Tim Wescott wrote:
Anthony Fremont wrote:

Some material I read suggested keeping Xl of L1 at ~300Ohms, the
series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have
any thoughts on that? Right now I have way too much inductance for
3.5MHz by that theory, and judging from other circuits I've seen.
10uH seems to be the going thing for around 4MHz?


That sounds more or less right. With a Clapp oscillator the main tank
is isolated by the series cap, so more of the energy is kept in the
coil and C3, and less of it shows up in C1, C2, and the transistor.

If you're driving a balanced mixer you want to have an LO signal that
doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but
for a casual receiver that's the least of your worries. Since you're
operating at a fixed frequency it may be a good idea to just feed the
oscillator output into a single-tuned resonant circuit to clean it up,
then send it on to the mixer.

Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1
to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed
the tank circuit voltage down as well. Which brings up a question, with the
tank now completely DC blocked from Vcc and Vss, where does it get it's
energy. I assume that it must come thru the gate. How does that happen?
:-? My circuit is much like Figure 1 here, without the diode though:
http://www.electronics-tutorials.com...scillators.htm

Anthony Fremont wrote:
Tim Wescott wrote:

Anthony Fremont wrote:


Some material I read suggested keeping Xl of L1 at ~300Ohms, the
series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have
any thoughts on that? Right now I have way too much inductance for
3.5MHz by that theory, and judging from other circuits I've seen.
10uH seems to be the going thing for around 4MHz?



That sounds more or less right. With a Clapp oscillator the main tank
is isolated by the series cap, so more of the energy is kept in the
coil and C3, and less of it shows up in C1, C2, and the transistor.

If you're driving a balanced mixer you want to have an LO signal that
doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but
for a casual receiver that's the least of your worries. Since you're
operating at a fixed frequency it may be a good idea to just feed the
oscillator output into a single-tuned resonant circuit to clean it up,
then send it on to the mixer.


Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1
to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed
the tank circuit voltage down as well. Which brings up a question, with the
tank now completely DC blocked from Vcc and Vss, where does it get it's
energy. I assume that it must come thru the gate. How does that happen?
:-? My circuit is much like Figure 1 here, without the diode though:
http://www.electronics-tutorials.com...scillators.htm


It comes from the source, through the coupling capacitors -- Cfb-a and
Cfb-b in your link.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
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