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 Post subject: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 2:36 am 
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I've built the following modulation monitor for viewing my two tube transmitter audio on an oscope.

Attachment:
modulation monitor.png
modulation monitor.png [ 106.89 KiB | Viewed 324 times ]


I used a smaller value of capacitor and the diode is a detector diode from a defunct solid state radio chassis. The headphones were replaced by a 2K resistor and the coil is a small inductor like what is normally used in a speaker crossover.

What should the displayed signal look like?


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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 3:29 am 
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Tube Radio wrote:
What should the displayed signal look like?


With your scope set in the audio range 100% modulated sine wave should look something like this

Image

Here is 125% modulation of a 1KHz tone:

Image

EDIT: I just realized I misread the OP's posting.


Last edited by Macrohenry on Jun Wed 12, 2019 4:12 am, edited 1 time in total.

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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 3:54 am 
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That is the normal output of the transmitter I get on my scope without any modulation monitor circuit.

With the modulation circuit as I built it I was getting a display that was mostly a positive waveform with a very small negative waveform as though it was 1/2 wave rectified.

I disconnected the 2K resistor and I get just an audio signal along with a little of the carrier with the audio much greater than the carrier.

I'm wondering if it is the diode I used. Have no clue what it is.


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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 4:20 am 
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In the headphone circuit what you should hear is the effects of distortion if over 100% and low audio if less than 100%. The diode rectifies the RF leaving mostly audio.

Turn the sweep down to 5kz, modulate the transmitter with 1khz, as the input gain to the transmitter audio is increased the effects of of over-modulation will appear as defined breaks in between the envelopes. Use the same diode detector circuit but do provide a load for the headphones.

An alternate is to use an RF probe on the scope.

BTW Your the digital clock guy and have electrostatic scope tubes lying about. Taking links and voltage dividers, no horizontal or vertical amps needed or sweep to make a trapezoid modulation monitor. using a phase shift between the horizontal & vertical circuits a doughnut pattern is produced. No diode or RF probe is used and no loading of the scope tube deflection plates.

There should be hookups on the web for this simple scope tube monitor. I have one using a 3" Millen scope directly connected to the plates for 6 meters. I had to monitor when I was doing FAX. Holding 95% modulation at 2.5khz.

No need to tie up the bench scope...

YMMV

Chas

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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 1:33 pm 
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I may have to look into that.

I do have several working scopes though so tying one up won't be much of a problem.

Think I will look for the proper diode, capacitor and then try the 2K or a little higher load again to see if the circuit now works better.

Also I'm pretty sure that circuit was meant to work with much higher powers than the 100mW my transmitter puts out so a three turn coil possibly won't be good enough.


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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 4:26 pm 
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Referencing that monitor circuit, it delivers audio to the headphones.

For BC band the coil could be a generic RF coil with a fixed or variable cap shunted to resonate at or near the home transmitters frequency. Such a tuning effect would make it far more sensitive and allow for an amplitude adjustment. Use a 1N34 diode.

When an audio tone is used to modulate the scopes internal trigger could lock the trace. For other programing the trace will be dancing around.

You can also lock the trace with an external trigger to the programming input to the transmitter.

Earlier, an RF probe would do the job too.

I did my leaning curve as said on using scopes for RF purposes, wanting to be sure that the 2.5khz fax tone was not over-modulating. At that time there was a lot of 6 meter AM activity and many did complain about the FAX signal. I had to be sure it was not 'splatter".

Chas

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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Tue 11, 2019 7:21 pm 
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I may eventually invest in a good quality RF probe.

I built the circuit and at 10K load it seemed to be ok. Had to connect directly to the RF generator I tested it with at work to get it to do right given the low signal level. I got a bit of carrier on the output as well, but found increasing the load to 100K substantially reduced the carrier while barely affecting the audio amplitude up to 10KHz.

This is more for display purposes as I have a VU meter on the transmitter set to go into the red on over-modulation, although it is cool to see the waveform symmetry change based on the audio processing I use.


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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Thu 13, 2019 1:09 pm 
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The diode detector will also produce a DC level, since it rectifies the RF. The main component should be recovered audio. There is residual RF, which is filtered by the capacitor. The capacitor value is a trade off between rolling off the higher audio frequencies and effectively eliminating or reducing the residual RF.

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 Post subject: Re: Modulation monitor circuit question
PostPosted: Jun Thu 13, 2019 1:47 pm 
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So I could use the DC level to measure signal strength as well or just feed an op-amp to drive a meter after filtering out the audio.


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