EMRFD Message Archive 181

Message Date From Subject
181 2006-10-30 23:02:14 Rick VFO adjustments
Building a VFO that comes up 100 kHz higher than expected at 7 MHz is pretty good.
That's less than 2 percent error, which is easily explained by changes in stray capacitance
and lead inductance when you connect measured parts together in a circuit. I always trim
the VFO frequency range after building the VFO, often by squeezing or spreading the
inductor windings. When I build narrow range VFOs (100 kHz at 7 MHz is narrow) I usually
include an accessible variable capacitor to trim the frequency range after everything is

Since the earliest days of radio, variable oscillators have had alignment adjustments-
trimmer capacitors or slug-tuned coils that may be tweaked after the radio is built.
Inexpensive All American Five vacuum tube radios had waxed paper coils with dents in the
side to engage powdered-iron cores with a hexagonal hole that would screw in and out
with a plastic tuning tool. Collins radios had ceramic trimmer capacitors and screw
adjustable slug-tuned coils. Satellites have individually machined, gold-plated air
dialectric (vacuum after launch) precision variable capacitors.....but they all need trimming
after building the VFO.

One way to help understand why RF circuits that need frequency accuracy (for example,
oscillators and narrow filters) have to be trimmed is to realize that capacitance happens
everywhere there is an electric field (for example, between the turns of the inductors) and
inductance happens everywhere there is a magnetic field (for example, around every wire
that carries current). So measuring individual components before connecting them
together isn't enough to determine the total capacitance and inductance.

Once you have the frequency range you want, the next step is stability. You can start with
mechanical stability and a closed box to keep air currents away from the components.
Then proceed to thermal stability, and finally, Wes Hayward's simple and effective
temperature compensation techniques.

Have fun.

Rick KK7B
183 2006-10-31 14:40:52 Preston Briggs Re: VFO adjustments
200 2006-11-15 21:59:49 Preston Briggs Re: VFO adjustments