Hans says:
>I often asked myself how much noise sidebands are generated in the
>output transistors of a rig? Looking through test reports, I have a feeling
>that figures of synthesizer noise are better in the receiver than
>transmitter. Using VMOS fets should then give less noise, no?
In cellular phones, PA stages for the US IS136 system (D-AMPS) typically
have gains of 24 to 28dB. The noise output in the receive band is limited to
-80dBm in 30kHz, or -124.8dBm/Hz. With 28dB of gain, this means that the
noise figure of the PA is about 21dB if there's no exciter noise. All this
suggests that PA noise figures of the 15 to 20dB region shouldn't be too
hard to achieve. I believe IS136 PAs are considerably better than this, but
I think most of them are GaAs. However, close in ( say within 500kHz) ,
phase noise from the synthesiser is much more likely to dominate. A lot will
depend not so much on the PA stage, but on the transmit IF chain and up
converting mixer, especially since such chains are not generally designed
with low wideband noise as a design parameter. The old tube rigs with a
tuned circuit in the driver were rather better in this respect.
Most rigs not designed for transverter driving wind the gain back far too
early in the transmit chain to provide optimum noise. The best route for
many of the modern rigs would appear to be to run it almost flat out at 50
or 100 watts, and use a suitable power attenuator to drive the transverter.
73
Peter G3RZP
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