Hi Pete,
Many thanks for the kind comments, and for the added information. I
inadvertently forgot to mention the FCC legal limit. Industry Canada is more
generous; Advanced licensees are permitted 2.25kW PEP output for SSB, and
1kW DC input to the final PA or 750W carrier output for all other emission
types.
Overmodulation of a typical modern solid-state HF transceiver in AM mode
will drive the modulator circuit (or the DSP) to the end of its range. This
will cause peak clipping, which will show up as compression on peaks and
troughs of the envelope, as you describe. 90% modulation is pretty safe.
Overdrive of a linear amplifier in AM mode is complex. If the amplifier is
driven out of its linear operating region (such that the PA plate runs out
of voltage), flat-topping will occur on envelope peaks. If the drive is also
sufficient to force the PA devices into cutoff, crossover distortion will
occur; this will show up on the scope as distortion on envelope troughs. The
two forms of distortion may not necessarily occur at the same drive level.
On the very rare occasions on which I experiment with AM, I set the
resting-carrier output of my 100W exciter to 25W, apply the test tone and
adjust Mic Gain such that the sidebands are 7dB below the carrier on the
spectrum scope (or the spectrum analyser, if I can summon up the energy to
lug that beast into the shack.) This corresponds to 90% modulation.
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com]On Behalf Of ac5e@comcast.net
Sent: Saturday, November 22, 2003 20:05
To: tentec@contesting.com
Subject: RE: [TenTec] AM with the centurion amp
Adam's comments about the Centurion on AM are correct - but two more items
deserve mention.
The first is that 375 Watts of resting carrier is the legal limit under FCC
rules. As Adam says, that's completely equivalent to 1500 Watts PEP SSB.
The second is that overmodulation not only causes flat topping on positive
peaks, it causes "flat bottoming" on negative peaks as well. You should be
able to see a complete sine wave with no distortion on both positive and
negative peaks. That is, your scope pattern with sine wave modulation should
be a series of smooth sine waves with NO straight lines anywhere.
If you get a flat top, or get that straight line between individual pulses,
please back off on the audio gain. 90 percent modulation sounds great. 101
percent modulation sounds awful.
73 Pete Allen AC5E
> Hi John,
>
> Any linear amplifier will work in AM mode. You will need to de-rate the
> amplifier by 6dB or 75%; if it is designed for 1.5kW PEP SSB or CW output,
> you will need to set the drive for no more than 375W resting carrier
output.
>
> You should check the output envelope with an oscilloscope or monitor scope
> to ensure that there is no flat-topping on voice peaks. The best way to
> perform this test is to apply a 1kHz pure sine wave to the mic input,
adjust
> the Mic Gain until you just begin to see flat-topping, and back off the
> drive. At this point, the average power output (as measured on a Bird or
> equivalent power meter) should be 1.5 times the resting carrier output.
The
> peak amplitude of the envelope at 100% modulation is twice the
> resting-carrier amplitude.
>
> 375W resting-carrier is equivalent to 1.5kW PEP or 562.5W average at 100%
> modulation.
>
> Hope this is helpful.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
> -----Original Message-----
> From: tentec-bounces@contesting.com
> [mailto:tentec-bounces@contesting.com]On Behalf Of John
> Sent: Saturday, November 22, 2003 18:55
> To: tentec@contesting.com
> Subject: [TenTec] AM with the centurion ampo
>
>
> Hello all can any one tell me if the centurion can work on AM ? thanks
> John
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