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'[EE]: FM transmitter problems'
2002\06\08@194942 by Herbert Graf

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Hello to all, was wondering if someone might be able to helping me figure
out what wrong with the FM transmitter I have. First a little background...
:)

I built an MP3 player out of a Pentium PC and installed it in the car. I
currently have an audio line going to the front which I connect to a tape
adapter, the car radio has no audio in, and I don't want to modify it. This
system works, but is a little involved.

So, I had the "brilliant" idea of using an FM band transmitter instead. I
tried a few retail cheapies (read RadioShack) and none worked well, the
radio catches the signal but gets louder, and softer, then louder again, at
a freq of about 0.5Hz.

I decided I'd buy a cheap kit and build my own Stereo FM transmitter, I
decided on the CK222 by CanaKit (http://www.canakit.com). It is a simple stereo
transmitter based on the BA1404 with a single transistor acting as a power
amp. I assembled it and got it work, however it exibits the EXACT same
problem, the radio signal comes through but it alternates between really
quite and loud, and about 0.5Hz. I am truly at a loss as to WHY this is
occuring, I also tried my home theatre receiver, with the same results!@ :(

I connected my trustly HP 1710A scope to it (a 100MHz analog dual trace
beauty from the 70's!) and I don't quite understand the waveforms fully
(both are DC coupled, 0.1V/div). I made links to them here:

http://repatch.dyndns.org:82/06080001.jpg (20ns/div) 84k
http://repatch.dyndns.org:82/06080002.jpg (50ns/div) 109k

There are TWO signals, one at just below 50MHz (which doesn't seem to
modulate at all) and one at something less (which DOES seem to modulate),
the interesting thing is the lower freq signal doesn't change much if I
change timebase, which tells me there are some harmonics or something
happening since I'm at the edge of the scopes input bandwidth. I had the
transmitter set at 88.6 MHz.

If anybody has any ideas as to what might be causing this I'd love to know,
I have little RF experience (hence the buying of a retail product at first,
and then a kit) but would love to figure this out. Could it be the AFC
circuit in the digital tuners "hunting" for the not so perfect stereo
signal? I say this since if I short out the crystal on the board (thereby
removing the FM stereo pilot signal at 19kHz (I think) and causing the
receiver to go into mono mode) the problem magically disappears. Thanks in
advance for any help/advice. TTYL

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2002\06\09@003216 by Dave Tweed

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Herbert --

> http://repatch.dyndns.org:82/06080001.jpg (20ns/div) 84k
> http://repatch.dyndns.org:82/06080002.jpg (50ns/div) 109k

> There are TWO signals, one at just below 50MHz (which doesn't seem to
> modulate at all) and one at something less (which DOES seem to modulate),
> the interesting thing is the lower freq signal doesn't change much if I
> change timebase, ...

I'm not sure what's wrong with your transmitter, but I think the "two
signals" you're seeing are simply an artifact of having the scope intensity
turned up too high. You're seeing the "50 MHz" signal during the retrace,
giving the appearance of a lower-frequency signal.

Even with a wideband (+/- 75 kHz deviation) FM transmitter, at VHF
frequencies, you're not going to "see" the modulation on a scope.

> ... I'm at the edge of the scopes input bandwidth. I had the
> transmitter set at 88.6 MHz.

The scope should give you useful qualitative results at that frequency.
Just don't trust the vertical calibration for specific amplitude values
too much. Look for relative changes.

> ... would love to figure this out. Could it be the AFC
> circuit in the digital tuners "hunting" for the not so perfect stereo
> signal? I say this since if I short out the crystal on the board (thereby
> removing the FM stereo pilot signal at 19kHz (I think) and causing the
> receiver to go into mono mode) the problem magically disappears.

Interesting. BA1404-based designs tend to be notorious for unwanted
coupling of RF through the power supply and to the audio inputs. I would
suspect that you're seeing some sort of effect of RF getting somewhere it
shouldn't (perhaps the MPX BALANCE inputs?). Search the web for BA1404 and
check out the notes that various people have accumulated on this chip.

-- Dave Tweed

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2002\06\09@012609 by Rick C.

