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 Voice480)460-2350 | |
| E-mail Address at Website Fax480)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 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 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 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

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

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 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