GPS Disciplined

10 MHz Oscillator


constructed by

James Miller G3RUH

This frequency standard is based on the article A GPS-Based Frequency Standard by Brooks Shera W5OJM, QST 1998 July, ARRL. Pages 37-44.

Here I describe my implementation and show photos of the equipment.

I've built two systems; my first was based on the inexpensive Toyocom TCO-613 5 MHz ovenised oscillator that appeared on the UK surplus market for typically 20 ($35) in the Pye/Philips HS400 unit. I'd wholeheartedly recommend that approach; for peanuts you got a strong enclosed chassis, mains PSU, an excellent crystal oscillator and plenty of empty space to incorporate the GPS receiver and Shera controller. If you cocoon the oscillator in 20-25mm polystyrene sheet, performance is more than adequate for most needs.

Nevertheless, I wondered just what could be achieved with a top class oscillator such as a rubidium resonance or a Hewlett Packard 10811 crystal unit. I didn't really want to wear out a rubidium standard, since it would be on continuously, so opted for the crystal oscillator.

Research showed that a double-oven version of the HP 10811 was inside the HP Z3801A GPS Time/Frequency standard. This unit was (around 2001) readily available from the advertisements in QST and on eBay for typically $250. Z3801A details.

A unit was acquired, the HP oscillator and associated PSU board removed and the remainder discarded. The PSU board takes in 54V DC (it's from telephone equipment) and generates +/-15V for the inner oven and oscillator with plenty to spare for the Shera controller and there's lots of 5V for the GPS engine. Also on-board is the outer oven control system. Perfect!

GPS Engine
I've been using the Rockwell 12-parallel channel Jupiter GPS engines for some years now, and opted to use the -T version which is optimised for static users in time/frequency applications. Rockwell Semiconductor Systems became Conexant Systems Inc. in 1999, and the GPS marketing interests are presently (2002) owned by Navman. The Jupiter-T is pin and software compatible with the Motorola UT+ engine.

Jupiter engines also have a precise 10 kHz output synchronised to GPS time which can form the basis of a really simple 10 MHz standard.

Controller and PIC
The controller board was obtained from A&A Engineering (PCB #217), whilst PIC and 18-bit DAC were supplied by Brooks Shera (d. 2013 Mar 16, Santa Fe, USA) whose support of this project was outstanding.

Manipulating the DIP switches inside the enclosure was found to be a tiresome business on my first controller, so for the present system the filter "N" settings were brought to the front panel via a push-button 'thumbwheel' switch.

On my first system only the basic frequency was output; 10 MHz to the front panel. Almost immediately I realised that sub-frequencies would considerably enhance the system's utility so for the present system a PCB containing several 74HC390 decade dividers was built. 10 MHz, 5 MHz and 1 MHz have dedicated BNC sockets on the front panel, but 1 Hz to 100 kHz are selected singl via a 6-pole thumbwheel switch.

[2020 Jul 19: I have qty 5 spare PCBs - please email].
Another option would be this PIC-based divider from Tom van Baak.

I wanted a compact system, as space is at a premium here, and selected a simple, neat aluminium cabinet that measures 260W x 90H x 250D (10.25" x 3.5" x 10"). It's OKW Enclosures Ltd Unicase 2, part M5502110, available in the UK from RS Components 222-115 [this is not a RadioShack number] and in the USA from Allied Electronics P/N 272-0119. There's just sufficient headroom for the oscillator and its mounting standoffs; about 0.5 mm clearance. All PSU components and the oscillator are mounted on the base; GPS engine, controller and divider PCBs are mounted on an internal shelf. See photos.

The double-ovened HP 10811 is very stable with respect to environmental temperature, and a filter setting of 5 is normal. The DAC control voltage shows little change from day to day; a millivolt or two. Without absolute test equipment such as Caesium frequency standard it's difficult, or at least, very time consuming, to characterise the fine structure of phase and frequency of a standard such as this. Work continues, [Update 2008 - I now have the test equipment ...] but suffice to say that with frequency accuracy and stability useful to 10 GHz or more, this system is outstanding. Thanks again, Brooks.

Beware! Constructing this HP 10811 based unit to such a high standard came close to $1000; my first system, more sensibly engineered, cost $250 - about the same as a Z3801A. Yes, I know what you're thinking; please don't ask ...

I like to document my projects properly; saves much trouble later. Manual in PDF format, 63kb.

Here are some pictures of the system taken before the front panel lettering was applied. Click on the images for high resolution 1600x1200 versions, typically 300 kb.

Front view. From left to right; status LEDs; Filter "N" selection; DAC output voltage; 10 MHz, 5 MHz and 1 MHz outputs; decade selector for 1 Hz-100 kHz BNC output (R).

Interior view showing shelf with PCBs.

PCB shelf and front panel. Navman Jupiter-T GPS engine is in the foreground, and the decade divider PCB is adjacent. Both are slightly smaller than a credit card. The large PCB is the Brooks Shera controller. The two SMB connectors are 10 MHz_in and EFC_out and go to tails on the HP 10811 oscillator unit.

Shelf removed, revealing power supply components. From rear left: HP 10811 double-oven oscillator; fused IEC mains connector; DE-9S connectors for GPS and Controller ASCII; GPS antenna MCX input socket; 36Vac/50VA mains transformer; 54V DC supply 4700µf 64V capacitor and bridge rectifier attached; LM7812 voltage regulator for oscillator 12V supply and 6 pin connector; LM7812/7912 regulators for inner oven supply; PSU board 54V DC to +/-15V, 5V DC with outer oven controller in rear left corner. Blue capacitor is 2200µf additional filtering for +15V and -15V rails (see documentation).

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Last updated: 2020 Jul 19