HagaaarTheHorrible wrote:
Hi there,
I hope this is the correct way to post here and that its not been covered
before. I've read some threads about frequency dividers, but didn't really find
all the answers I'm looking for.
I'm trying to generate a 10kHz sine wave thats as clean as possible, as it will
be a reference signal to measure jitter. The idea was to use a 10 / 10.24MHz
Crystal Oscillator (since they are easy to get) and divide it down to around
10kHz (exact frequency is uncritical). So I've been looking for low phase noise
frequency dividers, and now have a few questions.
1. Will this way result in worse or better jitter/phase noise specs than
generating a 10kHz Signal from the start? (meaning, is the additional noise
added by the divider worth more or less than the reduction of phase noise by
the division ratio)
The problem would lie in finding a 1kHz resonator with sufficiently high
Q to achieve low phase noise and hence jitter.
2. If the suggested 10MHz->10kHz way is the correct one, would a conjugated
regenerative divider be the best way to get to 10kHz or are there less noisy
alternatives?
Conjugate regenerative dividers are currently considered the lowest
noise technique but using a custom mixer perhaps using diode connected
BJTs (eg 2N2222's or 2N3904's) will be required for frequencies below 1MHz.
CMOS dividers can have reasonably low PN floors but a properly
configured ECL divider can have lower close in phase noise.
You could try using something like a 74AC4040 driven by a 10MHz input
signal.
However lower phase noise can be achieved by using cascaded decade (or
divide by 16) dividers with input bandpass filters (reduces aliased
input noise) for each stage.
3. I've read through several threads about the divider designed by David
Partridge, havnt found any real specs for it though. Is there any measurement
data available?
Rough measurements indicate a cycle to cycle jitter of around 5ps rms or so.
Most of this is probably due to the input comparator.
Its difficult to measure such low jitter even a wavecrest 2075 (and its
cousins) isnt really good enough.
Measuring phase noise at frequencies below 1MHz or so is outside the
range of a Timepod or most mixers.
Thanks in advance for any replies.
Hag
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Bruce
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