geoff@hls0.hls.oz (Geoff Bull) (09/25/90)
I want to build a 60 MHz VCXO.
Can I simply replace the capacitor on a MC1658 VCO with a capacitor
and crystal in series (as shown below)?
_
| |
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----| |-----------| | | |---
| | | | | | | |
| | | |
| - |
| |
| |
| |
| |
------------------------------------------------
| Cx Cx |
| |
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| Motorola MC1658 |
| Plessey SP1658 |
| |
| |
| |
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If this would work, why? Any better ideas?
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Geoff Bull (Senior Engineer) Phone : (+61 48) 68 3490
Highland Logic Pty. Ltd. Fax : (+61 48) 68 3474
348-354 Argyle St ACSnet : geoff@hls0.hls.oz.au
Moss Vale, 2577, AUSTRALIAwhit@milton.u.washington.edu (John Whitmore) (09/25/90)
In article <935@hls0.hls.oz> geoff@hls0.hls.oz (Geoff Bull) writes: > >I want to build a 60 MHz VCXO. >Can I simply replace the capacitor on a MC1658 VCO with a capacitor >and crystal in series? > Not really. The 1658 is a multivibrator, and few (if any) crystals will operate reliably with all the frequencies that it'll be generating (it MIGHT work if you put a series LC in, to limit the out-of-band power). Usually, one uses a damped crystal (put a resistor across it if you have to), with a simple oscillator like | +-------------+ | | | / +--|O|--+---| NPN transistor | | | = = \ | | | GND GND GND Resistors omitted to confuse the neophytes. (I hope the ASCII graphics isn't too terrible). Tuning would be by using a varactor diode/capacitor combination for one of the two capacitors. As nearly as I can remember, some crystals come specified for VCO operation (usually very narrow band). John Whitmore