[ARC5] Lopsided modulation

[Bob kb8tq]

Hi

If you sit down in class to design the tank circuit for a vacuum tube transmitter 
(yes it *was* a long time ago and the text books were from before I was born …) 
the ratio of unload Q to loaded Q comes up pretty quickly. The ratio of Qu/Ql is
significant in that it helps you estimate your losses in the circuit. More or less, 
you want a Qu that is as high as possible (hundreds if you can) and a Ql that is
as low as possible. How low you can go depends on the circuit topology and 
the impedance transform. With a pi network and normal tube loads, you get numbers
in the ten-ish vicinity. There now are calculators all over the internet that will 
do the math for you. No need for the graphs and manual methods ….

So, how does this actually *matter* in this case? Well, it you want a Ql that is 
high enough to impact sidebands < 0.1% off the carrier, the Qu needs to be
even higher still. If it’s not, all your power gets wasted in the tank circuits. That’s
not the way you normally do the design ….. :)

Bob



> On Feb 27, 2018, at 2:35 PM, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
> 
>   I would add, as I did in a previous post, that a _loaded_ final tank has rather low effective Q. It is desirable in most cases to maximize the Q of an _unloaded_ tank but when connected to a load the load impedance is imposed on the tank. That is why the dip in plate current at resonance becomes less as the loading is increased. At some point the plate resonance can be see more easily as an increase in power. If the amplifier is properly neutralized this peak should be coincident with the dip in current and can be easier to see when the load is near optimum.
>   I suppose a tank with low loading and mis tuned for the dip could be far enough off resonance and maybe sharp enough to reduce one sideband but I think it would have other problems at that point.
> 
> On 2/27/2018 11:27 AM, Tom Lee wrote:
>> Yes, the muddle is very odd. Bob said it succinctly and correctly: The shape of the modulation spectrum has no impact on symmetry of the sidebands. For pure AM or pure FM, you get symmetrical sidebands, period. As a professor, I leave it as an exercise to the student to compute how high a tank Q would be necessary to produce a pronounced asymmetry (hint: you will find it hard to contrive sensible numbers in most cases). By far the most common source is the simultaneous occurrence of AM and FM. It is difficult to build a modulator/transmitter chain that is completely free of incidental modulation by the unwanted mode.
>> Cheers
>> Tom
> 
> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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