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My first thought was to try another receiver like a portable Walkman, but you
said you tried another receiver with the same results. So, not a receiver
problem but I'd be sure with a third receiver anyhow. If both transmitters are
doing the same thing, then it doesn't sound like that could be the answer. FM
broadcast transmitters don't create anything weird that could cause this
phenomenon. The only thing left is your PC. Have you tried plugging in a set of
headphones to verify your source is stable? It could be something the PC is
doing to the sound card. Other than that, you have a pretty strange problem. I
use two of the Ramsey kit FM stereo transmitters. One is a tuned LC using the
BA1404 and one is a fully synthesized programmable transmitter that is very
stable. I have excellent results with both.
Rick

Herbert Graf wrote:

{Quote hidden}

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2002\06\09@024831 by Herbert Graf

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Actually after I sent the original message I tried it with my walkman, with
the same results. The source while testing has also not been constant, I've
tried my walkman, a CD player and the PC, all with the same results, so I
believe I've covered all those bases. You mention you used a BA1404 kit and
it worked fine, was that with a digital tuner as a receiver or with an
analog tuner? Thanks for any info. TTYL

{Quote hidden}

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2002\06\09@024841 by Tal Bejerano - AMC

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hi Herbert

I have the same kit , how can I assist you?
mine works fine.
did you double check the assembled kit ? all parts in place ? what about the
voltage supply? did you know there is two supplies?
6V and 12V ...

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\09@032204 by Tal Bejerano - AMC

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I use both digital and analog tuners with no problems at all
btw, we use the SAME kit ck222.

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\09@051859 by Scott Stephens

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From: Herbert Graf <.....hgrafKILLspamspam.....EMAIL.COM>
Subject: [EE]: FM transmitter problems


>I built an MP3 player out of a Pentium PC and installed it in the car.

Oh my, anyways try piping signals from two different sources into your FM
stereo transmitter L & R channels, and see if it creates the same .5Hz
fading. Perhaps there is some kind of wierd correlation between how L-R
signal is encoded on the CD, and encoded in FM stereo. Try switching L & R
channels too.

hit from yahoo search of "stereo FM transmitter subcarrier L-R"
http://members.tripod.com/~transmitters/stereo.htm

Scott

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2002\06\09@075145 by lintech

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On 9 Jun 2002, at 1:34, Herbert Graf wrote:

> Actually after I sent the original message I tried it with my walkman, with
> the same results. The source while testing has also not been constant, I've
> tried my walkman, a CD player and the PC, all with the same results, so I
> believe I've covered all those bases.
Try the transmitter with just one channel input - left or right.
It could be some sort of phase cancellation between the 2 channels.


George Smith

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2002\06\09@105545 by Herbert Graf

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> > Actually after I sent the original message I tried it with my
> walkman, with
> > the same results. The source while testing has also not been
> constant, I've
> > tried my walkman, a CD player and the PC, all with the same
> results, so I
> > believe I've covered all those bases.
> Try the transmitter with just one channel input - left or right.
> It could be some sort of phase cancellation between the 2 channels.

       I just tried that, with one channel disconnected (I did alternate) with
similar results. I checked the audio inputs with my scope and there don't
seem to be any problems there. However it appears there is a problem with my
scope now! :( It doesn't scan anymore, just happened, this sucks, might not
be able to debug this, or anything else!@!@ Thanks, TTYL

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2002\06\09@110210 by Herbert Graf

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Yes, actually I was more careful then I usually am when I assembelled it and
I am confident everything is everywhere it should be. I figure if it works
as much that it does I must have gotten some of it right!

I am amare of the two power supply possibilities (and actually I'm quite
impressed by the power supply design, they seem to have covered most if not
all the bases on getting it automobile ready). I am currently using the
battery supply (4 duracells) in order to eliminate noisy supply problems.

Thanks, TTYL

> {Original Message removed}

2002\06\09@140816 by Rick C.

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It's with a digital tuner. It works sufficiently with a stable temperature
environment but in an car, I wouldn't take a chance. That's why I went to the
synthesized unit. "Rock" solid and good range. I used it during a parade once
where about 100 Honda Gold Wings (motorcycles) were wanting to all play the
same song on their radios while on the parade route. I parked next to the
parade route, played a CD through my portable CD player into the synthesized
transmitter and it could be heard along the whole parade. They were very
impressed and the audience couldn't believe the sound of 100 Gold Wing
amplifiers.

You said the source was not constant too? That's gotta be a clue. Is it
varying? The same rate?
Rick


Herbert Graf wrote:

{Quote hidden}

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2002\06\09@152854 by Jeff Berosik

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>  Herbert Graf  wrote
>  I had the transmitter set at 88.6 MHz.

I have no Idea if this is part of the problem or not, but my car radios can not
tune to an even decimal point station.  My car radios will only tune in odd
stations like 88.5 MHz or 88.7 MHz.  (not divisible by 2 ??)

