[time-nuts] Phase Detector was --> Simple method for comparing 10 MHz signals

[M. Simon]

There is another way. 


Take your reference signal and divide it by convenient steps. Apply the ref and the divided ref to the input pins of a very fast 574. Use a high output current 245 (24 or 32 ma drive) with resistor inputs (to decouple from the 574) to drive an LED bar (or RC integrator). The 574 will run faster if the loading is light and resistive. 


Clock the 574 with the signal whose phase you are trying to measure. Or a divided down version of the same. 


The two signals should be integral multiples of each other. That includes the divided down versions. 


You can now tell if your test signal is "skipping phases" of your ref.

Example: 10 MHz   1 MHz  100KHz into the 574. Clock it with 32Hz from your 32,768 Hz clock osc. Adjust your 32768 until the 10MHz LED rarely blinks. Your 32768 is now better than 1E-7 if your 10MHz is on the button. At 20 ppm (the typical limit of 32768 oscillators)  the 100KHz LED blinks twice a second. 


Using an XOR requires you to have square waves. The above only requires edge alignment (it is in effect a type 2 phase detector). And you are not limited to signals whose frequencies are nearly equal. They need only be roughly phase coherent. 


Simon


 
Message: 6
Date: Wed, 9 Jan 2013 16:29:32 -0800
From: "Bob Quenelle" <BobQhome at live.com>
To: "Discussion of precise time and frequency measurement"
    <time-nuts at febo.com>
Subject: [time-nuts] Simple method for comparing 10 MHz signals
Message-ID: <SNT125-DS15824F3CF1B6C9F0B40B93B22A0 at phx.gbl>
Content-Type: text/plain; charset="utf-8"

I
 kept putting off buying a nice counter and finally decided to try a 
phase detector circuit to compare 10 MHz standards.  It?s not novel, but
 I like the results so far.  It lets me see things I couldn?t see 
before.  I thought the idea might be useful to some of us who are 
equipment-limited.  The graph shows an LPRO-101 as the white trace and 
an FE-5680 as the red trace, both compared to a simple GPS standard.   
The graph is just an example of a data collection run and doesn?t 
represent any particular level of performance.  It does show a lot of 
common mode change, indicating the GPS is changing during the run. Maybe
 I should say probably changing.  The whole breadboard circuit has 4 
IC?s.  The blue trace is a measurement of the case temperature of the 
GPS standard.

The circuit uses 1/2 of a 74HC4015 4 bit shift 
register for each channel.  The D input of each 74HC4015 gets the Q-D 
output inverted by a gate from a 74HC04, forming a divide by 8 ?Johnson 
counter?.  At the beginning of a run all 74HC4015?s are simultaneously 
reset.  74HC86 XOR gates are used as phase detectors.  One input of each
 XOR connects to the Q-A output of the GPS 74HC4015 and the other input 
connects to the Q-C output of the LPRO-101 or FE5680 74HC4015.  Using 
different taps gets the initial state of the XOR output close to 1/2 
scale and known slope.  The average value of the XOR goes from 0 to full
 scale for a phase change of 180 degrees.  180 degrees of the divide by 8
 corresponds to 400 nsec, +/- 2 cycles of 10 MHz.  

I already 
had a LabJack U6 data acquisition unit, which has several analog inputs 
and digital I/O.  Other similar products are available and inexpensive. 
 LabJack has free data-collection software so you can get a file usable 
by Excel or whatever without writing any code.  For me it was easy and 
cheaper to convert the phase signal to a voltage and read it.  This 
approach isn?t useful for comparing PPS signals and isn?t as accurate as
 using a good TIC.  I?m looking forward to the TIC design in progress, 
but this project seems useful for now.
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