Post by DanielJackson on Sept 7, 2019 16:33:50 GMT -6
Hello I have been involved in clandestine radio on HF in the 6.900 MHz region in the past and on 11 meters. To solve some problems encountered on 11 meters I devoted a group to the study of modulation techniques and so I met David Dowler and John Bartal the innovators of the Mauldulator Asymmetrical Modulation circuit. This circuit solved the problem of clipping the negative audio peaks and hence reduced adjacent channel interference and other unwanted in and out of band products. However as I began to work to analyze this circuit in LTSpice software analysis I discovered allot of total harmonic distortion. After some time I developed and alternate method for asymmetrical modulation that drops the THD distortion down to 2%.
In time I hope to have these circuits made available as kits, the versions I have now work with Galaxy DX series radios. In time I want to develop a circuit that can be used for screen grid modulation to accomplish this kind of modulation with vacuum tubes.
Hello I have been involved in clandestine radio on HF in the 6.900 MHz region in the past and on 11 meters. To solve some problems encountered on 11 meters I devoted a group to the study of modulation techniques and so I met David Dowler and John Bartal the innovators of the Mauldulator Asymmetrical Modulation circuit. This circuit solved the problem of clipping the negative audio peaks and hence reduced adjacent channel interference and other unwanted in and out of band products. However as I began to work to analyze this circuit in LTSpice software analysis I discovered allot of total harmonic distortion. After some time I developed and alternate method for asymmetrical modulation that drops the THD distortion down to 2%.
In time I hope to have these circuits made available as kits, the versions I have now work with Galaxy DX series radios. In time I want to develop a circuit that can be used for screen grid modulation to accomplish this kind of modulation with vacuum tubes.
Hey, very nice and welcome to the forum!
Do you have a version of this that will work with any generic AM transmitter as opposed to just the Galaxy?
Post by DanielJackson on Sept 9, 2019 10:53:53 GMT -6
Yes the circuit will work with most every modulation scheme, I have yet to design a version for vacuum tubes but screen grid modulation seems most feasible. The circuit uses Op Amp sections and a transistor or two at the output.
The first image shows the asymmetrical sinewave @ 1 kHz, the second show the circuit in the Op Amp sections that make it work and shift the bias of the audio all above 0V so that all of the audio sinewave is within the positive domain of an oscilloscope.
Looks like the biased clippers that have been used in audio processing of times past in the broadcast industry. My audio processor does something similar by biasing the positive OP Amp input voltage across a potentiometer wiper going to negative and positive rails of the OP Amp and feeding the negative input with audio. By adjusting it the voltage plus AC audio signal on the output of the OP Amp will simply go into negative and positive soft diode limiters with one side being biased further towards a rail to make it clip sooner either positive or negative depending on potentiometer placement. See middle right part of my schematic (labeled Assymetrical Clipper).. i.imgur.com/Il6ngIv.png The trade off is peak clipping distortion whereas your circuit will have some crossover point distortion where the diode goes into conduction which may be more audible.
Looks like your project is doing something similar by hitting the zener diode but yet still allowing some riding modulated AC across it and shunting it to be active with R7. Antique audio processors of the 70s would sometimes simply implement diode limiters with two in series for positive and a single opposite polarity for negative mod. peaks followed by an NRSC filter.
Problem with soft clipping/diode limiting like this is that it will need to be filtered well at the *output* to remove high frequency harmonics. There are tons of ways to do this but most methods cause filter overshoot which requires even more circuitry. For CB radio use though I guess no one really cares too much about a little channel bleed these days. You may not notice the distortion much on a scope but trust me it will be there and thus the need for low pass brick wall filtering the audio output. Hope your circuit is being preceded by some form of audio compressor or limiter or else it will probably sound like a fuzz box if pushed hard.
Would definitely be interesting to test though. If you give schematic values I might rig it up and run some tests if you'd like. Welcome to the forum!
Looks like the biased clippers that have been used in audio processing of times past in the broadcast industry....
I was working with David Dowler and John Bartal the fellows who developed the Mauldulator to make software files of the Mauldulator. Their circuit had a high degree of harmonic distortion and so I developed filters in that circuit to reduce that, I use those same filters in the circuit diagram I post to reduce the THD down to 2% as compared to 9 too 14% with the Mauldulator. So some of the Op Amp sections are devoted to filtering. I likes this more simpler method of using the loading circuit of a potentiometer with a diode and resistors since this method starts off with a lower level of THD however with gross adjustment the THD can increase.
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