[ARC5] Lopsided modulation

Wed Feb 28 16:27:30 EST 2018

Hi

I believe what you will find is:

	As you modulate the final, the RF load it presents to the VFO changes.

	As the load to the VFO changes, it’s frequency changes. 

Since it’s all at RF, there isn’t a whole lot you can do about it. It’s one of
the classic limitations of a very simple transmitter configuration. Less coupling
of the RF from the VFO means less drive to the final and (likely) less power out.
That’s not exactly what we’re normally after …..

Bob

> On Feb 28, 2018, at 3:46 PM, AKLDGUY . <neilb0627 at gmail.com> wrote:
> 
> I'm reading all comments and technical suggestions, but am overwhelmed and can't reply to everyone. I'm intending to increase the decoupling on the VFO line which is derived via 3 x 4K7 5W resistors (14.1K) from the 550V dynamotor B+. The MD7 modulator schematic sez 15K. The VFO is running at a shade over 250V.
> 
> Interestingly, the MD7 modulator schematic shows no decoupling on the VFO side of the 15K, so for the transmitter as a whole, the only decoupling present for audio is that provided by C55 (1.2 uF) at the dynamotor output.
> 
> The VFO decoupling capacitor needs to be rated at least 580 volts in case I do some testing with tubes removed, and preferably double that. Don't have anything appropriate, so need to buy a couple of 10uF 450V electrolytics and balance them with a couple of 47K 5W resistors that I have on hand.
> 
> Hope to get this done before the weekend and post the results.
> 
> Neil ZL1ANM
> 
> 
> 
> On 1/03/2018 8:31 AM, "Richard Knoppow" <1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>> wrote:
>    For the asymmetrical voice begin by visualizing a sine wave with a line to indicate the zero volts going along the center. Now, imaging a sine wave with one side flattened. That is asymmetrical. If the center line is still in the same place the voltage on one side is larger than the voltage on the other.
>    The human voice is generated by forcing air from the lungs past the vocal cords. The vocal cords act as a valve. Since the pressure is in one direction only the modulated air stream is mostly more positive pressure than the average air pressure. However, since the voice is also partially generated by a number of resonant chambers in the throat and head, which are after the vocal cords, the pressures are modified so that its not all more positive than the average air pressure. The displacement from average depends on the individual voice: some voices are very asymmetrical, some are more nearly symmetrical. I don't think any are more negative going because the voice starts out as a positive pressure from the lungs. The vocal cords can vary or modulate this pressure but can not reverse it.
>     If one picks up the voice with a purely pressure sensitive microphone the asymmetry is reproduced as an asymmetrical electrical wave form. However, since microphones have some resonances the symmetry or lack of it may be affected. Some microphones, namely the figure eight type or cardioid type are sensitive to either air particle velocity (figure 8) or both velocity and pressure (cardioid). The figure-8 pattern results from sensing a pressure difference between two points in the pressure wave so the microphone, in effect, performs a first order differential on the wave, and thus looses any constant pressure. That results in a waveform which may be asymmetrical but not in the same way the original pressure wave was asymmetrical. All this is made much clearer with a couple of drawings than it is in words.
>     In any case, if you have an oscilloscope attach a microphone to it and talk into it. You don't even need a modern scope, one that responds to audio frequencies is adequate. The scope should be DC coupled. It will show the asymmetry in the voice.
> 
> On 2/28/2018 10:17 AM, Tim wrote:
> Yep!
> I'm waiting for Prof. Lee to hit us with the all-defining modulation math.  (leaving it as an exercise for THIS student would not be productive!)
> 
> Also. I can imagine asymmetrical sideband power on either side of an AM radio carrier but I cannot imagine what an "asymmetrical voice" could be. ;o)
> 
> Tim
> N6CC
> 
> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>
> WB6KBL
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