[time-nuts] Question about precise frequency / phase measurement

EWKehren at aol.com EWKehren at aol.com
Fri Apr 20 11:53:59 EDT 2012


I have and use a Tracor 527E how ever the Austron circuit including counter 
 is a PCB board 2.2 X 2.5 inches and I have not seen a Tracor for $ 50. I 
think I  paid $ 800 fifteen years ago. 
Bert Kehren
 
 
In a message dated 4/20/2012 11:37:16 A.M. Eastern Daylight Time,  
eb at telight.com writes:

You may  want to look at how that was done many years ago with 
frequency difference  multiplication as in the old Tracor meters - I 
think the 528 was the main  one. They synthesized a 9 MHz reference 
from one input, and then  subtracted it from the other to get a 1 MHz 
result, which was used as a  reference for another 10 MHz PL 
oscillator. The 9 MHz was subtracted from  this, and so on to a next 
identical stage. The net result is that each  stage of this process 
multiplies the frequency difference by ten times.  These can be 
cascaded until you reach the limit of the noise performance  of the 
stage designs - they managed around 10,000 times fairly readily.  This 
method can be duplicated fairly easily with modern logic parts. With  
a setup like this you can produce a 1 or 10 MHz carrier that can be  
counted to very high resolution at one second gating - you just 
ignore  the carrier digits and look at the multiplied difference 
frequency digits.  Or, you can subtract the carrier and get just the 
multiplied difference -  but you have to keep track of the phase info 
to know if it's plus or  minus.

It's of course possible to use whatever frequencies and stage  
multiplication factors you want, but the tradeoffs are in making the  
numbers come out rationally (especially if you want a number of 
decade  multiplier ranges), and the precision and quality of the 
intermediate  frequency filtering and processing. About ten to one 
hundred times per  stage is within reason. For example, I have an 
experimental (way  unfinished) setup started with three stages of 1 
GHz PLOs for  multiplication, and a 990 MHz reference. This will give 
100X per stage,  reaching 10E6 difference frequency multiplication at 
one second gating,  presuming I can manage the phase noise 
sufficiently. A quick two-stage  setup indicated no problem reaching 
10E4, but that last 100X will be  tricky - I have to build it up for 
real first, with extremely clean power  supplies, shielded signal 
processing modules, and solid signal routing,  just to see if it's 
possible. I would not recommend this approach - I'm  only doing it 
because I happened to have all the main parts on hand. It  would be 
better to keep everything down to 100 MHz or less for processing  in 
ECL or ACMOS, and using crystal VCOs and  filters.

Ed


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