so it passes 101.3 and,
to a lesser degree, 122.7 MHZ.  The airplane flies right over your house so
the signal from it is _STRONG_ and has no trouble being heard.  Even though
it is AM and the music is FM, the receiver will respond to it and you may even
hear the voice talking to the tower.
       A few receiver designers were smoking something when they designed
their local oscillator circuits and decided to put it 10.7 MHZ below rather
than above the desired frequency.  These sets will hear any video or audio
carriers in the bands occupied by television channels 4 through 6 mixed
right in with the FM broadcasts.  The bottom line is that good receivers
are hard to design no matter what the frequency range.  The same problems
will be encountered at 35 MHZ that I just described.  Your best bet is to
probably modify an existing RC receiver to take several frequencies and use
the PIC to switch crystals.  Commercial scanners to do this are expensive
because they need to be very good receivers to filter out all the nasty RF
that is there that you don't want.

       I am sorry for the length of this posting, but RF topics seem to come
up from time to time and a little discussion about receivers might be helpful.

       A lot of the link receivers that are used in car alarms and remote
controls are regenerative type receivers and do not fit the situation I was
describing above.  They have their own set of pit-falls.

       The Radio Amateur's Handbook published by the American Radio Relay
League or any similar publication by the amateur radio society of a given
country will probably have plenty of designs for simple receivers that
work fairly well.

       Don't forget that your PIC and LCD display may generate signals also
which means they should be shielded and run at the lowest practical clock rate.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1997\07\30@122521 by Philippe

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>At 11:52 AM 7/30/97 +0100, you wrote:
>>I'm planning on designing a "channels in use" indicator
>>for R/C purposes. I know there are commercial units for
>>sale, but they are expensive.

Try:
       ftp://ftp.luth.se/pub/misc/rc and look at RX project

It uses MC3362 and PLL.

Good luck,
       Philippe.

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1997\07\30@131327 by Steve Smith
picon face
Look up a device called a PLL02 used on CB raido or LC7136... Ect these will
generate a frequency -10.7 mhz for the 27mhz band if u use a different IF  or
add a programable divider then you can give these devices a binaray pattern
input for channel selection. I beleve a simple signal strength threshold will
be ok as the modulation is FM and the carrier is presant all the time the
channel is in use an MC3357 (back of memory) is a NBFM decoder device and a S
meter signal is available from one of the pins use a comparator to generate a
'in use' signal line. I may have some more info on old CB stuff in the attic
but will have to look. The bits were cheap as I remember but this was 1982
last time I played with them . Channel seperation is 10Khz !


Cheers Steve...

1997\07\30@132350 by Mike Keitz

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On Wed, 30 Jul 1997 10:43:20 -0500 Martin McCormick
<.....martinKILLspamspam.....DC.CIS.OKSTATE.EDU> writes:
{Quote hidden}

There is such a thing as an "image reject mixer".  It requires two mixers
and two 90 degree phase shifters.  Except at microwave frequencies, it
isn't very practical.  Image rejection depends on filters.  For a given
cost, the effectiveness of a filter is roughly determined by the
percentage difference between the desired and undesired frequencies.  At
35 MHz it will not be difficult to filter out an image 21.4 MHz away,
this is a large percentage difference.  Operating the LO below the signal
(which is possible because the LO will not have to tune through a large
percentage) will put the image at 15 MHz where it will be very easy to
filter out.  The receiver should have a decent (maybe 2-stage LC) front
end filter on it to prevent higher order effects.  The filter can most
likely be fixed-tuned; the problem is a lot simpler than a general
purpose scanner which needs to track the filter through the entire band.

If the band 910 KHz away from the R/C band is known to be vacant, it is
possible to convert directly to 455 KHz, accepting almost no image
rejection but having the opportunity to try the LO on both sides of the
desired signal.  If it is heard on only one side it is an image and not a
genuine signal.  This is a very simple receiver but it has the major
disadvantage of making the LO swing through the R/C band, where it could
leak out and interfere with R/C operations.

Many R/C receivers use single-conversion to 455 KHz giving almost no
image rejection. So it would be desirable to make the in use scanner
respond to the same spurious frequencies that they do.  If a channel has
interference on an image it shouldn't be used.  A scanner with poor image
rejection may not be bad, if its purpose is to look for signals that will
interfere with R/C receivers.  If it has good image rejection it could be
programmed to look at other frequencies which are likely to interfere
with the R/C receivers.


>        A few receiver designers were smoking something when they
>designed
>their local oscillator circuits and decided to put it 10.7 MHZ below
>rather
>than above the desired frequency.  These sets will hear any video or
>audio
>carriers in the bands occupied by television channels 4 through 6
>mixed
>right in with the FM broadcasts.

