I don't think it's ever a good idea to switch the VFO on or off, because
of drift. Also, unless the VFO is turned on before the rest of the
transmitter and off after the rest of the transmitter, you'll get a bad
chirp if on CW. (It would probably be acceptable on AM, and possibly ok
on SSB, though.)

Often, a VFO will be heterodyned to the operating frequency. If so, you
can switch the crystal oscillator it's heterodyned with. If it's running
on the operating frequency, you can pull a trick I first saw W7ZOI do.
He switches in some extra C or L when not transmitting, to move the VFO
frequency far enough away so it's not a bother. You've got the potential
chirp problem here, too (although not nearly so badly as if the VFO is
switched from an off state), plus the potential for momentarily
transmitting out of band if your VFO is idling there. That can be
overcome with timing, making sure the VFO comes to frequency before the
rest of the transmitter goes on, and stays there until the rest of the
transmitter is off.

In the heyday of the vacuum tube, it was important to keep things
simple, since components were expensive and active components (tubes)
expensive, large, and power hungry. Fortunately, the bands weren't as
crowded, so drift, sloppy keying, and the like were better tolerated.
It's quite a trick to mix the techniques of yore and the signal purity
requirements and expectations of today. Good luck!

Roy Lewallen, W7EL

Active8 wrote:
hi:

i see a lot of transmitter schems where the VFO's power is switched with
the mic or key. and i saw a test where it took 5 minutes of warm up time
for a VFO to stabilize.

now, of course a good op will be checking to make sure he's on freq.,
but aside from reduced current drain on batteries, are there other good
reasons to switch the VFO providing it's not leaking into other parts of
the rig (if it's a tranceiver) and wreaking havoc?

yeah, this is for a homebrew project. as much as i like the idea of
powering down the VFO when it's not needed ...

thanks in advance for any info on this.

mike c