A High Performance Frequency Standard
- and the -
VNG in a Box

By Murray Greenman ZL1BPU

A trio of GPS projects, with LCD display and clock. Use an oven controlled oscillator for incredible accuracy. Time code generation.
 VNGBox Clock Core  VNG Time Code  Oscillator Control
 Telemetry & Commands  Phase Comparator  GPS Monitor/Display

INTRODUCTION

This design is an improvement on the popular GPSClock, and uses similar hardware. Most of the improvements are aimed at providing support for higher stability oscillators, especially the oven controlled type. The additions allow use of positive or negative slope OCXO references, provides time code generation, full holdover when GPS fix is lost, power failure lockout and default mode etc.

The design locks a 10MHz voltage-controlled OCXO to the GPS reference, in just a few minutes, and is capable of providing a low noise, accurate reference, with performance at the level of a few parts in 1010 (1e-10) or better, typically close to 1 in 1012 . In addition to controlling the reference, the design provides a local (independent) but GPS synchronized time clock, generates a 1kHz reference, and provides a VNG format time code. All these products are controlled to within 50us of UTC.

Of course the reference can be any other frequency from 8MHz to 16MHz, and can be buffered and used to operate dividers for other reference frequencies. (For OCXO frequencies below 8MHz, use a frequency doubler to operate the micro.) Oh - and did I mention that this is essentially a single chip design?

This is in reality three projects in one:

(The picture on the right is a natural light screen shot of the PLED clock display - it is blurred because of the long exposure time.)

In this discussion we'll call the device the VNGBOX, but unless otherwise mentioned, the comments apply to the 'High Performance Frequency Standard' and the 'Phase Comparator for Frequency Standards' as well.



The GPS Display (left) and VNGBOX (centre) prototypes in a recycled cabinet
New life for an old HP5326A counter!

The clock and reference continue to operate accurately when GPS is lost, and although the reference frequency will then drift as the oven oscillator ages, it should hold to within 109 for at least 24 hours (depends on oscillator quality). The controller it is held digitally at the current operating point until GPS returns.

Most GPS units continue to send 1pps pulses even when the GPS fix is lost, and these can be sufficiently inaccurate to cause the much higher accuracy OCXO to be forced off frequency. With this unit, the GPS fix can be monitored (by a separate micro) and the unit put into HOLD if the fix quality is poor, or reception is poor, thus preventing the unit from wandering off following the GPS 1pps when the fix is lost. It is best used with a Motorola Oncore or Navman Jupiter T GPS engine with TRAIM capability.

When the two micros are combined in a single unit, the GPS data and the telemetry from the VNGBOX can be combined into a single data stream for PC display of both GPS and local reference!

The clock is essentially bullet-proof and has perfect accuracy, even if GPS is occasionally or frequently lost. On repower after power failure, where local time will be lost, the device returns to a safe condition, locks to GPS and the time code is disabled. The time code is restored when the clock is reset. The unit should be used with a backup UPS power supply capable of 50VA or so.

Here is a summary of the special features of the VNGBOX:

Longer integration times give lower noise for better quality oscillators. If the oscillator wanders more, shorter integration times allow the unit to track the changes more quickly.



The VNGBOX 16 x 2 PLED display explained

The display shown in the photograph is a 16 x 2 unit. At the top left is the VNGBOX (locally set) time, in 24 hour format. The information shown on the right of the display is the reference phase (top) and feedback term, related to oscillator Electronic Frequency Control (EFC) voltage (bottom).

The rectangles (bottom left) flash in time with the time code, or are replaced by an error message when there is a fault condition. To the right are displayed the operating mode, and the flag byte (helps diagnose problems) and the feedback integration timer.

The basic VNGBOX is simple to build - two ICs and a couple of transistors. Very likely the GPS engine and reference oscillator will cost more than the rest of the parts. The LCD display is a standard type used everywhere. The unit is also fully functional without the display. A new organic LED or PLED display (as shown) can also be used with no circuit or code changes.

The unit can be this simple, or made as complex as you wish, by adding the extra features - a better OCXO, better regulated power supplies, active filters and amplifiers in the feedback path, and of course the GPS Display module (second micro).

Precision time keeping and the precision 10MHz reference are achieved through the use of a Voltage-Controlled Oven Controlled Oscillator. For reduced performance, but lower cost and power consumption (say for a portable VNGBOX), a Temperature Compensated Crystal Oscillator (VTXO) is suggested. These can cost $30 new, but can often be found on the internet at good prices. Just about any frequency from 8 to 15 MHz can be used, but unless you plan only to use the device as a clock and phase comparator, you MUST use a voltage-controlled oscillator. These devices usually have a control range from about 2 parts in 107 to 1 in 106. The design will handle both negative and positive control senses (but only positive voltages). Bipolar devices can be controlled using an offset technique.

