Regenerative superhet 'Göttinger Baby II'
In tube devices, a popular concept for homebrew amateur receivers has been to supplement an regerative receiver with a tunable converter. The regenerative receiver then worked on a fixed intermediate frequency, so that the regeneration in the search for receivable stations did not need to be constantly readjusted. One could thus largely incorporate the experience gained in the self-construction of simple regenerative receivers into the construction of a superheterodyne receiver. Optionally, even an existing 0V2 receiver could be extended to the superhet in this way. A working as regenerative superheterodyne receiver, which was already equipped with transistors, was developed in the DARC local section Göttingen from the late 1960s as a model device for homebrew. It was a small and portable device for the 80m amateur band, which was also used as a receiver for ham fox hunting. It was thought mainly for amateur radio newcomers and got the name Göttinger Baby. The first version was designed only for headphone reception and had less sensitivity. Also, the frequency stability was not as good as the here presented Göttinger Baby II, whose circuit is now to be discussed here in more detail. An RF stage with the cascode amplifier IC CA3028 from RCA was followed by a mixer stage with a BF224 RF transistor. The oscillator is another transistor of this type. In order to better recognize the cascode circuit, the integrated circuit is shown with its internal schematic. In combination to the ferrite antenna the switchable rod antenna allows a clear direction determination when used as fox-hunting receiver. The mixer directly supplied the 470kHz IF-regenerative detector with its output signal. Behind it, a lowpass filter limits the audio bandwidth, which is especially important for SSB and CW reception. All other stages are for audio amplification. When comparing the detector stage and the following two audio stages, it is clear that this part was copied from the Trabant KM from Radio RIM.
I was fortunate to be able to extensively test such a device in my SWL time, because it was kindly loaned to me by an OM. The sensitivity was very good. In the early evening, many amateur stations were already received with the built-in ferrite antenna. If one connected a long wire antenna instead of the rod antenna, the reception of weaker stations or DX reception was no major problem either. The disadvantage, however, was that at CW no single-wave reception was possible and so in SSB, the unwanted sideband was not suppressed. But this is a general disadvantage of the regenerative superhets. Especially in the case of the described device, the regenerative IF-detector tended to sync very much, so that stronger SSB stations were received rather distorted if the RF gain was not turned down. An automatic control the device did not offer. This made listening to SSB circles quite exhausting. Furthermore, the push-pull output stage operating almost without bias current had a detrimental effect on the sound quality, since at a relatively low volume setting, quite considerable takeover distortions of the final transistors occurred. In the device It was used a telephone earset as a loudspeaker. It contributed positively to the selection of the device during SSB and CW reception due to its small audio bandwidth, but the sound was not exactly a pleasure when taken together. Measured on the effort was really good at this receiver, however, the selectivity when receiving AM stations that were still occasionally heard in the 80m band. Although the circuit can provide many interesting ideas for self-built equipment, because of the described disadvantages I renounced replicating the such a device.