Mario Sanchez wrote:
Hi Bruce...
Thanks for your answer...
I have a couple of questions:
1. I am using a 1kHz beat frequency, and measure back2back period using a
Pendulum CNT-91 high resolution frequency counter. Does the DC stability of
the mixer still matter?
2. I have been comparing the AD831 with some passive mixer, but i can not
see any great improvement. I am basically following this procedure:
a. As I just want to compare the mixers, using only one of them I mix 10MHz
from a Rb with 10MHz+1kHz generated by a R&S signal generator referenced to
the same Rb.
For passive mixer, I amplify before the mixer, I have also tested different
input levels at both LO and RF signal.
b. Filter the high component.
c. ZCD - Hard limitter, using Collins-type amp.
d. Calculate ADEV at 1s.
Any approach on how to test the mixer?
My goal is to reach an accuracy of 5E-14 @ 1s
Regards,
Mario
Mario
1) dc drift (referred to the mixer inputs) does affect switching
thresholds or equivalently the mixer phase shift.
2) To characterise the mixer phase noise simply drive its inputs (LO
+RF) in phase quadrature with the same signal and measure the output
noise (after the low pass filter) using a spectrum analyser (eg sound
card plus high gain low noise preamp - I have some tested low noise
amplifier designs that may be useful). Then measure the mixer phase
sensitivity by driving one input at a frequency offset from the other
input by 1kHz and measure the zero crossing slope of the beat frequency
signal.
Then calculate the mixer phase noise spectrum from these results.
Typically for a 10534B or similar mixer the output noise is only a few
nv/rtHz @ 100Hz.
If you don't know the mixer output noise spectral density then its not
possible to optimise a Collins style limiter.
The higher the mixer output noise spectral density the lower the gain
and bandwidth of the first stages in an optimised Collins style limiter.
Your tests may well be limited by the phase noise of the 2 sources you
are using and not either mixer.
Its more important to have a low phase noise source driving the mixers
in phase quadrature than a low drift one when testing.
A good crystal oscillator may be quieter than your rubidium source.
Bruce
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