Introduction

The West German SEM-35 VHF FM manpack radio is available from various UK and EU dealers. This note gives a brief description of the set, its historical relevance, circuit and performance. The SEM-35 performance and characteristics are given in Table 1.

Table 1: SEM-35 Specifications

This was by force of circumstance because, especially in the early 1950’s, only the US had the finance resources and volume requirements to undertake and justify the work.

The ‘landmark’ sets that resulted were the RCA PRC-25 VHF FM backpack radio and the AVCO VRC-12 VHF FM vehicle set. Both sets covered 30 to 76 MHz and both employed transistors. The PRC-25 retained one valve in its transmitter RF output. The fully transistorised PRC-25 was later issued as the PRC-77.

It was the arrival of the transistor that allowed the complexities of the synthesiser to be engineered for field use with acceptable physical size, performance and power supply requirements.

The US evolved a circuit configuration for military, synthesised VHF FM transmitter receivers that was almost as significant, in its field, as the development of the superhet. It consisted of a two band receiver, synthesised in 50kHz steps buy a single band local oscillator. On the receiver low band (30 to 52.9MHz) the received frequency was local oscillator frequency minus intermediate frequency (IF). On the high band (53 to 75.95MHz) local oscillator plus IF frequency. The receiver first IF was 11.5MHz and each receiver band was twice IF frequency wide, i.e. 2 x 11.5 =23 MHz.

The use of a single band local oscillator simplified band switching, reduced component count and provided two working RF frequencies for each synthesised local oscillator frequency.

The transmitter frequency of the US radios was locked to the synthesiser frequency by the use of a crystal controlled 11.5MHz ‘side step’ oscillator that beat with the synthesiser output frequency to generate the transmitter frequency. This synthesiser configuration became the basis for subsequent VHF FM military radio designs.

Historical

Military radios do not just pop into existence. Most result from a steady, progressive improvement to existing designs. Some, however, are landmark designs that made a major step forward. These resulted from a comprehensive, innovative, well financed and thought out development that appraised all aspects of a radio including the man-to-set interface. In the case of VHF FM synthesised military radios the ground breaking design work was conducted in the US in the 1950’s and early 1960’s.

 It was not a prima ballerina requiring excessive workshop attention and, in an emergency, anyone could pick it up and use it without previous training. The PRC-25 was introduced to US service in small quantity in 1961 but it was nearly 4 years later before it entered volume production (Ref.1).

SEM 35 Development

West Germany had used the US designed PRC-10 series radios. By the early 1960’s they had a requirement for synthesised sets to update army

VHF FM field communications. Their home electronics industry was by then re-established after the war. They decided to develop the synthesised sets themselves as a first foray in the field.

It would use an 11.5MHz discriminator before the receiver IF filter to regulate the transmitter oscillator at 11.5MHz from the receiver synthesiser frequency. This would replace the PRC-25 side step crystal oscillator. It would be less accurate than the PRC-25 arrangement since it would rely on the tuning of a discriminator rather than an 11.5MHz crystal. However, the SEM-35 circuit achieved an acceptable transmitter frequency accuracy of better than + 3.5kHz from –40 to +60^oC.

The German contract was awarded to Standard Elektrik Lorrenz AG of Stuttgart. The end result was the SEM-35 manpack, the SEM-25 vehicle transceiver and the EM-25 vehicle receiver. The EM-25 receiver is basically the SEM-25 hardware in a SEM-25 case with the internal transmitter sections omitted. The EM-25 is, therefore, externally similar to, and easily confused with, the SEM-25. The sets entered service in the late 1960's.

The SEM-35 was primarily intended as a manpack but a vehicle mounting and a rebroadcast/relay adapter were produced for it and it could use the SEM-25 remotely tuned aerial base. These allowed the SEM-35 to fulfil vehicle, ground static and re-broadcast/relay roles.

All three sets share two common large-circuit modules with obvious advantages in spares holding and maintenance.

However, the requirement that one programme develop both a manpack and a vehicle set produced conflicting goals. As a result, the SEM-35 has features and components more appropriate to a vehicle radio. In this respect the most obvious vehicle type component is a large multi-gang permeability tuner reminiscent of the techniques employed in the auto radios of the day.

SEM-35 Detail

The set provides a total of 880 channels spaced by 50kHz. These are covered in two frequency bands. A high band from 47 to 69.95 MHz and a low band from 26 to 46.95 MHz. The SEM-35 has the same 11.5MHz receiver first VHF as the PRC-25 and each band could be 23MHz wide permitting a low band of 24 to 46.95MHz. The SEM-35 receiver can operate down to 24 MHz but the transmitter is automatically disabled below 26 MHz. The tuning display is a mechanical counter dial giving direct read out of frequency. the tuning arrangement does not permit two channels to be pre-set as on the PRC-25.

The transmitter output power can be switched for 150mQ or 1 Watt RF output. The receiver sensitivity is better than or equal to 0.5 microvolts for 10db signal-to-noise. Squelch is pre-set and selected by switch but it is not the US 150Hz tone squelch.

A battery box toggle clamps below the set and contains a holder for 12 off U2 cells, either normal BA 30 or rechargeable Ni.Cd. cells. On a 9:1 Rx:Tx ratio the BA 30 cells give a 14 hour life while the NiCd cells give 20 hours per charge. Battery exhaustion, i.e. the minimum battery voltage for operation is quoted as 13.2V. The battery box also contains a transistor inverter producing supplies of +6V, +1 6V, -17V and –30V DC from the internal battery or from a 24 Volt external source.

