W5OMR/Geoff wrote:
Hi Geoff,
Yes both statements are true. Remember that the PEP is 4 times the
carrier power with a 100% modulated AM signal.
if it's modulated with a sine-wave, that's a true statement.
A pair of 813s are good for about 600 watts PEP output. So that limits
carrier power to 150 watts if 100% modulation is used.
I understand that, but in a grounded grid arrangement, as it's already
been said here, there's the exciter drive power that's added to the
signal. So, in order for there to be 100% modulation, you must modulate
the exciter, as well.
The "efficiency at carrier" that I was referring to is the efficiency
of the amplifier at the power level that the carrier runs when any
amplifier is set up for AM operation. Like the 813s, 600 watts PEP out
requires that the carrier be operated at 150 watts output.
Thereabouts. Again, if you're modulating it with a sine-wave, then it
holds true that 100% modulation is 4x the carrier output. However, we
don't speak in sinewaves, as John/WA5BXO, Don/K4KYV and
Bob(Bacon)/WA3WDR have so eloquently stated in
http://w5omr.shacknet.nu:81/~wa5bxo/asyam/Amplitude%20Modulation.htm
If the amplifier has say 60% efficiency at the full PEP level then at
1/4 the power output the efficiency will be 1/2 of what it was at full
power or 30% in this case. This is not only true for AM operation but
for SSB operation as well.
As long as the amplifier is to remain linear this efficiency ratio
must exist. (see Orr's radio handbook. see "efficiency modulation")
If that efficiency ratio is not maintained then the amplifier is not
operating linearly. If you readjust the loading at carrier level to
increase the power then it will no longer be linear when the
modulation is applied.
Which, to me, would be another good reason why you shouldn't
plate-modulate a grounded-grid (aka 'linear') amplifier.
--
73 = Best Regards,
-Geoff/W5OMR
Geoff,
I am ONLY talking about a linear amplifier here. The drive signal is
already modulated. If you are modulating the final stage these
efficiency rules discussed here do not apply. Unless you would be
talking about grid modulation and then they "do apply".
It does not matter what type of signal is modulating the signal when the
linear amp is involved. Whether it is a sine wave or speech or anything
else. The PEP rule still applies for 100% modulation! You can not exceed
100% positive peak modulation if the carrier is set at any level greater
than 1/4 the PEP output capability of the amplifier. Unless you want to
operate it in a non linear mode.
73
Gary K4FMX
