EMRFD Message Archive 7245

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