The Phase Comparator


The VNGBOX design can also perform very nicely as a phase comparator and stand-alone clock. This is useful in the situation where a very good reference is available (for example a Rubidium source, other GPSDO or a non-adjustable high performance oven). All the other features of the design remain useful.

Absolutely no changes to the design are required, except of course the VCO/EFC (feedback) circuits are redundant, and the control mode settings are irrelevant.

In this arrangement, the Phase Comparator and companion PC software offer the following features:

If the unit is built in a small metal box with good feed-throughs, filters and bypassing, along with the reference oscillator, and with no display, it should be possible to build a completely leak-proof reference. This of course also applies to the VNGBOX and High Performance Frequency Standard, but with a display it is very difficult to keep all the RF in the box.

The secret to good phase monitoring is to set the PC software REFMON4 to a suitable plot speed. With 1s update, you can plot 1ppm oscillators and adjust simple crystal oscillators to frequency. For good OCXOs, use 10 second update rate to start with, and move up to 60 seconds or more. Very high performance oscillators can be monitored with 360 second or higher update rate.

For long term monitoring using the Phase Comparator idea, use a low power laptop computer or micro and write a simple program to take the phase data from the telemetry and plot it on a thermal printer. This might seem a low-tech, old-fashioned approach, but it's effective. Use only the last two digits of the phase, and every 10 minutes or so print that many spaces to the printer, followed by a '*' and CR/LF. You could print the time and date every hour, at the end of the line after the '*', for a long term phase plot. Here's an example:

          *
          *
           *
          * 02:00 23/03/07
          *
          *
           *
          *
           *
           * 03:00 23/03/07
           *
           *

Count the number of spaces at the beginning and again after a month or so (hint - use a ruler) and divide this number by 107 (or whatever your reference frequency is) and then by the elapsed time in seconds (number of days x 86400) to determine the offset and ageing rate of the reference.

You can also use the VNGBOX as a phase comparator by setting the operating mode to C04. This puts the EFC/VCO output to the centre of its range. The reference will operate in an open loop manner, so that it can be monitored directly for long term wandering and ageing effects. The technique is also helpful in tracking down sources of random phase variation, such as those introduced by changes in oven voltage, load, electrical noise or hum or draughts. This works best if you are able to electrically trim the oscillator close to frequency independent of the VNGBOX control loop.


Copyright Murray Greenman 1997-2009. All rights reserved. Contact the author before using any of this material.