Thursday, 2 August 2018

MRF300AN 71.5 MHz amplifier

Overall view of test setup

Output spectrum at 40 watts 50v supply

Device test jig
Thermal image of amplifier output matching at 250 watt level

I am currently working on an amplifier for the 71.5 MHz band based
on the new NXP MRF300AN plastic LDMOS power transistor. The
first picture shows the overall setup. The next is the amplifier
transmitting a QPSK signal at 40 watts output and the final a close
up of the test jig.

The idea of the test jig is that the device can be matched for optimum
performance ( IMD / Efficiency or some other criterion) then the device
can be removed and replaced by the SMA jig and the input and output
match measured using a VNA. This can then be used in a program such
as QUCS to design a suitable matching network, possibly for broadband
operation.

I also have a temporary licence for AWR microwave office which allows me
to try the NXP large signal model for this device.

Eventually I may use the amplifier to experiment with Digital Pre-distortion
but that is sometime in the future.

Results so far, with about 300 mW drive I have managed to obtain a CW
output of slightly over 200 watts with a supply of 6 amps at 50 volts
which is well in excess of what is required.As can be seen from the
thermal image of the amplifier the main hot spots are the live end of
the supply choke and the end of the output inductor connected to
the tuning capacitors. The hot line at the left is heat from the losses in
the RG58 cable I am using to connect to the Bird power meter.

The MRF300AN operates from 1.8 MHz to 250 MHz and has a claimed output
of 330 watts. The devices cost £30 each. The output patch leads in my
setup get quite hot as does the LPF (located is the die-cast box connected
to the bird power meter) so I am loosing power there.

While these devices are designed to cope with a 65:1 VSWR at any phase
I have managed to blow one up. That was down to my impatience of
using it with an inadequate heatsink.

The amplifier is more than adequate for it's intended use and that was
for a very clean amplifier for the UK experimental 71.5 MHz band.
We are limited to 100 watts ERP so the amplifier need not operate
above 25 watts. I originally ran the device at 30 volts but the performance
was poor at that level. At 30v I was not able to achieve more than 15 watts
of acceptable output. Increasing the supply to the full 50 volts made
a significant improvement.

A number issues were discovered, the main one being that careful
level settings have to be made. The test waveform was QPSK
generated from an HP ESG signal generator which fed a broadband
class A amplifier based on an old  CA5800. At various stages in
the test I manage to introduce IMD due to the signal generator,
the driver amp and the the Spectrum analyser which gave
a false indication of the actual level of the shoulders the amplifier
was producing.
  
It has been an interesting journey so far and I have learned a lot ........

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