Probably the Simplest GPS
Disciplined Oscillator possible !
Using the GPS Satellite system
offers the advantage of very accurate timing and by extension, frequency
control. The long term error is to all
intents and purposes zero, with time and frequency accuracy being comparable to
the international standard. The
traditional route is to use a relatively low cost GPS receiver module which
outputs a 1 Pulse per second signal (1 PPS) aligned to UTC. Basic GPS modules such as the Garmin GPS25
and Motorola Oncore have been around for
some years and are available at low cost.
It is possible to phase lock a divided down crystal oscillator to this
1 PPS signal and transfer its long term stability to , say, a 10MHz
reference which is subsequently used for deriving any LO and beacon
frequencies. The subsequent PLL system
is usually described as a GPS Disciplined Oscillator rather than locked, since
it is not, strictly speaking, actually ‘locked’ to the GPS system at all; just
controlled by it via the 1 PPS generated by software in the receiver module.
The first GPSDO to appear in the
amateur press was by Brooks Sheera
W5OJM, described in QST July 1998. A
lower stability simpler version, suitable only for low data rate signalling on
the LF bands, was published by myself in
Radio Communication October 2002. Both
of these, in different ways,
demonstrates the problem with using the 1 PPS signal. On all these receiver modules the 1 PPS
signal can have up to 1us variation from pulse to pulse, and this varies
randomly. Later modules reduce to a few
100ns, but it is still there.
Consequently, for a frequency
standard with a short term stability measured over a few tens of seconds, this
1 PPS jitter has to be averaged out over many hundreds or thousands of seconds
- so giving very long lock up times and loop tracking constant. Now, as the PLL has a time constant of many
tens of minutes or hours, the voltage controlled crystal oscillator has to be
stable over this loop time constant - particularly if it is to be multiplied up
to GHz where a short term wander of a few Hz
(parts in 10^-10) is noticeable.
So a good quality oscillator has to be used here - typically an ovenned
high spec standard in its own right.
This was the approach taken by W5OJM with a microcontroller based
digital PLL and loop time constant of hours.
My design went the other way, and accepted a poor short term stability
for LF use only, where the phase wander over a few seconds was inherently
averaged by the LF signalling interval.
Many manufacturers now offer off-the-shelf GPSDO modules with varying
specifications between these , ranging in price from a few hundred pounds, to
The Jupiter-T Solution
Which brings me onto a new GPS
module that makes a homebrew solution very much easier. The Jupiter-T module made by Navman
(originally Conexant) has an output at 10kHz ‘locked’ to GPS time. Initially I was sceptical thinking it
probably only consisted of 10000 pulses per second - which could have been no
better than the 1 PPS signal itself in the short term. However, after making extensive
measurements, came to the conclusion it really was quite a respectable signal -
in particular I could not detect any discrete sidebands at sub Hz
frequencies. This suggested a simple
GPSDO solution. By taking a simple low
cost 5MHz voltage controlled TCXO (VTCXO) module and dividing down to 10kHz, this can be phase
locked to the output from the Jupiter in an analogue PLL with a time constant
of a few tens of seconds.
The circuit diagram shows how
simple this can be. Obviously, without
the ability to be able to tell if the GPS receiver has locked up by reading the
NMEA or binary data it sends from its communication port, there is no way of
knowing if the system is functioning properly.
The Jupiter module does output its1 PPS signal and a nominal10kHz
immediately after switch on, but the timing of these is way off and the initial
case of no GPS lock can be inferred from the large frequency error. In fact this is so large that the PLL is out
of lock range and the resulting frequency is sweeping so wildly that it is
obvious. When the module does lock up
to GPS after a few minutes, the frequency and phase of the 1 PPS jump immediately
and abruptly to their correct values, with the PLL taking a short time after
this to stabilise. Although not shown
in the diagram, an LED connected to the phase pulse output of the 4046 chip
will slowly change brightness over a few seconds during the GPS lockup, and
then much more slowly as the PLL locks, eventually settling to a stable
The R/C values forming the PLL
filter are optimised to my particular VTCXO which has a sensitivity of 125 Hz/V
at 5MHz, and a required tuning voltage in the
0 to 1.5 Volt region. As the
bandwidth and tracking performance of the PLL depends on this filter, it is
worth spending a bit of time optimising the values in this area.
Figure 1 -
Circuit diagram of the prototype GPS Disciplined Oscillator
Some Test Results
The 200th harmonic of
the 5MHz output, at 1GHz, was monitored
on a UHF communications receiver in CW mode and the output at 1kHz monitored
with Argo to show short to medium term frequency shifts. All the local oscillators in the receiver were
locked to a Rubidium frequency standard that has been calibrated to an accuracy
of a few parts in 10-11. At
1GHz, a frequency shift of 1Hz corresponds to 10-9 frequency error.
Figure 2 shows the plot after the
system has locked up and been running for about 30 minutes. It can be seen that the frequency is
maintained usually within a couple of Hz, and randomly wanders over a mean
period of something like 20 - 30 seconds - this being a function of the PLL
bandwidth. The breadboard which
produced these results was lying open on the bench, and susceptible to
perturbations when I touched it - it is quite possible this trace would be
cleaner still if the unit was packaged in a screened metal box.
Figure 3 shows the effect of
disconnecting and then reconnecting the GPS antenna. Presumably, the quite fast variation during
the period of no GPS signal is the GPS receiver going through its search
routine to find the satellites. The
faint line that remains fixed at 1kHz exactly is caused by leakage from the
Rubidium source controlling the communications Rx.
Figure 2 Frequency tracking performance when locked,
VCXO output multiplied to 1GHz
Figure 3 Frequency tracking with loss and
re-acquisition of GPS signal - multiplied to 1GHz.
This design is based around one
specific GPS receiver - others with a 10kHz output may be available, but I
don’t know of any. This is the Jupiter
T GPS Timing Board. In the UK it is
available from TDC Ltd in Basingstoke, www.tdc.co.uk
, tel +44 (0) 1256 332800. Their Stock number is TU6-D120-041 The cost is
around £86 which is significantly higher than a navigation only receiver
module, but worth it for the frequency tracking capability.
The board is available with a
variety of GPS antenna connectors, but they are all fixed by a standard 5 hole
fitting and can easily be replaced by SMA, SMC etc.
Full data sheets are available
from the TDC website.