Varactor tuning
 

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.

p.
--

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

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.

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

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???

Joe
W3JDR


"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.

p.
--

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

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.

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

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???

Joe
W3JDR


"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.

p.
--

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

On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote:


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.

p.

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).

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote:


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.

p.

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).

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote:


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.

p.

Sure. Is this a coaxial ceramic resonator, or one of the low-freq
piezo things? What's the frequency, Kenneth? [1]

John

[1] old Dan Rather joke, sorry.

On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote:


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.

p.

Sure. Is this a coaxial ceramic resonator, or one of the low-freq
piezo things? What's the frequency, Kenneth? [1]

John

[1] old Dan Rather joke, sorry.

that would the more total capacitance but not any larger dC/dV.....

you need even better tricks :-)

Marco

"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.

p.
--

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

that would the more total capacitance but not any larger dC/dV.....

you need even better tricks :-)

Marco

"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.

p.
--

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

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