Jeff Berosik

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2002\06\09@161554 by Herbert Graf

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> It's with a digital tuner. It works sufficiently with a stable temperature
> environment but in an car, I wouldn't take a chance. That's why I
> went to the
> synthesized unit. "Rock" solid and good range. I used it during a
> parade once
> where about 100 Honda Gold Wings (motorcycles) were wanting to
> all play the
> same song on their radios while on the parade route. I parked next to the
> parade route, played a CD through my portable CD player into the
> synthesized
> transmitter and it could be heard along the whole parade. They were very
> impressed and the audience couldn't believe the sound of 100 Gold Wing
> amplifiers.

       Problem with that though is cost, the cheapest synthesized transmitters
I've seen are way out of my range.

> You said the source was not constant too? That's gotta be a clue. Is it
> varying? The same rate?

       By "not constant" I meant I varied the sources, I tried a CD player, a
walkman and my VCR, all exibited the same problem. Thanks, TTYL

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2002\06\09@162008 by Herbert Graf

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> >  Herbert Graf  wrote
> >  I had the transmitter set at 88.6 MHz.
>
> I have no Idea if this is part of the problem or not, but my car
> radios can not
> tune to an even decimal point station.  My car radios will only
> tune in odd
> stations like 88.5 MHz or 88.7 MHz.  (not divisible by 2 ??)

       You're right, the radio in the care only tunes "odd" decimal frequencies,
however my home theatre receiver can (A Sony DA50ES), I set it to 88.6 when
I got home since it was a nice empty area of the band. Is the fact that I
only see around 44 MHz in the waveform a clue? Perhaps the transmitter is
actually transmitting at ~44MHz and I'm just picking up a harmonic? I'll try
a scanner I have and see if I can pick up the transmitter at ~44MHz. Thanks,
TTYL

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2002\06\10@072354 by Alan B. Pearce

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> My first thought was to try another receiver like a portable
> Walkman, but you
> said you tried another receiver with the same results. So, not a receiver
> problem but I'd be sure with a third receiver anyhow. If both
> transmitters are
> doing the same thing, then it doesn't sound like that could be
> the answer.

Sounds to me like the signal going into your receiver is far too strong, and
putting the AGC around the RF stage into meltdown. Around 0.5Hz would be a
reasonable time constant for this.

Try putting an attenuator between your modulator and the receiver. Do you
have the modulator in an RF shielded box?

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2002\06\10@073838 by Rick C.

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I've never seen an FM receiver with AGC. RF intensity should have no effect on
FM audio levels. AM however, this would be true. FM receivers do have AFC to
keep the receiver from loosing lock.
Rick - Chief Engineer - WTRM-FM

"Alan B. Pearce" wrote:

{Quote hidden}

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2002\06\10@080358 by Alan B. Pearce

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>I've never seen an FM receiver with AGC. RF intensity
>should have no effect on FM audio levels. AM however,
>this would be true. FM receivers do have AFC to
>keep the receiver from loosing lock.

Post mixer I would agree with you, but I have seen gain control on the RF
stage to get good IM performance. I seem to recall some early FM IF chips
had a signal strength output to do this.

If there is no AGC at all that may even exacerbate the problem if he is
feeding a very strong signal into the receiver. The IM resultant may be
producing a similar problem.

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2002\06\10@092513 by Rick C.

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Some FM chips might have a pin to provide signal strength indicators by
measuring saturation of the IF, but I have never seen this line controlling
the gain of the RF. Don't forget, we're talking about a Part 15 transmitter
outputting in the milliwatts, and I can't see this level coming anywhere
close to overloading the input. And even if there were ample RF energy, it
would probably overload every stage of the receiver and do more harm of
environmental exposure rather than attenuating the audio level because of
IMD. I've taken a walkman up to the base of a 50,000 watt FM tower and never
had an audio attenuation problem unless I was listening to an adjacent
channel. ;-)

If you reference me to a domestic (consumer type) FM receiver model with AGC,
I would like to know so I can change my statement as to never seeing an FM
AGC circuit.

