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.
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.
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