Colpitts VFO No Work


Aug 11, 2014

 


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#1820 Aug 11, 2014

Morning Everybody,



I resolved to make a new radio transceiver. It's going to overcome the

overly wide receive bandwidth of my current transceiver and have

several other important features that should allow me to work CW with

a bit more ease.



The basic specs are that it is a 7 MHz transceiver with a crystal IF

filter at 4 MHz. I intend for its receive bandwidth to be less than 1

KHz (It's a CW transceiver) and I may improve the transmit power as

well.



Right now, I only have three stages complete on the receive board;

there's the 9v regulator, the VFO, and crystal-controlled BFO. The BFO

works although I'm not happy with the output waveform. Maybe I will

end up feeding a resonator with it in order to clean it up a bit. The

VFO does not oscillate at all.



Attached is a schematic. The buffer (MPF102 and 2N2222A) might be

changed as I'm sure it was designed to drive TTL loads. As far as the

VFO goes, I'm shooting for a target frequency of 3.0-3.2 MHz and am

getting nothing. I think that the value of the 10 pF capacitor should

be changed but I'm not sure. A lower value would better isolate the

tank circuit from the FET's gate but a higher value would allow more

signal through. Please advise.



Ed AE7TE



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