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.

Peter HA5RXZ

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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