Technical Topics > The RF Workbench
Someone had Corsair II AM TX experience?
Stretchyman:
Perhaps a picture or two showing this marvellous design in the flesh!
Perhaps not . ...
Charlie_Dont_Surf:
--- Quote from: Stretchyman on January 02, 2017, 1010 UTC ---A Scope would help?.
--- End quote ---
It's pretty infrequent but I actually find myself agreeing with Stretchy's comment from years ago. Miracles do happen.
Charlie_Dont_Surf:
--- Quote from: netsmo62 on January 24, 2017, 1318 UTC ---So I have decided to change completely the modulation type from "low level" to a more classic "high level" directly on the final mosfet, using a simple serial modulator
--- End quote ---
This is a positive step.
I may have missed it in this thread but I don't think we ever got a clear definition of what was termed "distorted audio". ("Distorted" is somewhat imprecise because it doesn't relate much about the nature of the problem.) This is why an oscilloscope image would have been very helpful.
In any case, one of the issues with the original design is that the modulated audio was probably getting into the oscillator and "pulling it". Modulated audio was driving the oscillator buffer (as the low-level modulator) and that buffer is - like it or not - part of the oscillator circuit. Its parasitics will affect the oscillator stage before it, changing the oscillator frequency, and the amount of oscillator shift will vary with the modulation. This would produce frequency or phase modulation along with the amplitude modulation and result in distorted audio. (I can't be sure that this is what is happening because I never saw oscilloscope images or heard recorded audio, but it is a safe assumption given the schematic.)
There are ways to fix this and my favorite way is to add an additional layer of oscillator buffering to better isolate the oscillator from the modulation. In effect this is what you have done by changing to a high-level modulator. That is, by modulating the final amplifier directly.
Good work. Congratulations.
redhat:
You could also replace the series modulator with a $11 class d audio amp board from amazon. Efficiency would be MUCH higher, and it makes more sense to go this route since you already have a 24V supply at your disposal.
+-RH
Charlie_Dont_Surf:
--- Quote from: redhat on February 11, 2021, 0215 UTC ---You could also replace the series modulator with a $11 class d audio amp board from amazon. Efficiency would be MUCH higher, and it makes more sense to go this route since you already have a 24V supply at your disposal.
+-RH
--- End quote ---
Not disagreeing with you (you're absolutely right about the efficiency) but some day I should write up my experiences trying to use a cheap Class-D amp as a series modulator, requiring the cheap amp to supply all the bias for the RF stage.
The cheap class-D amps that I am aware of don't like to drive loads below 1-2 Ohms, which is exactly what you would be asking it to do by becoming a series modulator. It gets "unhappy" when you do this and I damaged more than one by asking it to deliver x Amps continuously as DC bias for the RF final, in addition to modulated bias. Sometimes the damage was not catastrophic, sometimes it was catastrophic. (It's a good thing that the amplifiers are "cheap".) My proposed solution (which I never got around to trying) is to add 2 Ohms in series between the class-D amp and the RF amp. That way the Class-D amp is never seeing less than 2 Ohms and is less likely to be "unhappy". Of course, this is wasteful and might eliminate the efficiency gains you made by using a Class-D amp. :(
Well, I guess I have now written up my experiences on this, so today is "some day".
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