Saturday, 22 February 2014

VLNA23 revisited

Recently there has been much mention on the VHF/UHF reflector about strong signal 23cm performance.

Being a 2-stage preamp, that was designed primarily for EME work where high gain and low noise is a positive advantage, using the standard VLNA23 for terrestrial work can be an problem. I've always acknowledged this and recommend that a 10 to 16 dB attenuator is inserted after the VLNA23 to reduce system gain. However, the second stage in the VLNA may still be subject to strong out of band signals, enhanced by the gain of the first stage.

A few years ago I offered a single stage VLNA option, where the same PCB is used but the ATF54143 stage was omitted. This wasn't too popular and few were sold. I stopped offering it.

However, I continued to use this single stage version in my own 23cm system with great success until I started to remote the outside shack from the house shack. For this arrangement the signal had to go a total of 65m over FSJ4/50. I usually added back-in a second stage amplifier to overcome the extra cable loss!

Recent transmit problems with my 23cm system meant I had to remove the masthead amplifier. On test it still measured very close to the numbers marked previously on the preamp box.

But, on closer examination I noticed that this single stage VLNA23 did not have the later 'FET on stilts' modification. So, I modified it and ended up with a VLNA23 that exhibits 0.32dB noise figure with 18dB of gain and 6.5dB input return loss. No wonder I needed to add in the external amplifier........

18dB is a far more practical gain for terrestrial use ( 2dB better than the unmodified single stage version) and having eliminated the second stage should have a better dynamic range. I will perform a proper 2 tone IMD measurement in the next few days to determine the actual IIP3.

With a single stage it is possible to add an external band pass filter after the VLNA, but before the radio/transverter. If more gain is needed you can add one of the SPF Amps or PGA Amps after the filter to obtain more gain for more critical (long cable) situations. Less than 0.35dB noise figure with 18dB gain may allow contest stations to leave out an add-on second stage.

I can supply details of what to change for anyone who wants to 'de-sense' their two stage VLNA23.

Sam







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