EMRFD Message Archive 4046

Message Date From Subject
4046 2010-01-20 06:48:52 Tim K7HFD high-level low-phase-noise VFO
The K7HFD high-level VFO dates back to the 70's at least - it's in Solid State Design for the Radio Amateur. Google "K7HFD oscillator" and you'll see some schematics.

In the past couple years it's been in the ARRL Handbook too, praising its low phase noise but with some derogatory comments about its stability owing to heating the transistors at the high power levels and also remarking about how the high voltage levels beat up the poor little transistors.

I'm willing to believe that at the power levels involved that it may not key well (heating up and cooling down when keyed can't help any) but in my naive mind it would seem fine if you kept it running all the time.

Anyone used it in a receiver that needs +17dBm at the mixer? I'm gonna give it a shot with TUF-3's sometime soon.

4056 2010-01-20 13:20:26 w7zoi Re: K7HFD high-level low-phase-noise VFO
Hi Tim and gang,

The K7HFD (Linley Gumm) oscillator goes back to 1977 in SSD, page 126. The work was done in 74 or 75 and was not published (that I know about) until DeMaw and I decided to include it in SSD. There was a dual reason for this. First, it was a really good oscillator. Second, a method for measurement was presented that works well with any oscillator one might want to test. I've moved to the use of crystal notch filters instead of the crystal bandpass that Linley used, but that does not matter a lot, for either works.

I found the circuit to be stable when built with quality NP0 capacitors, followed by temperature compensation. We want to do this with any serious LC VFO that we build. The fact that there is a lot of power in the circuit does not really impact things a lot. The active devices are well decoupled from the resonator, so they are mostly out of the picture.

The circuit is presented as an idea, yet folks want to grab it and use it as a "catalog" circuit. It is really quite easy to back the power off if desired. Just increase the value of the emitter resistor in the diff pair. Be sure to use it with the output terminated. Without the termination, the output transistor saturates.

You probably intended to use the oscillator with a TUF-1H or -3H. A TUF-3 is a lower frequency variation of the TUF-1 that is still a level 7 mixer. LO injecti
4065 2010-01-22 10:43:31 Glen Re: K7HFD high-level low-phase-noise VFO
Have seen the K7HFD oscillator schematic reproduced too many times
without the numerous ferrite beads - these are important.
The oscillator can have VHF spurs without them, sometimes oscillating
at HF & VHF simultaneously. Even if not, the large switching signal
at the base causes such rapid turn-on (and turn-off) that VHF
transients can ring for quite a while. Using transistors with lower
Ft helps curb VHF spurs too.
Instability (at the HF frequency) is a comm