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From: "Bruce Griffiths" <bruce.griffi...@xtra.co.nz>
Sent: Wednesday, March 30, 2016 7:29 AM
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Subject: Re: [time-nuts] Oleg' s PN test Re: A new member & PN test set
One hidden issue you don't address is that operation of the 40uH inductor
at
frequencies above its parallel resonance may allow substantial RF at the
sum
of the LO and RF frequencies to appear at the opamp input.
120MHz at the 797 input will likely lead to RF rectification effects in
the
opamp input stage. The resultant offset will create a number of issues
including operation away from the quadrature point.
Unless you use something like a series string of inductors and/or a
conical
inductor the first parallel resonance of the 40uH inductor is likely to be
somewhat below 120MHz.
Ohhh... I do not like words like "substantial", "much more" and etc. I like
numbers and tests. ;)
So I looked at the Murrata inductors datasheets, and it appeared 40uH
inductor will have SRF in 10MHz region. But it does not mean that pi-LPF
will not work at the higher frequencies. Actually it mean that our LPF will
have response similar to the elliptic filters.
So let's draw the model with the inductor with self resonance at 10MHz and
well at 120MHz and 1MHz to see how bad the response is:
1MHz: http://skydan.in.ua/PNTestSet/1MSRF.png
10MHz: http://skydan.in.ua/PNTestSet/10MSRF.png
120MHz: http://skydan.in.ua/PNTestSet/120MSRF.png
As we can see the RF+LO product will be attenuated more than 60dB in all
cases. So, your comments? Would you like me to measure the RF voltage at the
AD797 input in the real "test set"?
However 50 ohms to ground at the LC filter output shouldn't be necessary.
A somewhat larger value should suffice.
I made some experiments trying to find the optimal value of the resistor at
the LC filter output. The phase detector gain grew along with the resistor
value, but so did the harmonics level. So I needed to apply more attenuation
to the input signal to stay in the linear region. The resulting "test set"
noise floor was almost identical for 50..300Ohm values (300Ohm was a bit
better at offsets grater then 2kHz and a bit worse closer). Large values
noticeably degraded the performance.
I suppose the noise floor can be lowered only if better LNA will be used
(currently the LNA noise dominates the PD noise), or if the levels on the
mixer will be increased (this will require higher level mixer and/or new
calibration routine if the mixer will not be in a linear region).
All the best!
Oleg
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