Last Updated on May 10, 2026 by John Berry
Overview
The Pye Philips L700 Fixed Link equipment was developed between 1979 and 1984 in Philips Radio Communications Systems Ltd, Cambridge, UK. It was an analogue link using frequency modulation that could carry 8, 12/24, 60/72 or 120/132 speech channels according to options selected. It was typically sold along with a base-band frequency division multiplexer (such as that from Granger Associates) to drop and insert speech channels at sites across a network.
The L700 was made in a 19″ rack-mounted and slimline equipment practice (SLEP) formats. The rack mount version is shown below.
The L700’s principal role was to carry traffic in the mobile service from controllers to distant base stations. The higher capacity variants were used to carry inter-exchange (PABX) telephony. Many hill-top mounted L700s were installed in complex region-wide mobile radio systems for the emergency services or fuel and power sector use worldwide.
The Pye Philips L700 Fixed Link was designed and manufactured to operate in five frequency bands:
390-470MHz
890-960MHz
1427-1535MHz
1650-1900MHz
1900-2310MHz
An 1800/2300MHz split pair of sub-bands was allocated to the UK’s emergency services and a cross-band variant done for that. The image below shows a typical installation.
RF Design
I designed all the 890-2310MHz RFs.
Here are various images of my lab designs and the subsequent service manual diagrams. I designed one version of each per band over a period of about two years from 1981-1983.
There’s a picture of production units at the foot of this page.
Front Ends
Image Filter, Rat-race Mixer and Thick Film IF Amps
Complete Down Converters
And here’s the corresponding circuit and layout for the front end, image filter, mixer and IF pre-amp. The IF frequency was 70MHz in all cases.
Interdigital RX and TX Filters
The branching filters for the lower bands were copies of those from the L300 predecessor. I designed filters for the 1800MHz and 2300MHz bands.
Varactor Multipliers
The 390-470MHz transmitter signal was multiplied to the required frequency with a varactor. The varactor was screwed down into the heatsink with its cap soldered to the surface track (bottom left).
I used the same design to obtain a local oscillator signal for the down-converter – though for the TX multiplier I replaced the output filter (seen left) with a directional coupler (below).
And here’s the corresponding circuit and layout for the TX multiplier that provided RF power output on the required band.
Firsts
The designs had novelty and several firsts.
- The use of Teflon as a printed circuit board substrate was in its infancy.
- Through-hole plating in Teflon was particularly difficult – again in its infancy – testing the skills of our guy in the PCB prototyping shop..
- Computer-aided S-parameter RF design using the Touchstone program was in its infancy. I used S-parameter design throughout (from a ‘special’ room in Advanced Development with a single dial-up terminal accessing Touchstone and SPICE running on distant computers somewhere in the Philips world).
- Foundations for Microstrip Circuit Design by TC Edwards (1st ed.) was my bible. This book is now in its 4th edition.
- The Teflon boards were stuck down with 3M (non-conductive) double sided tape! Made the assemblies easy to manufacture!
- Hence from 5, the boards used capacitive coupling distributed across the PCB to couple the ground planes.
Pye Philips L700 Fixed Link Production and Installation
The 390-470MHz version of the Pye Philips L700 Fixed Link went into production in the early 80s. The other bands followed in 1983 onwards.
It was developed and made by Philips RCS and branded Pye in the UK. Pye Telecom was the Philips mobile communications sales outlet there. Philips had acquired Pye in the late 1960s. It was sold branded as Philips worldwide.
The L700 in its SLEP format was used in several major UK-wide networks such as the RN1 civil defence speech and telegraph network.
In 1985 I returned to the lab to lead the final development and factory introduction of last variants of the L700, and of the FR/FX5000 base station and Fl5000 narrow band link. In 1988 I took over product management of all systems products, including links.
I would estimate that around 3,000 L700 terminals were made. The receiver downconverter, interdigital filters and varactor multiplier have also been used in several 23cm amateur repeaters.
And finally, a pic of production down converter, IF and transmitter.
The Pye Philips L700 Fixed Link continued in production until the mid 90s. Pressure on the radio spectrum for mobile services meant that in the late 90s such links were allocated to higher bands such as 7GHz, 13GHz and 22GHz. In any case, analogue had by then made way for digital. The spectrum used for links by the emergency services and fuel and power was re-farmed to make way for 3G mobile networks.
Thanks
Many thanks to:
Howard Davis, G6LXK, who gave me a 1900MHz down converter and a transmitter, ex-Scottish Police acquired at a ham radio rally. They are from the Tayside region.
Colin Stuart, MM1APS, for the handbook, and who gave me an IF module comprising IF amp, IF filtering, demodulator and base-band amplifier (designed before my time).