T
Tim Wescott
I doubt it. You really need to start with an circuit that wants tocolin said:sorry if this sort of subject has been covered here a lot but cant seem to
find what info i need ..
1) is there a way to get a cmos inverter xtal oscilator to 3rd overtone
without using an inductor? i tried a rc filter to the input of the crystal
but it wld oscilate at fundemental or not at all. the inductors seem to be
cuasing some jitter from vibration/interference. and what about 5th overtone
too ?
oscillate at the overtone frequency, then put the crystal in.
I'd only use a CMOS inverter with a 3rd overtone with a lot of
qualification work -- I think you'll get better reliability out of a
transistor oscillator. If your inductors are mechanically stout you
shouldn't be having vibe problems -- what kind of precision are you
trying to get, what are you seeing, and what kind of inductor are you using?
See the bottom of my reply for a suggested circuit.
Overtone operation tends to be much higher Q than fundamental, and it is2) also when u use a fundamental crystal at 3rd overtone what are the
diferences in performance? i notice that it has a very narrow margin of
adjustment compared to fundamental, how to detemin what is the best loading
capacitance?
best to run the circuit such that the crystal is running at it's series
resonant frequency. I would try to load the crystal with as little
capacitance as I could get away with, were I running the thing in
overtone mode.
3) is it normal for the 3rd overtone to vary so much in a fundamantal
crystal, and is the performance of crystals cut for 3rd overtone much better
(aside from initial acuracy )? and how much jitter can one expect ?
Crystal overtones tend to vary from the fundamental; crystals intended
for overtone operation are tested at the overtone frequency. I would
expect to see variation among crystals cut for the fundamental that
you're running on overtone, but I've never tried it myself so I don't
know how much.
the project im working on (mainly for interest) is time domain reflectometry
and im trying to see how much resolution i can get.
im using about 10-100 mhz signal and mixing this with a local oscilator to
get about 32khz IF this is done to efectivly multiply the delay by fsig/f IF
for the main signal i used a 10mhz crystal oscilator with a single gate
unbufered 74AC04 wich is buffered by another.
philps do a nice package with 1 unbufered and 1 bufered cmos inverter ideal
for this but unfortunatly i cant get them from farnel or rs
im using a PLL to generate the local oscuilator with a LC/varicap VCO (and
the same 2 single gate ic) and the beat frequency (derived with a single
gate xor) compared io a 32768 crystal oscilator. the IF signal is then
compared to this beat frequency to get the delay. im using a sa605 to do the
mixing and if.
however im geting quite a lot of jitter wich is as yet unacounted for. i
started using single gate ics everywhere becuase i had everything going
through one hex inverter and it was obvious there was too much interference
between devices in the chip, even for the low frequency if, and was cuasing
jitter when two signals transitioned quite close together.
i also noticed the LC VCO was constantly being adjusted and i was geting
0.1us jitter on the IF signal so i asumed this was the cuase, ( signal
strength was high). i was using 2 crystal oscilators before but this limited
my IF frequcny as to what crystals i cld find.
i decided to increase the signal frequency i only had 10 and 20mhz crystals
as i cldnt get much higher ones so i decided to try a 10mhz at 3rd overtone
to give about 30mhz.
however i was surprised that the resultant frequency was so far off the
fundamental, i know its not exact but i thoiugh it wld be a lot closer i get
a range of about 29.5 to 29.8 from a bag of 20 crystals. this was however a
bit usefull becuse i found 2 crystals just the right frequency to give a
diference of 32khz. and was still able to include it as the VCO in the PLL.
however i still get significant jitter, i had to use an inductor acros the
input to the crystal to get it to oscilate at 3rd overtone, i tried to get
it to oscilate at 5th but havnt been able to yet, i think the 74ac04 is fast
enough becuase the LC oscilator went well over 50mhz.
incidently i fnd that any trace of flux on the pcb makes things much worse,
its horible stuff !
many thanks Colin =^.^=
Here's the circuit that works for me. You _may_ be able to replace the
inverter with an inverter, but I've never tried it. The inductor and
variable cap should resonate slightly higher than the crystal overtone
frequency, and you're depending on the base-collector capacitance of the
transistor for feedback. I'm leaving out component values as an
indication that _you_ need to be responsible for it's correct operation.
The ARRL UHF Experimenter's Guide has another circuit that they highly
recommend (I think they hold my circuit below in high contempt), but I
haven't tried it, so I can't recommend it.
VCC
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GND | | GND
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GND GND
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