|[AMRadio] Re: Ranger... good news, bad news|
ne1s at neandertech.com
Thu Nov 9 13:05:58 EST 2006
Jack & list, see my answers & comments below.
Jack Schmidling writes:
> We speak of peak envelop POWER but a scope is actually displaying peak
> voltage. How do we relate this to 4 times carrier on a scope?
The RF voltage on modulation peaks is exactly twice that of the unmodulated
carrier, for 100% positive modulation. Since the RF load impedance is fixed
(and can be considered 100% resistive for the sake of argument, and will be
close to that practically anyway) when voltage doubles, current doubles too,
so hence the positive peaks indicate 4X the power. (P = IV = V**2/R, so if
the voltage doubles (as does the current) the power will quadruple).
> > I still do not understand. The final is tuned with a tank capacitor and a
> loading cap. On my amp, the optimal positions of these knobs only change
> with the frequency is changed. Changing the input level does not effect
> this tuning in any way that I can see.
Your amp then doesn't behave like any linear amplifier I've ever played
with. The tune and load settings for maximum output have always depended on
the input drive level for me, at any fixed frequency. There is a theoretical
argument why this must be, as well, but I'm having a brain cramp on that at
present (I USED to know, honest :>))
> However, today I changed the rf pickup from a loop on top of the tuner to
> a wire just running off away from the rig.
> Bottom line is, that it now looks like 100% modulation.
Good. Odd, but good.
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