Nogoton superhet module (2m-Band)
After the Second World War, VHF broadcast was introduced in Germany. Many radios were initially only designed for the AM broadcasting areas (MW, LW and SW). So attachment devices came onto the market towards the end of the 1940s, with which such receiverss could be expanded to include the VHF range. These were designed as complete receiving parts so that the audio signal was already present at the output. This could then be connected to the pickup input available on most broadcast receivers, which was actually intended for connecting a turntable. For the sake of simplicity, these devices mostly worked as super-regenerative receivers. Therefore they were actually AM receivers, so that the FM broadcast transmitters, which are frequency-modulated, had to be demodulated on the flank. Although this resulted in poor sound quality, but in this way it was possible to receive the increasing number of FM stations at a fairly cheap effort. The interest was great, as more and more programs were broadcast here that could not be heard in the AM ranges and that had a large range of music.
For experienced radio hobbyists, such receivers were also offered in the form of built-in modules. In the usual large wooden housings of radio receivers, such VHF modules could, for example, be screwed from the inside to a housing side wall and provided with a tuning button on the outside. With an next to it mounted switch, the VHF module could be switched on and off and at the same time the audio line from the pickup connection could be connected to the audio output of the VHF part. Since these receiver parts mostly worked with only two tubes, the power supply reserves of the broadcast receivers were usually sufficient to power these modules.
Soon the expectations on the sound quality of FM reception increased. In the 1950s, the proportion of radios with built-in VHF parts increased by leaps and bounds and the superhet principle prevailed here too. With a real FM demodulator, for which the ratio detector was increasingly popular, a quality could be achieved that clearly stood out from AM reception. So it is not surprising that VHF attachments had become more and more rare and for the still being produced devices higher quality standards were requested. Consequently the Superhet also prevailed here. The company Nogoton from Delmenhorst made a name for itself in Germany.
There was an initially produced superhet for the frequency range from 86 to 100 MHz, since the VHF broadcast range in Germany only reached up to 100 MHz at the time. An export version for the range from 88 to 108 MHz was also released. The receivers were built on an elongated sheet metal chassis and with very high quality components. The professional designed input section includes a double triode of the type E88CC, actually produced for commercial VHF receivers and for use in community antenna systems. Here it was connected as a RF pre-stage in a cascode circuit with an intermediate base arrangement on the input side. This enabled an input sensitivity that was excellent for the time. This was followed by a self-oscillating mixer stage with an EC92 triode. In contrast to the usual tube-equipped VHF parts, this did not work with a Meissner feedback, but rather like a single-stage Butler oscillator with feedback from the anode to the cathode. This type of feedback only became very important later in transistor technology, where the feedback then naturally takes place from the collector to the emitter. With this trend-setting concept, it was possible to achieve good frequency stability with little effort. In addition, the radiation of the oscillator frequency via the receiving antenna could thus be kept small. The barely retro-active cascode arrangement of the preamplifier did the rest. The self-oscillating mixer stage was followed by two IF stages (1xEF89, 1x EF80) as well as the ratio detector built on the duo diode EAA91. With the four-circuit IF filter arrangement between the mixer and the IF amplifier repectively the overall eight IF circuits, the receiver offered extremely good selectivity for broadcasting purposes, so that even weak stations working alongside stronger transmitters could still be received well.
Now, however, such a receiver was no longer suitable for retrofitting in an existing receiver device, since the additional heating current of over one ampere required for the five tubes could hardly be supplied by the existing power pack. This resulted in a new class of equipment, namely the FM tuner device as a component for use in conjunction with pure audio systems. VHF receivers equipped with Nogoton built-in superhet modules were therefore often used in connection with PA systems for sound reinforcement in larger buildings.
Later, the delivery program of these built-in superhet modules was expanded to include models for other frequency ranges. This included a version for the frequency range from 32.5 to 38.5 MHz for receiving wireless microphones for events. But the greatest importance is likely to have had a version for the 2m amateur radio, which was gaining in importance at the time. This was designed for the reception range from 143 to 147 MHz. Since the AM operating mode dominated here at the time, the receiver was designed accordingly. The circuit of the superhet for broadcast receiption was adopted largely unchanged. Apart from the differently dimensioned RF circuits, essentially only the demodulator and the audio output were constructed differently. As the circuit diagram shows, the AM demodulator, which works with a germanium diode, is followed by an impedance converter connected as a cathode follower with a second EC92 tube, which was installed at that point, where in the broadcast version the EAA91 for the ratio detector was placed. So the construction could take place on the same chassis. The impedance converter offered the possibility of being able to connect a transistorized audio amplifier directly. For mobile radio systems, this could then also be used to modulate the transmitter.
The connection for displaying the signal strength (S) is high-impedance and designed for direct connection of a strength display tube (e.g. "magic eye"). It is noticeable that the entire IF amplifier was taken over from the FM version and has no control whatsoever, so that, depending on the field strength, large differences in volume and distortions can be expected with strong stations. Even then, the for broadcast radios good selectivity was actually insufficient for amateur radio with its smaller channel spacings and the receiver was therefore unsuitable as a station receiver. For use in mobile or portable radio systems or as an entry-level object for those who are interested in VHF technology, it was still the better alternative compared to the supergenerative-receiver. As a result of the single conversion with the high intermediate frequency of 10.7 MHz, the bandwidth of the IF section was around 100 kHz. At least the existing IF output offered the possibility of operating the receiver as a converter with a behind connected shortwave receiver tuned to 10.7 MHz. In this case the selectivity is essentially determined by the behind connected receiver. Soon Nogoton also brought out an improved version that was designed as a triple conversion superhet. The second and third mixer stages were equipped with ECH81 tubes and the therein contained triode systems worked as crystal-controlled oscillators. With a final intermediate frequency of 450 kHz, this model was able to achieve quite usable selectivity.