Re: [Discuss-gnuradio] Gauging interest for an SDR PA

[Louis Brown]

I was thinking the user could just screw on coaxial low pass filters.  For 
instance, the VLF-xxxx line from Minicircuits are available from 80 MHz to 6 
GHz, at about $22 each.  An output filter bank would be doable, but it would 
probably be 0.4 dB insertion loss per switch as S band, so it adds up pretty 
quick.  Peregrine makes some SOI switches which are essentially DC coupled and 
have P1dB=40 dBm, so that may be an option, along with relays.

In band filtering may be too specific, and get costly as frequency increases.

Ha ha, yes, burning things out would be a problem.  Can't protect from some 
user error.  Even TX/RX monopoles with one meter separation may blow that 2nd 
stage amp on the WBX card.  As far as the amplifier itself, these 48V GaN parts 
have 160V breakdown, so running them at ~30 VDC rails should make them pretty 
robust.  Allot of GaAs parts breakdown at only 2.5 times the rail.  Conversing 
with a guy from Nitronex, he said 37 dBm with a 30 VDC rail should be feasible 
without any matching at the output.

Efficiency would be poor, probably at most 20% at P1dB, but that would be the 
trade off for that much bandwidth.  I figure true SDR (with the computer) is 
already inefficient to begin with.  Heat could be taken care of with a 
processor cooling fan.  There could be space left for the user to add matching 
to shape up the drain waveforms, with user adjustable gate bias, just limited 
enough so that nothing would be cooked.  Maybe instructions of where to solder 
parts for 2m, 70cm, ham bands etc.

I have had good use with the RMS detectors from Linear Tech and Analog Devices. 
 Typically 0.2 dB variation over 5% bandwidth when used in an analog ALC loop.  
There may have to be a LUT to flatten the detection form VHF to S band.  They 
should be pretty insensitive to modulation bandwidth as long as it is a 
fraction of the RF bandwidth, and the video filtering is low enough.  Maybe not 
a true ALC, but I/O available to feed back to the SDR board.  Linear Tech now 
has a detector with built in ADC and 3 wire interface.  It would be rough power 
detection as not much coupler directivity could be had for that bandwidth, 
really non at all; just a resistive tap with some equalization to flatten the 
response.

I can see how marketing as a radio would be an issue.  It would have to be test 
equipment, yet, anyone can freely buy a multi-KW, multi-GHz PA.  I'm not sure 
how the FCC views that stuff.  ITAR would be a concern to with the GaN; could 
be a show stopper.  Anyway, just a thought for a project.  I can key up 
repeaters now directly from the WBX card, but I have a 10 dBi antenna 20 feet 
up.  5 watts would allow desktop experimentation, again, given nothing is blown 
up.