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