[time-nuts] Oleg' s PN test Re: A new member & PN test set

Oleg Skydan olegskydan at gmail.com
Wed Mar 30 11:11:51 EDT 2016


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From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
Sent: Wednesday, March 30, 2016 7:29 AM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at 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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