Sunday, 8 November 2015

First steps with amplifier distortion investigation

Power Sweep
This is my first tentative steps at looking at distortion in PAs using the HP8753E.
Tentative really is the most appropriate word as one slight error and the VNA could
be compromised. The maximum input power the receivers can accept before
damage is only 20 dBm. It is important to have a lot of attenuators in place.

The above graph is that of a RA60H1317M operating at 146.5 MHz.
The output starts at around 12 watts  and ends around 70 watts.

The top trace is the phase difference across the power sweep and the bottom trace
the gain. I didn't calibrate the gain to take into account the 30 dB attenuator or the
effects of the directional coupler so it is a relative value rather than an absolute one.

The drive was also too high, next time I attempt this I will use the 70 dB stepped
attenuator in the test box and I will also properly calibrate for the attenuators
and couplers.

To measure the output I used a 20 dB directional coupler and a 30 dB Narda
attenuator which reduces the 70 watts (48 dBm) down to about - 2 dBm.
The maximum measurement input level of the VNA's receivers is 0 dBm, above
that they start to go into compression.

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