My References


I am fortunate to have some very nice references and frequency measurement equipment, some commercial, some homebrew. These enable me to check, compare, calibrate and adjust my own equipment and that of friends. Some of this equipment has been acquired recently, while other items have been lovingly collected and restored over the years.

Primary References

Hewlett-Packard Z3801A Stratum 2 GPS Receiver
This well-known unit was built into a rack enclosure by the late Ted Minchin ZL1MT, and includes power supply, 50V backup supply (for 24 hour duration power-failure holdover performance), and a ZL1BPU Superclock, which runs off the reference. A divider system and distribution amplifiers serve 10MHz, 5MHz, 1MHz, 100kHz sine waves at 1V RMS into 50Ω, as well as 1pps pulses. The distribution amplifiers include individual crystal filters for each frequency in order to provide superbly clean signals.

I use Symmetricon's Satstat software for general monitoring, and have written my own specialized interrogation software named EFC2 for displaying status and time, logging and plotting EFC and 1pps values on an old DOS laptop PC. Z38XX by Ulrich Bangert also works well with the Z3801A, as does Con Wassilieff ZL2AFP's excellent Nixie Clock.

Agilent/Symmetricom GPSR-A Stratum 2 GPS Receiver
Originally designed by HP, and made under licence by Chongho in Korea, the Z3815A used the famous (or infamous) HP E1938A oscillator. There were manufacturing problems with this oscillator, and later examples of the Agilent Z3815A like mine were reworked with an add-on board to replace the E1938A with a 5MHz Milliren MTI-260 OCXO. This unit was manufactured about 1998 - 2000 and was used in cellular base stations. It operates from 20 - 50V at about 20W. I use it with software especially written by Con Wassilieff ZL2AFP, although it also operates happily with Satstat, GPSCon and the Nixie Clock.

Samsung GCRU-D Stratum 2 GPS Receiver
This unit was built in 2004, and was designed for use in Samsung's CDMA cellular base stations. The performance is excellent, and uses a C-MAC STP2145A OCXO with an amazing low ageing rate. Little is known about the rear panel connections, but the main 10MHz outputs are buffered sine wave. There are other outputs at 29.4912MHz and 4.096MHz. The unit operates from 20 to 50V at about 15W.

Trimble NTGS50AA Stratum 2 GPS Receiver
The NTGS50AA is a low power (5W) unit intended for Nortel Metrocell cellular base stations. While physically large, it is a single board design with integrated GPS receiver, with plenty of room inside for home-brew accessories. It has front panel outputs at 10MHz and 1pps/2. It operates from 50V, and I have built it a regulated linear supply, and have an APC RS-500 UPS which supplies back-up power.

Homebrew "VNG-in-a-Box"
An unusual GPS Disciplined Reference which also replicates the much-loved VNG Australia radio time code as an audio signal. It uses a Navman Jupiter-T GPS timing engine as reference, and uses a single-chip micro (advanced version of the GPSClock) to lock an HP 10811A 10MHz oscillator. The audio tones are generated using Direct Digital Synthesis. A second micro monitors the GPS module and asserts a hold command to force the VNGBOX into holdover when the GPS fix is poor or missing. This second micro also generates a Morse ID which can be inserted into the time code. See the VNGBOX page for more information.

Homebrew GPS Disciplined Reference and Clock
This largely experimental unit is centred around an excellent NDK oven oscillator operating at 4MHz. I modified the oven in a very simple manner to provide electrical control over 0.1ppm. Although a single oven oscillator, the aluminium case has good thermal mass, and when well lagged, the performance is virtually free of ambient variations. The oscillator drives a divider to 2MHz, which then operates a parametric crystal multiplier of my own design. This multiplier is like a 10MHz crystal oscillator phase modulated (and therefore locked to) the 2MHz reference signal. The multiplier exhibits exhibits low thermal phase variation on its 10MHz reference output. The 10MHz reference drives the micro which operates the GPS-locked phase control loop, based on a design similar to the GPSClock. A DDS tone generator replaced the display, and was used as a test bed for the VNGBOX time code.

Homebrew GPS Disciplined Clock
Built to the GPSClock design, this is in fact the prototype unit. The reference oscillator is a Rakon 10MHz V-TCXO, which achieves about 1ppb (1 part in 109) accuracy, although the short-term thermal performance isn't stunning. The clock is disciplined to 1pps from a Marconi CMC Superstar GPS module, and has a NiCd backup battery pack with 24 hour hold capability (the display back-lighting goes out when power goes off). The design will hold current settings when GPS is lost. The clock can be synchronised to within 500µs of GPS 1pps and keeps time easily to within 1ms/year.