The clock transmits serial telemetry data every second, in addition to providing the LCD display. The serial data can be used for time logging, or setting other clocks. The phase information can be used for calibration or long-term tracking of the reference oscillator performance. A special companion monitoring program has been designed for this unit.

SOFTWARE

The firmware and PC software for this project is well tested, versatile (any reference frequency can be used), comprehensive (includes software for both for micro and PC) and is inexpensive. See the Micro Projects page for purchasing details.

A completely new PC program, REFMON4, has been developed for this project. While the telemetry is compatible with the older software (RECORD2F), the new software has a number of improved features.



Oscillator performance monitoring with REFMON4
Click on image to view full size.

Note the values for offset and variance calculated by the program from this week-long long recording. The oscillator phase (green trace) has not varied by more than 1us (1e-10 or 1 part in 1010) in that time. - That's equivalent to less than 1 milliHz movement at 10MHz in over a week! This recording was made with a 4MHz NDK OCXO, adapted for voltage control, output divided by two and then multiplied by five to operate the micro at 10MHz. (Here's another example using the HP 10811A oscillator). on the The display therefore also demonstrates the very low thermal phase shift in the oscillator and multiplier. You would expect to see a daily variation in the EFC (feedback voltage), and there is no obvious daily effect. Over one week you also see very little effect of oscillator ageing, which would cause a linear trend in the EFC voltage.

The vertical scale represents 0 - 25ms with 100ns resolution. The purple trace is the EFC feedback voltage. The pink trace is the p-p variance accumulated from the start of the recording, while the pale blue trace is the accumulated offset from the start of the recording (impressive - 6.89 parts in 1013!). These two graphs are plotted with a logarithmic scale - read the vertical axis as powers of 10.

Under the main graph (just below the zero line) is a GPS status line. This is normally white, but will show blue if the GPS signal is missing, red if the GPS fix is lost, and green if either the GPS telemetry is lost or misunderstood by the PC. On the slow PC used for this recording, this latter effect clearly happens at least once in most groups of 1200 samples.

The display of all graphical elements is updated every second, even at very slow chart speeds. All events recorded are overdetermined (new pixels are plotted every second, even though the graph horizontal speed is one pixel per 20 minutes) and so in this example there are 1200 samples at each location - any noise on the oscillator will show up very quickly. The main graph is scrolled to the left when the right margin is reached.


The sub-graph and repeater boxes

Under the main graph is a small sub-graph which operates at higher horizontal speed (one sample per second). The vertical axis is the same for phase, but has full resolution (1mV) for EFC feedback. The vertical axis is limited in size, so the data wraps around. Under the graph is a single dot which moves along to indicate the current recording point. The horizontal range is about four minutes. This graph is not scrolled - the data simply restarts plotting at the beginning, and overwrites the previous data. This display is very useful for initial oscillator setup, and you can also quickly spot any tendency for the oscillator to move when you open the door and walk into the room! The EFC trace is fascinating to watch, as the integrating error feedback is accumulated.

Finally, under the sub-graph are the two 'LCD repeater' displays. The left small box shows a repeat of the VNGBOX display. It shows local clock time (time code time) at the top left and oscillator phase top right (in hex). The bottom line shows the operating mode, flags and integration time and EFC feedback (in hex).

To its right is the repeat of the GPS Display. It is not copied from the GPS Display display (if you see what I mean), but is reconstructed from the GPS telemetry. This means that you don't need the GPS Display micro for this to work. The upper line shows the current GPS time (tends to be slightly delayed behind the VNGBOX display). On the lower line of green text the date, latitude, longitude and fix quality (DOP, satellites in fix / satellites in view) are displayed in rotation. If no GPS telemetry is multiplexed with the VNGBOX data, this box does not show.

In summary, the new features of REFMON4 include:

When using REFMON4, every valid telemetry sample is used (i.e. every second), even at very low plot speeds. This means that the graph is overdetermined, and any noise will show as a thickening of the plotted phase line.

 VNGBox Clock Core  VNG Time Code  Oscillator Control
 Telemetry & Commands  Phase Comparator  GPS Monitor/Display
 Processor Schematic  RS232 Schematic  Oscillator Schematic
Copyright � Murray Greenman 1997-2009. All rights reserved. Contact the author before using any of this material.