Post subject: Colpits Oscillator help. Posted: Sat
Oct 06, 2007 9:26 pm
Joined: Wed Sep
19, 2007 6:58 pm
I am designing a Cplpits oscillator
for my senior design project. I want it to oscillate at two frequencies
so I plan on switching in and out a variable capacitor within the
tuning ckt. below is my circuit. The two frequencies are 15.5 and
16.5 Mhz. I was planning to simply use a fet as an analog switch
but I am having a very hard time finding a NFET with a low enough
drain to source capacitance. Does anyone have a specific NFET in
a non surface mounted package that could work for this application
or another suggestion as to how I can easily switch in and out this
capacitor? In the schematic below I have a black box as the switch.
the varicaps are to compensate for the discrete components I will
be using. Thank you.
Post subject: Posted: Mon Oct 08, 2007 5:42 am
Joined: Fri Feb 17, 2006 12:07 pm
You might be better off using a forward-biassed
Schottky diode rather than an NFET. The diodes are fast, and the
Scottky has a very low on-resistance so the Q is not badly effected.
Your circuit has a path to ground for the switching current, and
you just need an RF choke towards the bias supply. When OFF, the
circuit just sees the one capacitor, and when ON it sees both in
parallel. You might also consider PIN diodes, but I seem to recall
that most are poor performers at lower frequencies such as 16MHz.
A more elegant solution is to use a varactor diode switched
between the voltage bias that gives the two values of capacitance
required. Perhaps the frequency stability with this method would
not be as good as the diode switch however.
Post subject: Posted: Mon Oct 08, 2007 11:51 pm
Joined: Wed Sep 19, 2007 6:58 pm
Thank you I will investigate the diode idea further.