There are a couple of reasons to do this, it puts the image a larger
precentage away and it makes the LO easier to build because it works at a
lower frequency.  The energy in a TV signal is concentrated at the video
and audio carriers, so to an FM receiver much of the TV band is "quiet."
Most FM receivers run LO low for whatever reason.  Most users listen only
to strong local stations with them so image response is not a major
problem.

 The bottom line is that good
>receivers
>are hard to design no matter what the frequency range.  The same
>problems
>will be encountered at 35 MHZ that I just described.  Your best bet is
>to
>probably modify an existing RC receiver to take several frequencies
>and use
>the PIC to switch crystals.

One could use an existing receiver and replace the crystal with a
frequency generated by a PLL circuit.  It would also be necessary to tap
into some point which provides an indication of the energy in the
received signal.  The AGC line in an AM receiver is a good place to do
this.  If using an FM receiver, the IF chip would need to be one which
outputs a signal-strength function.  The discriminator coil could be
omitted.

1997\07\31@035004 by Mal Goris

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Oyvind Kaurstad writes:
> I'm planning on designing a "channels in use" indicator
> for R/C purposes. I know there are commercial units for
> sale, but they are expensive.=20

[snip]

> The hard part is obviously the RF design, and I wonder if anyone
> has any cheap and clever ideas on this.
>
> I live in Norway, and the RF frequency in question is 35.xxx MHz.
>
> The channel spacing is 10 kHz.

Everyone else who responded to this seems to think you were asking how
to build a superhet receiver. I sort of thought you were asking how to
cheaply build 40 or 80 receivers. If that is what you were asking,
I'll give you my ideas on how to do it. I'll assume you have 80
channels like in the US. To do this job cheaply you must reduce the
hardware. That is, build one single receiver that mixes the entire
35.XXX band down to baseband. You can decide whether it is best to use
a heterodyne or superhet receiver. Once the signal is at baseband, low
pass filter it to 80*10=800 kHz, sample and hold and 8-bit digitize
with at least 1600 kHz sampling. Store, say, 512 samples in your
micro's memory and then FFT it. You could probably choose a window
function that gives a broad mainlobe and low sidelobes because you are
not really interested in resolution but in detection. Use a threshold
scheme to decide whether there is a signal each channel. I am
assuming, with all of this that the R/C carrier always exists. That
is, a space is not indicated by no transmission. If it is then maybe
wavelet transforms would be a better choice than the FFT.

This is a pretty brief outline of what I would do but I think you
would need to understand the theory pretty well before attempting
it. One thing I can say is that I don't think I would attempt the
512-point FFT on a PIC.  Speed is not a concern so I would just choose
any old micro that has a proper stack. I think that an FFT of that
size can be most easily done with a recursive function that would go 9
or 10 levels deep.

Mal Goris
--
http://www.nfra.nl/~mgoris/

1997\07\31@041639 by Oyvind Kaurstad

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{Quote hidden}

I do know how FFT works, and I agree, it would not be recommendable to
try doing it on a PIC. At least not the '84.

-But I could of course use a laptop PC to do the job, but then the project
-will become very expensive, and that is undesirable.

-But mixing the entire 35.xxx band down to a lower frequency is probably
-a good idea.

-Thanks to everyone who responded!
-I did get some new ideas.

-¯yvind

1997\07\31@043505 by Oyvind Kaurstad

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<SNIP>

>>are hard to design no matter what the frequency range.  The same
>>problems
>>will be encountered at 35 MHZ that I just described.  Your best bet is
>>to
>>probably modify an existing RC receiver to take several frequencies
>>and use
>>the PIC to switch crystals.

>One could use an existing receiver and replace the crystal with a
>frequency generated by a PLL circuit.  It would also be necessary to tap
>into some point which provides an indication of the energy in the
>received signal.  The AGC line in an AM receiver is a good place to do
>this.  If using an FM receiver, the IF chip would need to be one which
>outputs a signal-strength function.  The discriminator coil could be
>omitted.

This is a very good idea.
But there may be a slight problem. The receiver is designed
to have a crystal connected, not a "frequency generator".

This may or may not pose a problem, depending on how the
circuitry is designed.

Maybe it is possible to make the PLL circuit behave in a crystal-like
manner?

One would have to get the schematics of the receiver to know.

But making a similar receiver without the servo-control options should
not be impossible. Only a signal-strength indication would be required.

-¯yvind

1997\07\31@121719 by Mike Keitz

picon face
On Thu, 31 Jul 1997 10:35:16 +0100 Oyvind Kaurstad
<EraseMEoyvind.kaurstadspam_OUTspamTakeThisOuTNOFAC.ABB.NO> writes:
><SNIP>
>
>>>are hard to design no matter what the frequency range.  The same
>>>problems
>>>will be encountered at 35 MHZ that I just described.  Your best bet
>is
>>>to
>>>probably modify an existing RC receiver
[...]
{Quote hidden}

My first attack would be to replace the crystal with an inductor and
varicap, so the crystal oscillator becomes a VCO.  Arrange to take a
sample of the oscillator frequency back to the PLL chip and the RF design
of the project is done.