Secondary References

Hewlett Packard 5065A Rubidium Standard

A very nice, if rather old, Rubidium standard. There seems to be plenty of life left in the tube, which was installed in 1978, and it is running continuously and provides a very good low noise 5MHz reference, which I use to lock HF receivers. Drift is very low, and the unit exhibits very low phase noise.

This particular unit has an interesting history. It was purchased in the early '70s to provide the system timing reference for TVNZ's television network, and was pensioned off when it failed (probably out of spite!) after replacement of studio equipment with new digital studios and matching Trimble Thunderbolt controlled Neve Sync Pulse Generators. I affected the necessary repairs and recalibrated the unit using GPS - it took more than a week to achieve calibration to 1e-12.

Frequency Electronics Inc FE-5650A option 48

This little unit (smaller than your fist!) is a complete Rubidium Standard and DDS Synthesized Signal Generator. The generator operates from the ~50MHz precision signal generated by the physics package, and can be programmed from 1Hz to 20MHz in ~0.01Hz steps. I use the unit for the following applications:

  • As a precise versatile signal generator
  • As an exciter for HF precision transmissions (used for propagation measurement)
  • As an exciter for 80m transmissions for frequency measurement tests.
  • As a super-stable LF exciter, computer controlled and ASK/FSK keyed.
See the FE-5650A Data Sheet for more information. Option -48 has a different output connector and requires separate 5V and 15V power supplies. Some modification is required to access the variable frequency output.

In its original telecoms application the FE-5650A option 48 was used to provide a 1pps signal. When access to the internal DDS control is achieved, the nominal 8.8388608MHz signal driving the 1pps divider provides the variable frequency output, and the 1pps output should be ignored.

I have built my unit into an old instrument case, along with power supplies and a broadband buffer amplifier followed by a low pass filter. This provides a reasonably flat sinewave output from 5kHz to 20MHz. I wrote my own control software. See the FE-5650A page for more intormation.

OCXO References

I have several oven-controlled references for general purpose use. These are calibrated and can be loaned out when required. Most operate at 5MHz, using recycled Transit satellite navigation references. These oscillators have very good short term performance and (now that they are old) have impressively low ageing rate. One has been GPS locked by a micro, using a thermal control technique (pulse-width modulation of the oven heater!) which gives barely enough control range, but results in an oscillator with very low phase noise that will stay in GPS lock for several weeks without calibration. The other two are free-running, and have dividers for general purpose outputs. One is a portable unit which is often loaned to friends in need of a reference better than 1 part in 10-8.


I have two old frequency meters - a BC221 and an LM-10. Both are in working order, with calibration books. When you consider their age, at the time they provided impressive performance , but are now well out-classed. The calibration books for these units were calculated and printed by a very early electro-mechanical computer which was first developed to generate trajectory tables for artillery.

Frequency Measuring Equipment

  • HP 5315A ratiometric frequency, period and time-interval counter, 8 digit, 1200MHz, with 1ppm TCXO reference.
  • Philips PM6668 portable ratiometric auto-ranging counter, 7 digit, 1000MHz, with 1ppm TCXO reference.
  • DSE kit-constructed frequency/period counter, adapted for 1200MHz operation, direct counting to 80MHz. Eight digit with over-spill. External 1MHz reference, 100 sec gate time maximum. Used for reference calibration.
  • Home designed 1200MHz direct reading frequency counter, software calibrated, 9 digit with over-spill. Has telemetry for logging purposes.
  • Racal RA6790/GM communications receiver, locked to primary standard, 1Hz steps 20kHz to 30MHz, used to measure off-air frequency in conjunction with a PC spectrogram display. Resolution 25mHz.
  • 'Clicklock' software phase plotter by Con ZL2AFP. Can perform long period integrating phase and amplitude plots on very weak signals up to about 1MHz. Receiver oscillator and PC sound card errors are included in a GPS 1pps locked loop. This technique can resolve to 1 part in 108 in just over one minute, 1 part in 1011 in about one day. With this software and the above receiver I can easily resolve the hourly 45 phase steps on the WWVB (60kHz) carrier.
  • Tektronix 1421 TV Vectorscope. Use for phase comparison at 5MHz.
  • Homebrew micro phase comparator. Operates at 5MHz and indicates phase with 16 bit resolution (20ns). Plots phase on PC in real time using homebrew software. Compares phase to GPS 1pps or TV 50Hz frame sync.

Copyright M. Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.
home HOME