EMRFD Message Archive 180
Message Date From Subject 180 2006-10-30 18:18:03 Preston Briggs VFO I built a VFO over the weekend, following Rick Campbell's design from his article
describing the Binarual I-Q Receiver. I wanted it to cover the range 7.0 to 7.1 MHz and I
had a 50 pF variable cap.
I began by winding a tapped coil, 22 turns on a T-37-6 core. Measuring it showed 1.62
uH, a little higher than Campbell's example. Next, following the sidebar on page 4.4 of
EMFRD, I did a day of algebra and picked values for the capacitor network.
Measuring my variable cap showed a range of 5 to 46 pF. Based on NPO parts I had (and
measured), I picked 31 pF for C1, 12 pF for C2, and 299 pF for C3 (C1, C2, and C3 refer to
the sidebar in EMFRD). I tried these values out with a small Matlab program, to help verify
my arithmetic and everything looked pretty good.
Next, I built the thing in an ugly Manhattan style and tried it out. It worked right off, a
relief, and pretty close to specs. The tuning range was good, but it ran about 100 KHz
fast. I played with the value of C1 a bit, eventually settling at a value of about 47 pF.
So, in the end, while I'm satisfied with the resulting VFO, I wonder about the error. Is this
just the sort of thing that happens with real parts, and we must always expect to trim? Or
perhaps it's due to my construction practices (all those stray capacitances from the
Manhattan pads)? Measurement error (the size of the various caps)?
What do y'all think?
ps., if you don't know about it, the GNU Octave program is a free version of Matlab and
seems quite fine for the sort of plots we see in EMFRD.
182 2006-10-31 11:19:40 Graham Haddock Re: VFO Preston:
Could be variation in real world parts as you surmise.
How did you measure the inductance of the inductor (wound T-37-6)?
If you did not measure it at 7 MHz, then that would be the largest
error source. The inductance of the iron core toroids walks around
a lot with frequency.
184 2006-10-31 14:47:35 Preston Briggs Re: VFO