The set uses a 1m laminated tape aerial and a 2.5m self-erecting whip like the PRC-25 but unlike the US PRC ancillaries each whip has a tapped cup at its base that screws over a stud type antenna mount on the set front panel. The set stud base has a sprung piston at its centre which, when depressed, operates switches to select the appropriate antenna matching circuits.

With no whip fitted the set RF output is switched to the 50 ohm BNC socket for connection to the remote antenna base. The cup on the bottom of the 1m tape antenna fully depresses the plunger while the cup on the bottom of the 2.5m whip only depresses the plunger by about 50% of its travel.

A similar antenna base is used on the West German SE-6861 SSB manpack radio.

The set employs transistors throughout but because of its design age the transistors are a mix of PNP, NPN, germanium and silicon types.

Circuit Description

The first mixer is a diode ring of 4 x OA90 germanium diodes with 50 ohm port, mounted on a small, unscreened printed circuit board (PCB). The receiver first IF is 11.5 MHz with a 30 kHz bandwidth crystal filter. The first receiver local oscillator covers 35.50 to 58.45 MHz in one band.

The squelch circuit has a relay output.

The receiver first local oscillator is frequency stabilised by the synthesiser. A signal from the receiver local oscillator goes via a buffer amplifier to the first mixer of the synthesiser. The local oscillator signal for this mixer is generated from a 1 MHz crystal oscillator. The 1 MHz oscillator output is clipped and shaped to make it rich in harmonics and then passed through a LPF to the mixer. The LPF lets through the 1 MHz harmonics from 1 to 11 MHz but attenuates the higher harmonics to ensure that they do not appear in the receiver tuning range.

The synthesiser first IF is a bandpass amplifier from 46.50 to 47.45 MHz. Only one of the 1 MHz harmonics can beat with the receiver local oscillator to produce a signal in the first synthesiser IF. As with the receiver local oscillator, the 1 to 11 MHz (0 to 11 MHz) beats with the receiver local oscillator on a sum and difference basis to cover the full 22.95 (23) MHz single band tuning.

The local oscillator for the second synthesiser mixer is an overtone of crystal oscillator with one of two frequencies spaced by 0.0500MHz, i.e. 33.425 or 33.925 MHz. The overtone oscillator frequency is remotely selected from switches on the 50kHz tuning selector via decoupled control lines that enable the appropriate oscillator output. The two frequencies are used for alternate bands of 0.500MHz, e.g. one for receiver tuning from 26.00 to 26.45 MHz, the other from 26.5 to 26.95 MHz, then back to the first from 27.00 to 27.45 and so on. Both overtone frequencies are midway between receiver channels and are attenuated by the receiver IF crystal filter should they leak into the receiver RF input circuits.

The second synthesiser IF is a bandpass amplifier from 13.075 to 13.525 MHz. The local oscillator for the third synthesiser mixer is controlled by one of 10 crystals spaced by 50kHz from 14.525 to 14.575 MHz. These are ganged to, and selected by, the 50 kHz channel selector switch. In combination with the switched overtone oscillator for the second synthesiser mixer they provide the 20 off 50 kHz channels per MHz, i.e. the first 10 channels on one overtone oscillator then a further 10 on the second overtone oscillator.

The IF output of the third mixer is 1.45 MHz. This feeds two discriminators, a wideband discriminator to capture the signal initially and a narrow band crystal discriminator for the final frequency lock. The discriminator outputs feed a varactor diode in the receiver local oscillator to complete the synthesiser loop.

Table 2 shows typical synthesiser internal frequencies for the SEM-35 tuned to 26.5 MHz. As with the receiver RF amplifiers, there are two transmitter RF sections.

Table 2: Synthesiser frequencies

Groans and Whinges

The SEM-35 suffers from having shared a common development programme with the SEM-25 vehicle radio. For vehicle duty there is ample power available from the vehicle battery and the designer can be lax about designing for energy efficiency. This mindset carried over into the SEM-35.

The PRC-10 series valve sets that preceded the SEM-35 required a total battery power of 8.4 watts in transmit to generate a nominal 1 watt RF output. The SEM-35 requires 10 watts of battery power to generate a similar RF output. On the receiver front the position is better but still not good. On receive the PRC-10 drew 2.6 watts while the SEM-35 draws 2 watts.

Because of the lack of valve filaments and HT supplies most transistor versions of valve sets should require about 20% of the valve set power. It goes against logic that a transistor transmitter should require a larger power supply than its valve equivalent for the same power output.

substantial life costs for battery procurement and re-supply and/or additional portage weight. It was within the technology of the day to double or triple the battery life and similarly reduce battery re-supply costs. This could have given significant payback over the operational life of the set.

On the receiver of the SEM-35 example examined the local VHF broadcast stations ‘broke through’ at frequencies around 30MHz. For example the local music station on 89.9MHz was picked up when the SEM-35 was tuned to 27.7MHz. At that frequency the SEM local oscillator was 39.2MHz and its second harmonic 78.4MHz. The 78.4MHz beat with the 89.9MHz station to produce the 11.5MHz first IF frequency of the SEM-35.

When the SEM-35 was developed there would not have been the current profusion of VHF FM entertainment stations and the breakthrough would not have been as obvious.

The strength of the break through varied with hand proximity to the SEM control knobs. Further investigation showed that the control knobs, shafts and bushes were not making contact with the set front panel. There were star washers under their shaft bush nuts that should have scraped through paint and earthed the controls but the quality of painting (thought to be powder spray epoxy) had been too hard and durable to allow the star washers to cut through.