oops I omitted an important phrase.
Bruce
Bruce Griffiths wrote:
There are 4 principal sources of noise
1) The GPS receiver
2) The 4046 Phase detector
3) The opamp
4) The OCXO
In the short term the GPS receiver noise will dominate.
The GPS receiver noise will dominate the sort term phase detector output
noise
In the long term the 4046 phase detector noise and drift together with
the OCXO noise and drift will dominate.
Again add the phase "the phase detector output noise" to the end of the
sentence.
Unless you make an extremely poor choice of opamp the 4046 phase
detector noise and drift will be much larger than that of the opamp.
This actually depends on the phase detector comparison frequency at 1PPS
phase detector (plus divider drift) of 1ns/C is equivalent to 5nV/C
input offset drift at the opamp (about 1/10 that of a chopper stabilised
amplifier). At 100Hz 1ns/C is equivalent to about 500nV/C at the opamp
input or about 10x that of a good chopper stabilised (or "zero drift")
amplifier.
Bruce
John Foege wrote:
Hi All,
Quick question for the more experienced members here with GPSDO
design/operation. Let's assume I'm using a 4096 phase comparator chip
followed by some kind of long time constant lowpass loop filter,
whether it be analog or digital, is not of concern for the following
question.
Obviously using a 74HCT4096 would mean that my EFC voltage range would
be approx. 0-5V. If I wanted to use an OCXO with say a 0-8V EFC
voltage range, then I would be inclined to simply use an op-amp
amplifier with a gain of 1.6 to scale the EFC voltage accordingly.
But not just any op-amp would do I take it? High-speed would of course
be of no concern. Also low-offset would be of little concern, as the
PLL would work to correct this, and it therefore seems to be
negligible. However, the part that's got me thinking is noise.
Obviously any noise at the ouput of the amp would adversely affect the
frequency stability of the OCXO.
I thought the best way to control this would be to use an extremely
low noise op-amp employing a rather large compensation cap to give me
a rather small bandwidth, perhaps only a few hundred hertz.
Anyone have experience with this? Assuming I have an OXCO with a max.
pulling range of 1ppm or 1e-6 over a 10V range, then I effectively can
pull 1e-7 per volt. This translates to 1e-10 per millivolt and 1e-13
per microvolt. Assuming that is a logical conclusion, then for a good
OCXO, in which I can at best hope for 5e-12 stability for tau=1s (e.g.
HP10811A), I would strive to to keep the noise at such a level that it
is an order of magnitude better than the best short term stability
figure. Accordingly, then I should shoot to keep any noise under 1
microvolt?
I don't have much experience with noise calculations. I know it is
specified in nV/sqrt(Hz) generally. Translating this to something
practical is basically the assistance I'm looking for here.
I would appreciate anyone being able to teach me a bit more about this.
Thank you in all in advance.
Sincerely,
John Foege
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