[AMRadio] Huff and Puff my VFO

Jim Candela jcandela at prodigy.net
Mon Feb 11 13:26:45 EST 2008

resend after my first try bounced...

Hi all,

   I have been playing with "Huff and Puff VFO" circuitry for some time
 now, and more recently I have  purchased a PIC based commercial
 product from Cumbria design in England. I purchased an inexpensive kit that transforms my Lakeshore Band Hopper VFO drift rate to that approaching DDS stability.
   See the link below for info specific to my installation:


Cumbria design x-lock web page:

The band Hopper VFO runs continuously and covers 5.0-5.6 mhz.

   These H&P stabilizers in general lock a VFO to a frequency band that
 is +/- so many hz. The Cumbria PIC design I used keeps the Band Hopper
 within +/- 10 hz, and usually much better than that. Some issues worth
 noting are:

* The VFO lock is disengaged when the drift rate exceeds 40 hz /
 second, and then locks again in 2 seconds after the knob tuning is done. This
 can also happen when the VFO takes a jump on it's own from something
 like a bad tube socket contact, or a large change in filament voltage.
 With my band Hopper I needed to tighten the tube socket to tube pin
 contacts, and regulate the filament voltage to exactly 6.0 volts DC. 

* A vacuum tube variation to the H&P requires adaptation of the
 varactor tuning circuitry. I used dual 1N4005 diodes in series back biased. If
 I used a varactor diode or led as a voltage variable capacitor, the
 peak signal level in a tube VFO would overload the diodes when they
 avalanche. HV diodes like the 1N400x series are good for 5-30pf each at
 about 0-10 volts back bias (max cap at min voltage).
* This design is intended for VFO's that run continuously, and not ones
 that sit idle until needed. This later category excludes VFO's in many
 Boat Anchors. There is a variation of the H&P that acts like a
 frequency counter and servo's the VFO to match a desired frequency. Go to
 Google and search for "Huff and Puff VFO".

Here is a link at an early H&P attempt I tried on the BC-458 VFO:



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