Technical > Ham Radio > FM Highpitched tone, ideas needed

FM Highpitched tone, ideas needed

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TorDek: "DSLR... Here, were not just experts... were also vindictive bastards..."

I am no genius, but I would suspect the internal components of the transmitter, including filtering of possible internal RF.

I have heard this same noise on some equipment for electric guitars. Turn the gain up and the high pitch sound would go away.
(but then it wouldn't be the sound you were trying to achieve)

Also, are you using good shielded cables all around?
Extra grounding of equipment may also help.

And if you are using their external antenna, is it a good distance from the transmitter and stereo receiver?

Its a very familiar sound to me. Restrict the power going out and the noise appears. Crank it up and its gone. Just like one of the pedals for my guitar did. (which I ended up returning)

Hope this helps a little. Appears to be a neat unit, but I am really suspecting the internal electronic parts on this one.

Keep us posted of your progress, and good luck.
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Long you live and high you fly, and smiles you'll give and tears you'll cry,
and all you touch and all you see, is all your life will ever be.

Robotics, I have tried their external antenna and my own a good distance away, but I get the same issue. The only cable I'm using is the power, antenna feed line and a USB cable (since the unit has its own sound card). I've tried disabling that sound card and using straight audio into the transmitter, but no change. Its grounded to a metal PC case.

They have also sent me a new unit, which does the exact same thing, and I have their engineers baffled, they are unable to reproduce the issue with the cards in their lab with the same settings. I've tried putting it in the other end of the house, but same effect.
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TorDek: "DSLR... Here, were not just experts... were also vindictive bastards..."

reply to Cjaiceman

reply to Cjaiceman
For what it is worth that tone is right about 8.8 kHz, but it is a moving target, I was able to get a couple of good readings of 8.8 and 8.9 however the noise floor is so high, my meter is choking up trying to grab a frequency lock.

None the less if, we put together the frequency measured of 8.8 kHz, and even though it is not 9.5 kHz, which would be an expected subharmonic of the 38 kHz sub carrier used to deliver the L-R needed for stereo or a subharmonic of the 19 kHz pilot tone used to indicate the presence of stereo, and the fact it disappears when mono is enabled, I strongly believe it is related to something in that transmitter is not linear, and when operating under reduced power is distorting either the 19 kHz pilot signal and or the 38 kHz sub carrier producing an audible subharmonic.

And just to add a twist, the 38 kHz sub carrier it transmitted as a suppressed carrier double sideband AM signal which may not be playing well with the others under reduced power.

Now there a few areas which can cause the non-linearity, the modulator and stereo injection, the power amplifier (PA), and the antenna / feed line, going out on a limb we can probably rule out the antenna and feed line, as the problem worsens with a power reduction, that leaves the modulator and stereo injection and or the PA, all of which are problems you have no real way of solving.

Wayne
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"It is sobering to reflect that one of the best ways to get yourself a reputation as a dangerous citizen these days is to go about repeating the very phrases which our founding fathers used in the struggle for independence." - Charles A. Beard

What baffles me though is they have sold a lot of these units, and they have never heard of them doing this. They tried and were not able to reproduce the issue in their lab with these units, and I have 2 of these units doing the exact same thing. I'm pretty stumped...
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TorDek: "DSLR... Here, were not just experts... were also vindictive bastards..."

reply to Cjaiceman
You are probably hearing a sub harmonic of the pilot tone. Don't know if there is going to be an easy way to fix that. I would first suspect poor power filtering on the card. But I would really have to put it on the bench and go through it front to know for sure.

Also.. note the unit is NOT part 15 compliant in any shape fashion or form unless perhaps it was put into a well shielded dummy load.

