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.


Cheers,

Phil Hobbs

Paul Burridge wrote:

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

Right. I might make the series inductors symmetrical, if I needed the
tap point to be near signal ground (e.g. with a centertapped coil or a
differential pair driving it). Last time I used this trick was in a
160-MHz phase shifter. The two inductors will generally be about the
same size for best results with a hyperabrupt varactor--5 minutes with a
math program will give you the right values. Generally you need to keep
the reactance capacitive if you're resonating a crystal against this
combination--there are multiple operating frequencies otherwise, since
the resonator will look capacitive almost everywhere.


Cheers,

Phil Hobbs

Paul Burridge wrote:

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

Right. I might make the series inductors symmetrical, if I needed the
tap point to be near signal ground (e.g. with a centertapped coil or a
differential pair driving it). Last time I used this trick was in a
160-MHz phase shifter. The two inductors will generally be about the
same size for best results with a hyperabrupt varactor--5 minutes with a
math program will give you the right values. Generally you need to keep
the reactance capacitive if you're resonating a crystal against this
combination--there are multiple operating frequencies otherwise, since
the resonator will look capacitive almost everywhere.


Cheers,

Phil Hobbs

Paul Burridge wrote in message . ..
Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

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.

Cheers,
Tom

Paul Burridge wrote in message . ..
Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

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.

Cheers,
Tom

On Wed, 10 Dec 2003 00:26:34 GMT, "W3JDR" wrote:

Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
changed.

Oh bugger. Well how about using varactors with a higher C/V ratio?

If you're not already using a "hyper-abrupt" type of varactor, you should
look into one. They offer a wider capacitance range.

Not sure what you mean by that term, but imagine it amounts to simply
a type with a higher capacitive reaction to applied voltage - as I
mentioned above.

What type of varactor are you using, and what's the frequency of the
resonator? What's the application...linear frequency modulation like FM or
data keying like FSK???

I'm currently using BB149A diodes, but I've got some BBY40s as well,
which might offer more shift per volt; I haven't checked the spec yet.
The fundamental frequency is 8.00Mhz and I need to pull it by +/-32khz
for tuning rather than modulating purposes.

--

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

On Wed, 10 Dec 2003 00:26:34 GMT, "W3JDR" wrote:

Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
changed.

Oh bugger. Well how about using varactors with a higher C/V ratio?

If you're not already using a "hyper-abrupt" type of varactor, you should
look into one. They offer a wider capacitance range.

Not sure what you mean by that term, but imagine it amounts to simply
a type with a higher capacitive reaction to applied voltage - as I
mentioned above.

What type of varactor are you using, and what's the frequency of the
resonator? What's the application...linear frequency modulation like FM or
data keying like FSK???

I'm currently using BB149A diodes, but I've got some BBY40s as well,
which might offer more shift per volt; I haven't checked the spec yet.
The fundamental frequency is 8.00Mhz and I need to pull it by +/-32khz
for tuning rather than modulating purposes.

--

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

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


Paul, That will work, but will double *both* min and max capacitance.
But I'm puzzled: "I can't get enough shift with the available bias
voltage" implies you're on the *low* end of capacitance (highest
voltage).

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
responsive ones but they're SMDs and I really hate messin' with 'em.
:-(
--

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

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


Paul, That will work, but will double *both* min and max capacitance.
But I'm puzzled: "I can't get enough shift with the available bias
voltage" implies you're on the *low* end of capacitance (highest
voltage).

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
responsive ones but they're SMDs and I really hate messin' with 'em.
:-(
--

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

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