VFO stability question
 

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

In article ,
says...
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.

Mike-

One of the effects of keying the VFO by keying the power, is a distinct chirp.
Another method used in the vacuum tube days, is grid-block keying. A negative
voltage is applied to the grid of the oscillator tube, and shorted to ground to
turn on the oscillator. This approach would apply to an FET oscillator as
well.

Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

that's what i was thinking. key the amps and maybe dump the VFO into a
terminator inside the RFI shield thru a good pin diode switch.

mike

73, Fred, K4DII

In article ,
says...
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.

Mike-

One of the effects of keying the VFO by keying the power, is a distinct chirp.
Another method used in the vacuum tube days, is grid-block keying. A negative
voltage is applied to the grid of the oscillator tube, and shorted to ground to
turn on the oscillator. This approach would apply to an FET oscillator as
well.

Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

that's what i was thinking. key the amps and maybe dump the VFO into a
terminator inside the RFI shield thru a good pin diode switch.

mike

73, Fred, K4DII

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

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

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

Oscillators should run continuously for maximum stability. This is true for
crystal oscillators as well as VFO's. However it is sometimes desirable to
give up
some stability for other advantages such as reduced power drain.

Bill K7NOM

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

Oscillators should run continuously for maximum stability. This is true for
crystal oscillators as well as VFO's. However it is sometimes desirable to
give up
some stability for other advantages such as reduced power drain.

Bill K7NOM

(Fred McKenzie) wrote in message
Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

I had a rig in which I kept the oscillator running on key up. "VXO 6 watter"
from "QRP Classics". Great rig. The "backwave" never caused difficulty in
copying. I got about 5 W out (keydown) and about 15 milliwatts out (keyup!)
This provided some fun: If a station gave me a good report, I'd ask if they
could hear the REALLY QRP oscillator on key up. Many could!
73 Bill CU2JL
http://planeta.clix.pt/n2cqr

(Fred McKenzie) wrote in message
Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

I had a rig in which I kept the oscillator running on key up. "VXO 6 watter"
from "QRP Classics". Great rig. The "backwave" never caused difficulty in
copying. I got about 5 W out (keydown) and about 15 milliwatts out (keyup!)
This provided some fun: If a station gave me a good report, I'd ask if they
could hear the REALLY QRP oscillator on key up. Many could!
73 Bill CU2JL
http://planeta.clix.pt/n2cqr

In article ,
says...
(Fred McKenzie) wrote in message
Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

I had a rig in which I kept the oscillator running on key up. "VXO 6 watter"
from "QRP Classics". Great rig. The "backwave" never caused difficulty in
copying. I got about 5 W out (keydown) and about 15 milliwatts out (keyup!)
This provided some fun: If a station gave me a good report, I'd ask if they
could hear the REALLY QRP oscillator on key up. Many could!
73 Bill CU2JL
http://planeta.clix.pt/n2cqr

was there any shielding around the VXO or a terminator to keep the
leakage down?

mike c

In article ,
says...
(Fred McKenzie) wrote in message
Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak through and be radiated by the antenna. You can
probably imagine a CW tone that gets louder when keyed, but the pitch doesn't
change. When such a station is nearby, it is hard to discriminate between the
strength of the keyed and unkeyed signal. If you use this approach, you should
pay attention to keeping the oscillator from bleeding through. One method
might be to key all of the following stages, for example.

I had a rig in which I kept the oscillator running on key up. "VXO 6 watter"
from "QRP Classics". Great rig. The "backwave" never caused difficulty in
copying. I got about 5 W out (keydown) and about 15 milliwatts out (keyup!)
This provided some fun: If a station gave me a good report, I'd ask if they
could hear the REALLY QRP oscillator on key up. Many could!
73 Bill CU2JL
http://planeta.clix.pt/n2cqr

was there any shielding around the VXO or a terminator to keep the
leakage down?

mike c

Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak
The VFOs in all my old Txs stay on all the time. I use a simple RIT
circuit to move the VFO off freq on receive and key it with either
plus or minus depending on the way the rest of the Tx is keyed.
Works neet...
73 W7ZFB

Keeping the oscillator running and keying subsequent stages will usually
produce a more stable signal. However, it is not unusual for some of the
oscillator's signal to leak
The VFOs in all my old Txs stay on all the time. I use a simple RIT
circuit to move the VFO off freq on receive and key it with either
plus or minus depending on the way the rest of the Tx is keyed.
Works neet...
73 W7ZFB

On 13 Jul 2003 16:59:47 GMT, WA44NBI wrote:

You have good advice from Fred K4DLL.
I have built a lot of gear and would not even think of keying an oscillator.
73s
Bob WA4NBI

__________________________________________________ ______________

Sissy! :-)

The trick is to make the oscillator run at essentially the same power level
whether it is oscillating or not. This means class A operation and you key
the feedback loop instead of the oscillator itself. That way the tube or
transistor has the same dissipation at all times.

It's a bunch of work to design and probably not worth it, but it *can* be
done. (Just kidding about the sissy part).

--
73, Bill W7TI

On 13 Jul 2003 16:59:47 GMT, WA44NBI wrote:

You have good advice from Fred K4DLL.
I have built a lot of gear and would not even think of keying an oscillator.
73s
Bob WA4NBI

__________________________________________________ ______________

Sissy! :-)

The trick is to make the oscillator run at essentially the same power level
whether it is oscillating or not. This means class A operation and you key
the feedback loop instead of the oscillator itself. That way the tube or
transistor has the same dissipation at all times.

It's a bunch of work to design and probably not worth it, but it *can* be
done. (Just kidding about the sissy part).

--
73, Bill W7TI