EMRFD Message Archive 9204
Message Date From Subject 9204 2013-10-03 20:21:36 rcbuckiii Colpitts VFO
I'm working on a 5.0-5.5 MHz colpitts VFO. Just the normal FET standard design using a J310 and without the diode at the gate. It settles down to about 10 Hz drift per hour after a 30 minute warm up. I follow the oscillator with a buffer stage and a feedback amplifier to get the output up to 10 dB.
I used the calculations from Fig 2, page 34 of SSD. These calculations fall pretty much in line with other references I've looked at. I used 680pF+680pF for the gate to source to ground capacitors. For the coupling cap to the inuctor and other caps I used 270pF.
The question I have is are these three capacitors very critical as to reactance values at 5 MHz or 7 MHz? The reason I ask is because the schematic for my Kenwood TS-820S shows 150pF + 150pf and 69pF for these three caps. The VFO in the Kenwood runs at 5.0 - 5.5 MHz and is very stable. Did they use these values to minimize any effect on the resonant L/C components thereby minimizing drift?
9232 2013-10-11 07:15:50 ha5rxz Re: Colpitts VFO
When I saw those 680pF values it was an eyebrow raising moment, they do seem a bit large. If I was building a 5 MHz Colpitts I'd have 2 x 150 pF in my circuit.
9245 2013-10-13 13:54:45 ka7exm Re: Colpitts VFO
Sorry, message 9244 had no text. It may have been encoded and Yahoo did not catch the encoding on the message. Please re-post. 73. Roger KA7EXM
9249 2013-10-14 20:48:46 ka7exm Re: Colpitts VFO
Sorry rcbuckii, your most recent posting has text encoding problems (we cannot read it online). Please re-submit by pasting text in plain format into the yahoo editor window. Thanks!, Roger KA7EXM
9250 2013-10-14 21:48:27 rcbuckiii Re: Colpitts VFO
Roger, I'm just clicking the Reply butt
9251 2013-10-14 23:58:10 blumu Re: Colpitts VFO Yes, same again. See my recent note.
The text opens as blank, but appears
under "forward" in OE.
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