Sunday, May 9, 2010

low phase noise VFO

Here is my low phase noise VFO. It is the differential transistor
pair VFO from the 2010 ARRL handbook, scaled to a frequency range of
10-10.5 Mc and combined with a surplus VFO chassis/vernier dial gear
reduction that i salvaged out of an old FAA modulation tester.

Photos from the top:
1. VFO
2. insides
3. connected to the rest of the modules that make up my 20 M SSB
radio project.

Results:
Works great! I found that cheaper 2n4124's (or equivalent) NPN
transistors work better than UHF low-noise transistors. This was
indicated in the ARRL handbook, but, i chose to ignore it and hoped
to improve phase noise performance by using the low-noise UHF
transistors. The problem with the UHF transistors was that they
would output broad-band oscillations on occasion causing instability
in the VFO. So, best approach is to follow the directions from the
handbook and go with the cheap transistors for this type of VFO.