**EMRFD Message Archive 7245**

MessageDateFromSubject7245 2012-02-13 08:04:30 Ben Pharr Converting VFO circuit to VCO I'm working on a VCO for my previously mentioned 80m direct conversion

receiver. I was hoping adapt this Colpitts VFO circuit from the GQRP

site: http://www.interalia.plus.com/q_tech9.htm

to a VCO.

So far I have the circuit built exactly like this, except no variable

capacitor. It's oscillating in the neighborhood of where I'd like it

to. I was hoping I could just put a varactor diode (or two in

parallel) right where VC is in the schematic. However, it occurs to me

that the inductor will be a short for the DC tuning voltage.

How can I change the circuit to keep the DC out of the inductor? The

first thing that occurs to me is to double C3 and put a capacitor of

equal value in series between the the varactor and the inductor. This

would keep the series capacitance the same, presumably maintaining the

frequency characteristics of the circuit. Is my reasoning sound? Any

other recommendations?

On a related note, can someone point me to a set of equations for

computing these values? I managed to figure it out experimentally this

time (w/ help from GQRP's "cookbook" values), but it'd be nice to be

able to get close mathematically next time. I've found a couple of VFO

calculators online, but they must use a different circuit, because

their numbers don't match the GQRP cookbook values or my experimental

results.

Thanks,

Ben

WF5N7246 2012-02-13 08:35:27 Ron McCurdy Re: Converting VFO circuit to VCO Hi Ben,

May I suggest that you go to Jim Tonne's website and download "BitXTune" and

the tutorial.

http://tonnesoftware.com/ Then, find an FQP2N80 on Ebay or email me your

address and I'll send you a couple. They work from @400 pfd to 690 pfd @ 12

vdc to 0 vdc. That's a larger range and you may find you like it.

Otherwise just use any varactor and substitute it's capacitance into the

"BitXTune" software.

73,

Ron, KE5QDA

KE5QDA@cox.net

I'm working on a VCO for my previously mentioned 80m direct conversion

receiver. I was hoping adapt this Colpitts VFO circuit from the GQRP

site: http://www.interalia.plus.com/q_tech9.htm

to a VCO.

So far I have the circuit built exactly like this, except no variable

capacitor. It's oscillating in the neighborhood of where I'd like it

to. I was hoping I could just put a varactor diode (or two in

parallel) right where VC is in the schematic. However, it occurs to me

that the inductor will be a short for the DC tuning voltage.

How can I change the circuit to keep the DC out of the inductor? The

first thing that occurs to me is to double C3 and put a capacitor of

equal value in series between the the varactor and the inductor. This

would keep the series capacitance the same, presumably maintaining the

frequency characteristics of the circuit. Is my reasoning sound? Any

other recommendations?

On a related note, can someone point me to a set of equations for

computing these values? I managed to figure it out experimentally this

time (w/ help from GQRP's "cookbook" values), but it'd be nice to be

able to get close mathematically next time. I've found a couple of VFO

calculators online, but they must use a different circuit, because

their numbers don't match the GQRP cookbook values or my experimental

results.

Thanks,

Ben

WF5N

[Non-text portions of this message have been removed]7247 2012-02-13 08:37:09 John Greusel Re: Converting VFO circuit to VCO Ben,

You might take a look at the Epiphyte 3 kit that Norcal offered.

The circuit is still available online.

I built a 2B version and the varactor diode/VFO circuit for that works great.

It's really stable after a minute or two of warm-up.

John

KC9OJV

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