Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

On Wed, 10 Dec 2003 14:39:22 +0000, Phil Hobbs
wrote:

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an LC
oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from a
decent varactor.

Thanks, is this the kind of thing you mean?



+-------+
| |
| |
| |
C| |
L1 C| |
C| |
| |
| |
V |
D1 - |
| C|
Applied DC control voltage | C| L2
Line --------------------+ C|
| |
| |
D2 - |
^ |
| |
| |
| |
+-------+

View in FP font.

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Wed, 10 Dec 2003 14:39:22 +0000, Phil Hobbs
wrote:

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an LC
oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from a
decent varactor.

Thanks, is this the kind of thing you mean?



+-------+
| |
| |
| |
C| |
L1 C| |
C| |
| |
| |
V |
D1 - |
| C|
Applied DC control voltage | C| L2
Line --------------------+ C|
| |
| |
D2 - |
^ |
| |
| |
| |
+-------+

View in FP font.

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Wed, 10 Dec 2003 19:53:30 +0000, Paul Burridge
wrote:

On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson
wrote:

I'm using a 555 timer to generate a sawtooth waveform to feed the
diodes, so I get a constant frequency sweep at the vcxo's output. Main
problem is the limited voltage output range; starts above zero volts
and peaks well before supply rail. So not much of a ramp; just around
4 or 5 volts, I guess. I could try changing the diodes for more

The 555 thresholds are set to charge and discharge the timing cap at
1/3 and 2/3rds the supply voltage. If you using that directly the
easiest way to get more "swing" is :

Use an opamp to translate the voltage lower and add some gain
(use at least 12v on the opamp). That can get you a Tuning voltage
that is near 0 to near 12V (that should help).

OR use a higher Vcc on the 555, say 12v. That will get your total
swing to about 4V and the low will be 4v and the peak will be 8v.

There are tricks that can be used to "offset" that 1/3 and 2/3 point
but the total swing is usually the same. That can help as operating
the Varicap closer to 0V will allow you to use more of it's
capacitance range though it's usualy less linear at the bottom.

Myself I'd use an opamp to create a saw generator and then I can
control the swings.

Allison

On Wed, 10 Dec 2003 19:53:30 +0000, Paul Burridge
wrote:

On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson
wrote:

I'm using a 555 timer to generate a sawtooth waveform to feed the
diodes, so I get a constant frequency sweep at the vcxo's output. Main
problem is the limited voltage output range; starts above zero volts
and peaks well before supply rail. So not much of a ramp; just around
4 or 5 volts, I guess. I could try changing the diodes for more

The 555 thresholds are set to charge and discharge the timing cap at
1/3 and 2/3rds the supply voltage. If you using that directly the
easiest way to get more "swing" is :

Use an opamp to translate the voltage lower and add some gain
(use at least 12v on the opamp). That can get you a Tuning voltage
that is near 0 to near 12V (that should help).

OR use a higher Vcc on the 555, say 12v. That will get your total
swing to about 4V and the low will be 4v and the peak will be 8v.

There are tricks that can be used to "offset" that 1/3 and 2/3 point
but the total swing is usually the same. That can help as operating
the Varicap closer to 0V will allow you to use more of it's
capacitance range though it's usualy less linear at the bottom.

Myself I'd use an opamp to create a saw generator and then I can
control the swings.

Allison

Winfield Hill wrote:
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.


Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.


Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:

Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.


An inductor in series with the varactors, then another one in
parallel with the series combo can get you a very wide range of
impedance from
a decent varactor.



Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

Winfield Hill wrote:

Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.


An inductor in series with the varactors, then another one in
parallel with the series combo can get you a very wide range of
impedance from
a decent varactor.



Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Last time I used this was with an MV104 common-cathode dual hyperabrupt,
to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature
hybrid in the usual way, coming in the 0 degree port, coming out the 180
degree port, and hanging matched reactances on the 90 degree ports.

Each section had its own inductors, and the cathodes were bypassed
heavily (1000 pF) to ground so that the two sides didn't interact too
much. The component values were 45 nH in series and 43 nH in parallel.
It was linear to within +-4 degrees, and the one section gave phase
shifts from 12 to 164 degrees, both dramatically better than I could get
with a bare varactor.

The idea is to have the varactor resonate with the series inductor just
off the low-voltage end of the range, and have the series combination
resonate with the parallel L just off the high-voltage end of the range.
Since the series-resonance doesn't even notice the parallel L, the
design equations decouple nicely, too. You adjust the placement of the
resonances to get the range and linearity desired.

Cheers,

Phil Hobbs

On Wed, 10 Dec 2003 23:28:06 GMT, wrote:

Myself I'd use an opamp to create a saw generator and then I can
control the swings.

Thanks, nospam. I can't imagine why I didn't think of this before. Got
just the circuit lying about for a 741 sawtooth too!
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill