Hummmmm

[K9ACT]

I had the Class D working well enough on 160 at 100w to make a few contacts but now a gremlin has jumped in and I can't seem to find him. Here are the symptoms:

1. Large hum content on output inversely proportionate to freq. Not heard on 80 but unacceptable on 160.

2. Changing nothing but the VFO freq and receiver frequency, I can see and hear it decrease as I move up in 500kc steps.

3. Using a lab supply in place of the modulator produces a clean sig with no hum on both bands.

4. Using the modulator on 80 meters produces a clean sig with no hum.

5. Power level is not a factor. As soon as I start applying HV (10V?) the hum begins.

Any ideas?

js

[kf1z]

Possible PWM filter is not happily filtering ?

Could measure the PWM generator frequency, do some math and see where the harmonics lay.....
Then transmit close to any of those frequencies, and see if your hum Peaks ?

example.....190khz .... 10th harmonic is....... 1.9mhz

Just a wild stab....
but I can't think of a reaason the RF amplifier would grumble on 160 meters. and since it's clean on a bench supply... then that sort of rules out the rf deck.

Also, even while on 80 meters.... tune up the band by the frequency of the PWM generator... the 160-190khz whatever it is.. with your reciever... or have someone close by search around for you on a SDR... panoramic display...
So transmit on 3700... have someone look at 3700 + pwm freq .. 3860 to 3890kc


If you have excessive signal there, then the PWM filter is almost certainly a problem.

That's really all I can think of at the moment...


Make sure you're running the proper current and voltage too....... for your 5 ohm filter ( 35 volts@7 amps .... 32v @ 6.4 amps etc )

I do not know for sure how far off you can be from the designed impedance for the filter before it really becomes ineffective.

Also checking your PWM frequency to make sure it's in the correct range is a good thing too..

[Gilly]

Hi Jack

I wonder what frequency the hum is....say 60/120Hz...
What happens if you remove the Mic and preamp from the Modulator.....
Does the power for the TX driver and Modulator driver come from the same power supply....

Have you checked the waveforms of modulator and TX drivers using a CRO.....

I think you use an air wound coil for the first inductor of your Modulators filter......I have never used one only CH777060 cores but they look like they might radiate energy to some extent....
Maybe some power supply lines are too close to that coil.....

Could you post a photo of your equipment......


Wayne

[steve_qix]

Probably an obvious question, but how are you demodulating your signal to hear the hum? Does the hum appear on an oscilloscope if you monitor the RF output? Do other people on the air hear the hum on 160, but not on 75, or do you only hear it (this one may be tricky, as it would be advisable to get the same person to give you a report on both bands)?

Let us know

Regards,

Steve

[K9ACT]

Probably an obvious question, but how are you demodulating your signal to hear the hum? Does the hum appear on an oscilloscope if you monitor the RF output? Do other people on the air hear the hum on 160, but not on 75, or do you only hear it (this one may be tricky, as it would be advisable to get the same person to give you a report on both bands)?

Steve

Those are very good questions and part of the reason I have not responded to the other suggestions.

The only thing I can say for sure is that the SDR display is very different on the two bands. On 80 it is nominal but on 160 I can only describe it as dirty. Hum would appear as energy close in to the carrier but this is over the whole signal like a 5X increase in the noise.

The RF out is clean and the RF envelope shows no hum.

Something else has come up since posting to cloud the issue. When I connected the scope to the mod output, it went away. Unlike the last time when I unknowingly connected the scope ground to the floating ground and blew up all sorts of stuff.

Until I can find the gremlin, I have a wire running from the scope ground to the mod ground and the hum is gone but the SDR views still differ as above.

Now for the real problem, I will start a new thread.

js

[steve_qix]

Ok, I think I see what's happening.

It is almost impossible to listen, using a receiver, to a transmitter operating in the same facility. The inevitable ground loops will interject hum onto the demodulated signal.

The only sure way to monitor your signal for amplitude modulated hum is with a direct-connected (in the transmitter output) low distortion RF pickup of some kind, preferably feeding a modulation monitor or other high quality audio system with headphones.

The problem you are having is very common - I have certainly had it myself many times, and so have most AMers I know who try to monitor their signals using a local receiver. Of course the other problem you may also see when doing this is driver pickup, and that will really give false readings !

Regards,

Steve