Rick Karlquist wrote:
Bruce Griffiths wrote:
Richard (Rick) Karlquist wrote:
On 11/23/2012 9:38 AM, Bruce Griffiths wrote:
NIST have shown (at least at 10MHz) that the high level mixers they
tested are noisier than the ZRPD1.
Bruce
http://tf.boulder.nist.gov/general/pdf/2556.pdf
Where all the high level mixers measured have a higher phase noise than
lower level mixers/phase detectors like the ZRPD1.
Bruce
This data is surprising. I am wondering if it is being corrupted
by AM noise on the source? They do not mention what the source is.
They could be measuring AM noise suppression differences in the
phase detectors.
Rick
Another potential issue is the relatively high input current noise of
the IF amplifiers (see the paper's Reference 9).
In the cross correlation setup used the IF amps both see the noise
produced by the sum of the IF amp input noise current flowing in the
mixer output impedance.
This noise common to both IF amps isn't rejected by cross correlation.
It can be more effective to use IF amps with very low input noise
current that may have a higher input voltage noise.
The effect of the amplifier input voltage noise is reduced by cross
correlation.
For offset frequencies >=10Hz a well designed low noise amplifier with a
discrete JFET input stage can have comparable voltage noise to the IF
amp used with much lower input current noise.
Bruce
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