Pye/Philips L700 Fixed Link

Overview

The Pye/Philips L700 Fixed Link equipment was developed between 1980 and 1984 in Cambridge. It was an analogue link that could carry 8, 12/24, 60/72 or 120/132 speech circuits. It was typically supplied with a base-band multiplexer (such as that from Granger Associates) to combine and break out the speech circuits.

The L700 was made in a rack-mounted and slimline equipment practice (SLEP) formats. The rack mount version is shown below.

Pye/Philips L700 Fixed Link

The L700’s principal role was to carry mobile-service traffic from controllers to distant fixed 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 emergency services or fuel and power sector use.

The Pye/Philips L700 Fixed Link was designed and manufactured to operate in four frequency bands:

390-470MHz
890-960MHz
1427-1535MHz
1650-1900MHz
1900-2310MHz

The 1800/2300MHz band was a split pair of sub-bands allocated to the UK’s emergency services.

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.

Front Ends

Pye/Philips L700 Fixed Link front-end

Image Filter, Rat-race Mixer and Thick Film IF Amps

Pye/Philips L700 Fixed Link rate-race mixed the IF pre-amps

Complete Down Converters

Pye/Philips L700 Fixed Link down-converter

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.

Pye/Philips L700 Fixed Link front-end, image filter and IF pre-amps circuit
Pye/Philips L700 Fixed Link front-end, image filter and IF pre-amps layout

Interdigital RX and TX Filters

Varactor Multipliers

The 390-470MHz transmitter signal was multiplied to the required frequency with a varactor.

The same design was used 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.

Pye/Philips L700 Fixed Link varactor multiplier circuit
Pye/Philips L700 Fixed Link varactor multiplier

Firsts

The designs had novelty and several firsts.

  1. The use of Teflon as a printed circuit board substrate was in its infancy.
  2. Through-hole plating in Teflon was particularly difficult – again in its infancy.
  3. 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).
  4. Foundations for Microstrip Circuit Design by TC Edwards was my bible.
  5. The Teflon boards were stuck down with 3M (non-conductive) double sided tape! Made the assemblies easy to manufacture!
  6. 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.

The L700 in its SLEP format was used in several major UK-wide networks such as the RN1 civil defence speech and telegraph network.

The Pye/Philips L700 Fixed Link continued in production until the early 1990s. 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. The spectrum used for links by the emergency services and fuel and power was re-farmed to make way for 3G mobile networks.

I would estimate that around 1,000 L700 terminals were made. The receiver downconverter, interdigital filters and varactor multiplier have also been used in several 23cm amateur repeaters.

Request

I’ve tried to get hold of old production equipment. So if you have anything… do get in touch.

Many thanks to Colin Stuart, MM1APS, for the handbook.