If its not Part 15 compliant, its really close. I don't have the 3W unit, I have the 1W unit (same price as the 100mW unit). I also run it at 5%, which my power meter shows it putting out about 55-60mW (depending on the frequency). I also had a small 50 Ohm attenuator from a project a couple years ago that I put on, and the meter shows that its putting out about 20mW into a 0dB antenna. The whole thing is below ground level in my basement and you lose the signal about 75-100' away from the house even with a very sensitive car radio. Also considering the fact that I live on 20 acres of land in a fairly rural area, and my closest neighbor is over 1200 feet from me, I don't think its bothering anyone. The frequency I have chosen is in line with the FCCs FM channel listing, allowing at least .4 MHz before running into another stations main frequency (ie, 104.7MHz as my frequency, 104.3MHz is the closest on the lower side, and 105.1MHz is the closest on the upper side), I can receive both of those stations without any interference, even sitting next to the unit. I have also scanned from 50MHz to 150MHz and found no spurious harmonics that didn't go away after getting more than 1-2 feet away from the unit. I'm not really worried about interfering with anyone because there isn't any power behind the unit, and I'm out in the middle of no where. I also don't run the unit 24/7, just when I want to listen to music around the house.
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TorDek: "DSLR... Here, were not just experts... were also vindictive bastards..."

Part 15 compliant, for an FM transmitter, means a measured field strength of 250 microvolts/meter at 3 meters. That's not a very strong signal.

That said, you're probably not going to bother anyone.

BTW: Nice job scanning 50-150. Most non-compliant FM transmitters draw complaints because of interference to the 108-136 Mhz AM aircraft band. Hopefully, that transmitter card has a good low-pass filter. If you didn't hear anything up in the aircraft ad, it probably does.

reply to Cjaiceman
I just wanted to update everyone about the situation. I got this from the manufacturer:

Guess what, I just went through 30 boards and I did find this weak tone in a few of them.
 
This is only noticeable when you set power very low, 20-30% of the max. If its set to full there is no tone.
 
The culprit is that some digital noise of the CPU is getting into the signal, but only when power is set low.
 
It is quite easy to reduce this tone sufficiently without soldering, all it takes is a ELKO capacitor 100-470uF rated at at least 5V, such as this one here:
http://search.digikey.com/us/en/products/ECA-0JM471/P5114-ND/244973
I believe many of you have something similar at home. 470uF worked great here in the lab.
 
You don't even have to solder it to the board, just cut the legs short (half an inch or so), than plug it into socket provided for RDS daughter board.
Align the negative pin to the socket marked GND and positive pin to the socket marked +5V.
 
We will be installing these capacitors on all boards starting today, just in case.
 
Thanks for catching a bug, it is appreciated!
 

It's nice to see that they found what I suspected.. (poor power filtering).

Also, I wasn't all that worried about the part 15 compliance, and in your case I doubt anyone else would be either. I just don't like it when those selling this stuff to unknowing people try to pass it off as being legal, when they know it's not. I was just making sure you knew it wasn't part 15 compliant.

Now had you been in Florida, and did like they said running an outside antenna at the full 4 watts, you could find yourself not only fined by the FCC, but also arrested by the locals and hauled off to the slammer with felony state charges.

One other thought on how to easily get rid of the tone, get a 10db attenuator rated for 10W and put that in the RF out line. Once you have that you should be able to run it at full power, and get around your same coverage. A 10db Attenuator will give you 10% out when the card is set to 100%.

That's still 400mW, though, isn't it?
I thought the "power" allowed was based on field-strength measurements, so that it wouldn't matter what you ran for an antenna as long as the field-strength readings were within the rules.
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One man's Magic is another man's Engineering.

Hmmmm...but if the tone is on the modulated carrier, how is cutting down the radiated power going to get rid of the tone?
Just make the overall signal weaker so it's not so noticeable?
--
One man's Magic is another man's Engineering.

Perhaps you missed the part where he said if he was running anything above 70% power the tone went away. Running through the attenuator was a way to turn up the power to where the tone vanished, with out increasing his coverage area.

yep, I missed that part!