Simple GPS Stabilised

10 MHz Oscillator


constructed by

James Miller G3RUH

A well kept secret of the Rockwell/Conexant/Navman Jupiter GPS engines is that they have a 10 kHz output, synchronised to GPS time. So, with a few additional components you can phase lock a 10 MHz VCXO to the 10 kHz, and you then have a simple lo-cost frequency standard with remarkable performance.

The GPS engine shown is Navman Jupiter-T, part TU60-D120-041. Alas, in 2005 Navman discontinued manufacture of the Jupiter-T and my stock is exhausted. I can no longer supply the Jupiter-T.

However the standard Jupiter receivers TU30-D140-xxx and TU30-D410-xxx also have the 10 kHz output and can be used OK. It's a similar size, but the connector and pin-out is different. These receivers are often available on eBay. Before purchase, make sure that the firmware is v1.18 onwards. This is important; very early models ca. 1995-1996 e.g. v1.16, have substantial frequency jumps every 30 seconds, and the first production (e.g. v1.03) were not even synchronised to UTC. The firmware version is printed on the label, and is also emitted at start-up in an ASCII plaintext NMEA message.

The interface PCB (part 0082-004) is no longer available.

Here's the schematic (latest version of PCB); click on the diagram for full size.

RS-232 interface and 10 MHz PLL schematic. My design used a DIL packaged C-MAC (now RAKON) VCXO with typical sensitivity 600-1000 Hz/Volt. However, you can substitute your own oscillator, on or off the PCB (see below).

PCB parts List (Part 0082-004)
Ref         Qty    Description                       Farnell     Notes
X1           1     10 MHz VCXO or OCXO, DIL-14 package - or use wires from other
U1           1     SP233ACP-L                        938-6572
U2           1     74HC86                            380-490   or 109-4310, 959-1729
U3,4         2     74HC390                           380-787   or 173-9940
U5           1     74AC14                            147-0853
L1           1     10u Inductor                      117-3864
C1,3         2      10u 6.3v tantalum                970-8200  or 110-0451
C2           1     100u 6.3v tantalum                970-8243  or 110-0455
C4           1      10u 35v  (for 12V operation)     970-8570  or 110-0496, 165-0978
C5-8         4     100n 100v polyester               121-6445  or similar
R1           1     160k 1/8W MF12                    934-2702  or similar
R2,3         2     22k  1/8W MF12                    934-2842  or similar
R4           1     470R 1/8W MF12                    934-3245  or similar
R5-8         4     200R 1/8W MF12                    934-2770  or similar
Reg          1     Voltage Regulator LM7805CT        966-6095  For 12V operation
D1           1     1N4001 diode                      956-4993  For 12V operation
Q1           1     BC547, BC107 etc NPN              109-7287  or 146-7869, 101-7673 etc
Terminals    2     Red    Vero    RS 262-2220        873-1209
            16     Black  Vero    RS 262-2214        873-1195
DIL Socket   1     20-pin DIL socket                 428-5608

Note: Farnell Electronic Components is a world-wide supplier. But none of the parts is unusual, and will be available from your favourite dealer.

Using An External Oscillator

Here is a system using a better quality oscillator.

 IsoTemp System

GPSDO using an Isotemp OCXO 134-10, often available on eBay. The ovenised oscillator has considerably better performance than a simple DIL packaged device, but at the cost of greater power consumption and increased size.

The Isotemp oscillator has an EFC range 0 to +8 Volt and the little circuit mounted on its pins is a simple DC amplifier with gain x1.6 to magnify the phase detector output from 0-5V up to 0-8V. Many people asked for this PCB, not realising that it was hand-made, masked, etched and drilled, one-at-a-time. So I had to have a batch made properly, 2-sided max copper and solder-resist (part 0083-001).

This OCXO has a sensitivity of 2 Hz/Volt (i.e. 3.2 Hz/Volt at the DC-amplifier input). Suitable PLL filter components are R1=160k, C1=100µF, R2,C2 not fitted.

Because the OCXO 10 MHz output is AC coupled, add pull-down 4k7 to 0V and pull-up 4k7 to +5V. These connections are to the OCXO output, and can be most easily mounted on the underside of the (green) PCB. The latest PCB (0082-004) has pads for these resistors.

In response to requests I supplied many of these IsoTemp-based GPSDOs built and tested. But I have no more IsoTemp oscillators. A photo. The manual.

Simple GPSDO Project - Summary of available items

   Component Parts (supplied at cost) including world-wide shipping
   PCB: GPS Interface/PLL        part 0082-004                  0 remaining
   PCB: DC-Amplifier for Isotemp part 0083-001                  0 remaining

These items are available until stock is exhausted. No more PCBs will be made.

Methods of payment

New OCXO-based GPSDO

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Last updated: 2022 Jun 29