Rick

"Alan B. Pearce" wrote:

{Quote hidden}

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2002\06\10@100953 by 4HAZ

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We here in the communications industry have seen 2 uses for the Received
Signal Strength Indicator. The first is connecting a voltmeter to aid in
aiming a beam. The second is an internally programmable function that serves
to conserve power by reducing output power according to the strength of the
incoming signal on handheld units, it can conserve battery power when 2
handhelds are talking to each other, but it can also cause a handheld to cut
back too far when talking with a mobile. Obviously this feature should be
used with care, only on non-repeater frequencies, and only in applications
which do not use 50w mobile units.
$.02 KF4HAZ - Lonnie
----- From: "Rick C." <rixy@

> Some FM chips might have a pin to provide signal strength indicators by
> measuring saturation of the IF, but I have never seen this line
controlling
> the gain of the RF. Don't forget, we're talking about a Part 15
transmitter
> outputting in the milliwatts, and I can't see this level coming anywhere
> close to overloading the input. And even if there were ample RF energy, it
> would probably overload every stage of the receiver and do more harm of
> environmental exposure rather than attenuating the audio level because of
> IMD. I've taken a walkman up to the base of a 50,000 watt FM tower and
never
> had an audio attenuation problem unless I was listening to an adjacent
> channel. ;-)
>
> If you reference me to a domestic (consumer type) FM receiver model with
AGC,
{Quote hidden}

RF
> > stage to get good IM performance. I seem to recall some early FM IF
chips
> > had a signal strength output to do this.
> >
> > If there is no AGC at all that may even exacerbate the problem if he is
> > feeding a very strong signal into the receiver. The IM resultant may be
> > producing a similar problem.
>

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2002\06\10@113428 by Alan B. Pearce

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>If you reference me to a domestic (consumer type) FM
>receiver model with AGC, I would like to know so I can
>change my statement as to never seeing an FM AGC circuit.

Hmm, well I am well out of RF stuff these days, so cannot do this, and it is
a fair while (as in a couple of decades) since I had anything to do with it.
Things will have changed significantly in that time.

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2002\06\10@120728 by Peter L. Peres

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On Sun, 9 Jun 2002, Herbert Graf wrote:

>
>        Problem with that though is cost, the cheapest synthesized transmitters
>I've seen are way out of my range.

You can improve a tx with a PLL. F.ex. one from Philips (TSAxxxx). I2C
control, use a PIC to drive it. It is fairly easy if you already have
varicap tuning.

Peter

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2002\06\10@121532 by Harold M Hallikainen

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       That's an interesting possibility. A friend built a little FM
transmitter and drove it from his computer sound card. The poor low pass
filters on the sound card output caused interference between the D/A
sampling frequency and the stereo encoding. I think it just resulted in a
lot of noise on the FM. He added an LPF chip from Linear Technology and
it got a lot better.

Harold


On Sun, 9 Jun 2002 02:20:21 -0500 Scott Stephens <KILLspamscottxsspamBeGonespamATTBI.COM>
writes:
{Quote hidden}

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2002\06\10@125356 by Peter L. Peres

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On Mon, 10 Jun 2002, Alan B. Pearce wrote:

>>If you reference me to a domestic (consumer type) FM
>>receiver model with AGC, I would like to know so I can
>>change my statement as to never seeing an FM AGC circuit.
>
>Hmm, well I am well out of RF stuff these days, so cannot do this, and it is
>a fair while (as in a couple of decades) since I had anything to do with it.
>Things will have changed significantly in that time.

I don't have a reference to a type but I have two tuner blocks scrapped a
long time ago from higher end FM receivers that have AGC inputs on the
first stage. They *may* have been used with just a switch (dx/local). BUT
the books say that a high end FM strip will have AGC to drive the tuner
(and allow listening to adjacent strong stations without IM), the S meter,
the squelch mute system and AFC defeat during tuning, to defeat stereo and
to switch to lower bandwidth (in IF) with low signal. I suspect that some
higher end FM receiver components (the kind that cost ~$1000++) may
actually do some or all of this.

Peter

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2002\06\10@131715 by Peter L. Peres

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Your problem is that the tx is not crystal controlled (not the stereo
crystal), or that it is crystal controlled and its PLL is being jammed
heavily by something you supply.

Similar problems occur with non-crystal-controlled FM transmitters where
the receiver's PLL goes crazy trying to decide which channel you're on.
Since stereo means larger bandwidth (on higher end receivers).

You can try in this order:

1) Receive in an inexpensive stereo FM receiver at some distance (10
meters or more). If it's ok you need to cut the Tx power, you are
saturating the radio.

2) Find and disconnect the Tx PLL and see if the problem goes away. The
station will drift but a good receiver should be able to lock onto it for
a couple of seconds. If this helps, you may need a trimmer on the Tx
crystal to pull it to an exact frequency that the receiver PLL likes.

3) Force mono operation on the receiver, and confirm that the problem goes
away. Then go to stereo, and on the Tx try to change the level of the
19kHz carrier. It is likely too high or too low (more likely).

hope this helps,

Peter

PS: For such a low distance you should have 1mW or so of power. The Tx
probably has a hundred times more. This is WAY too much.

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