EMRFD Message Archive 7245
Message Date From Subject 7245 2012-02-13 08:04:30 Ben Pharr Converting VFO circuit to VCO
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
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
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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