> As a general rule if you can reduce your amp driving power in
> half you gain a 3db reduction in the transceiver IMD.
This is one of those "general rules" that is very wrong when it
comes to solid state transceivers. Almost all modern power
transistors have a "saddle shaped" IMD curve. When power output
is reduced significantly below the optimum point IMD increases.
If power is increased above optimum IMD increases rapidly.
For example, look at the 2SC2879 used in the Yaesu Mark V Field:
http://www.datasheet4u.com/download.php?id=71199. The IMD vs. Power
Output curves show a "sweet spot" around 55 to 65 watts per transistor
before filter and transformer losses of about -38 dB. With the losses
in the transformer, lowpass filter and antenna tuner, this represents
just about the transceiver's rated 100 watt output. Decreasing power
to 40 watts per transistor (1.4 dB) increases IMD by more than 6 dB!
Increasing power output to 80 watts per transistor (~1 dB) results in
an 8 dB IMD increase and pushing to 100 watts (~2 dB) per transistor
results in a nearly 14 dB increase in IMD.
If your amplifier requires substantially less drive than your
transceiver produces you are better off using an attenuator on
its input rather than reducing the drive in the transceiver.
73,
... Joe, W4TV
> -----Original Message-----
> From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of Robert Kirkland
> Sent: Sunday, August 31, 2008 9:04 AM
> To: amps
> Subject: [Amps] IMD and driving power
>
>
> As a general rule if you can reduce your amp driving power in
> half you gain
> a 3db reduction in the transceiver IMD. Less in, less out. Bob,w5pvr
>
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