04 November 2009

Doubling to 6m and filtering

Previous measurements on my base oscillator for a simple 6m TX were encouraging and I started thinking of making a DSB transmitter instead of AM. I added a parallel LC circuit at the output of my EX-OR mixer/doubler and started seeing an improvement on the oscilloscope.

Then I checked at the spectrum analyzer and results were encouraging.

Frequency After LC [dBm] Delta [dBc] Before LC [dBm] Delta bef [dBc]
50,35 7,5
75,53 -29,5 -37

100,7 -14 -21,5 -6 -11
125,88 -17 -24,5

151,05 -33 -40,5

The 50 MHz output on 50 ohm has increased to 7.5 dBm (through the 7dB pad means 14.5 dBm = 28 mW) and I can tune the delay line to almost kill the 75 MHz product while keeping all other harmonics more than 20 dB below the carrier. Before adding the LC the second order harmonic was down 11 dBc, another positive improvement.

Now I have too many options:
  • try it on the air as a CW transmitter
  • build a PA and AM-modulate it
  • feed this signal to an NE602 and generate DSB
In any case I need to start working on the low frequency part.