In message , Bill Turner
writes
On Fri, 22 Oct 2004 19:34:26 -0700, Roy Lewallen wrote:

So a power meter that's really reading the voltage should stay at the
carrier level, provided that its time constant is comparable to or
longer than the time constant of the AC coupling of the modulation
signal. I'd expect this to be the case for a typical meter and typical
audio modulation.

_________________________________________________ ________

Isn't the S-meter in a typical receiver voltage driven? They do kick
upwards with modulation.

--
Bill W6WRT


Try making a simple RF voltmeter consisting of a diode detector driving
a sensitive moving coil meter (say 100microamp fsd) or a multimeter
reading volts. If the RF source doesn't already have a DC path to ground
(ie it is AC coupled), you will also need to add a shunt resistor or RF
choke. Make the resistor as low as possible without killing the RF too
much.

Apply sufficient RF to give (say) a half-scale reading. Now modulate the
signal with audio (to 100% if possible). The meter reading will stay the
same. Now add a fairly large capacitor across the meter (say 1uF). The
reading will increase. If the RF source impedance is low compared with
the resistance of the meter, the capacitor will discharge very little
when the diode is 'off', and the cap will charge up to the peak voltage.
With a good modulation waveform, 100% mod will double the reading.
(Note: Any capacitor value between 'far too small' and 'more than
enough' will give a reading somewhere between.

I did this about 30 years ago to add a simple mod depth indicator to and
old signal generator which already had an RF level meter.


Anyway, all this talk about power and PA efficiency tends to complicate
the explanation. Just think of the spectrum of the signal.
No mod = 0dB reference = the carrier.
Add mod. Sidebands appear each side of the carrier. The carrier level
stays at 0dB.
Set mod at 100%. Each sideband should be 6dB below the carrier level.
Total power = 0dB + -6dB + -6dB = 1 + 1/4 + 1/4 = 1.5
(Note: This is regardless how the modulation was applied).
Peak power can be got from the voltage waveform. The peak RF voltage
doubles with 100% mod. The peak power is proportional to the square of
the voltage, so it quadruples.

So, to sum up... for AM:
100% mod, power increases x1.5 wrt 0% mod.
100% mod, peak power is x4 wrt carrier.

Ian.

--