04 January 2012

Measuring RX sensitivity with simple tools

Yesterday I had the 'scope ON and it occurred to me that I could visualize the output of a simple 20m CW RTX I had designed and built back in 2005.

While it measured 100mA at 12V input key down, I could see a relatively well shaped sinewave of 7,4Vpp, which should equate to ~140mW on 50 ohm (ouch! That's 11% overall efficiency!).

Later on, I wondered if and how I could measure the receiver sensitivity, which I remember sounding good in my ears. I did some internet searches and I would need a 14 MHz source in the ballpark of  -100dBm. My lowest power generator is FT-817's 500mW, +27dBm: 130dB is too much attenuation to be implemented! Even commercial fixed attenuators don't go beyond 30dB (source JFW catalogue). Should I  build a Colpitts oscillator and compute its output power with the oscilloscope? Are there other techniques to generate really low power signals? I have the advantage that I need a single frequency...

Fellows over at GQRP reflector suggested three already tested circuits/products:
  • Elecraft's XG2 (-107 and -73dBm)
  • Norcal's S9 (-107 and -73dBm)
  • a test generator published on SSDftRA book
Both commercial kits documentation include schematic and parts list. They are also quite similar, so I will go with either circuit, depending on parts I can locate. I will tend to use 1% precision parts whenever available.

To complete the test, on the other side of the receiver I would probably use a computer software doing audio spectral analysis and look for a 20dB S+N/N signal. Or a DVM.

More ideas came up around the little RTX too...