If that doesn't work out so well, remove the feedback components so the
R/C receiver's crystal oscillator becomes an amplifier.  Then feed in an
external signal.


>One would have to get the schematics of the receiver to know.
>
>But making a similar receiver without the servo-control options should
>not be impossible. Only a signal-strength indication would be
>required.

Start with a receiver of the AM type.  It AC-couples the detected pulses
to the pulse demultiplexer and DC couples the average carrier level to
the IF amplifiers to adjust their gain so the pulses are always about the
right level.  This AGC line should be suitable for signal presence
detection.  It will detect any signal: AM, FM, or PCM, since they all
occupy the same bandwidth.  It may be desirable to speed up the time
constant to allow faster scanning.

1997\07\31@121731 by Mike Keitz

picon face
On Thu, 31 Jul 1997 10:22:00 +0100 Oyvind Kaurstad
<oyvind.kaurstadspamspam_OUTNOFAC.ABB.NO> writes:
{Quote hidden}

The major problem with this is the dynamic range of an 8-bit converter,
about 30dB.  Since the device is to be used at the flying field there
will be many nearby transmitters but still a need to detect far-away
ones.  A strong local signal will dominate the ADC and make signals more
than 30 dB weaker undetectable.  A MHz-sampling, 12 or 16 bit ADC which
would start to address this problem is still an expensive proposition.

This type of receiver is a good idea when several signals of
approximately equal strength are involved, or there is a need to rapidly
identify newly-appearing signals.

To me, the sweeping superheterodyne seems a more viable solution for R/C
since the band is narrow enough to use a fixed-tuned front end and a
simple VCO.

>>You could probably choose a window
>>function that gives a broad mainlobe and low sidelobes because you
>are
>>not really interested in resolution but in detection. Use a threshold

[...]


>I do know how FFT works, and I agree, it would not be recommendable to
>try doing it on a PIC. At least not the '84.

Thie first problem is the PIC's (any model) lack of sufficient RAM to
store the original samples, let alone the transformed results.

1997\07\31@123243 by hael Coop (AT)

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part 0 2590 bytes
The analogue detail that he needs to know is the RF channel spacing (the .XXX in the original note) - then he can start looking for a receiver that has the prerequisite master oscillator and divider circuitry to hit all those R/C channels while under digital control.

Any more than that, and sorry, I'm not an analog person !

Regards
MC


{Original Message removed}


'R/C RF scanner design. Suggestions?'
1997\08\01@034834 by Mal Goris
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Mike Keitz writes:
> >>Once the signal is at baseband,
> >low
> >>pass filter it to 80*10=3D800 kHz, sample and hold and 8-bit digitize
> >>with at least 1600 kHz sampling. Store, say, 512 samples in your
> >>micro's memory and then FFT it.
>
> The major problem with this is the dynamic range of an 8-bit converter,
> about 30dB.  Since the device is to be used at the flying field there
> will be many nearby transmitters but still a need to detect far-away
> ones.  A strong local signal will dominate the ADC and make signals more
> than 30 dB weaker undetectable.  A MHz-sampling, 12 or 16 bit ADC which
> would start to address this problem is still an expensive proposition.

Actually, the dynamic range of 8-bits is 48 dB, 6 dB per bit. On top
of that the 512-point FFT will give another 23 dB of dynamic range. So
you will have a total dynamic range of 71 dB. You are right about the
problem of having a strong signal in a channel next to a very weak
one. The sidelobes of the strong signal may be much greater than the
weak signal. But a good choice of window or even longer FFTs will
avoid this problem. If the purpose of this device is to decide whether
it is safe to use a channel then there would be a safety level
threshold. You would not need to actually detect signals at -71 dB, I
guess that -30 dB would be sufficient. Any signals in the same channel
that are more than 30 dB weaker than your own won't really affect
you. After all, the US regulations on R/C transmitters only require
out-of-band transmissions to be -25 dB.

> To me, the sweeping superheterodyne seems a more viable solution for R/C
> since the band is narrow enough to use a fixed-tuned front end and a
> simple VCO.

I think that is a good idea.

> >I do know how FFT works, and I agree, it would not be recommendable to
> >try doing it on a PIC. At least not the '84.
>
> Thie first problem is the PIC's (any model) lack of sufficient RAM to
> store the original samples, let alone the transformed results.

As the minimum sample rate is 1600 kHz an external memory with DMA is
essential.

Mal Goris
--
http://www.nfra.nl/~mgoris/

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