FM double conversion receiver for 144-146 MHz
Various multiband radios of Japanese production were offered from about 1970 as control or monitoring receivers, among others, in trade magazine advertisements. In most cases, the 2m band could be received already in FM mode. In the area often marked as PB2 (= Police Band 2), frequencies were usually adjustable in a range of 136 to 174 MHz. With the IF bandwidth of over 200 kHz designed for broadcasting, it was possible to tune FM amateur radio stations with sufficient sensitivity, but they were often difficult to separate from each other.
I had at that time a device as you can see in the picture. It had a very useable input sensitivity, which certainly could been improved significantly by a smaller IF bandwidth. The study of the circuit diagram showed that a separate tuner was available for each VHF band. Range switching was achieved by switching these tuners. The seperate tuners were constructed like input parts of a little higher class FM radio receivers, designed with RF pre-stage, mixer and separate oscillator. This first moved me to modify the VHF part of a mechanically similar built cheap Japanese portable AM/FM-radio by changing the resonant circuit elements to receive the 2m amateur radio band.
The entire unit was built on a board, including the VHF tuner, similar to the adjacent photo. Although the input circuitry with RF preamplifier and self-oscillating mixer was much simpler here, the reception results after replacing the RF preamplifier transistor with a good RF type were quite similar to those of the desribed multiband radio receivers.
Motivated to further attempts, I converted a Japanese car radio with variometer tuning and separate oscillator. Probably above all because of the more RF equitable construction of the input unit, the reception was noticeably better, especially with regard to the pervading of FM broadcast stations and other incorrect mixtures, than with the multiband radio.
Even better results were achieved with a tube radio, which had the at that time in Germany usable standard VHF input arrangement. This used a duo triode of type ECC85 and again worked with a self-oscillating mixer. These promising attempts encouraged me to build a complete transistorized double conversation superheterodyne for 2m. This should get an input circuit with self-oscillating mixing stage. For a transistorized VHF input circuit with self-oscillating mixing stage specified in "Das Spulenbuch" (the coil book) by Hans Sutaner, a frequency constancy of ± 15 kHz achievable with temperature compensation was specified in the temperature range from -20 ° C to + 50 ° C. Extrapolated to the conditions in the 2m band, a value of about ± 22.5 kHz is obtained for the frequency stability. Accordingly, such a circuit could also be operated in combination with a narrow-band FM intermediate-frequency part if one accepts occasional retuning after switching on or after major changes in temperature.
To investigate the useability of such minimal tuners for narrow-band FM, I first undertook initial preliminary tests. To this end, I connected the IF output of a cheap VHF receiver with self-oscillating mixer ("Aircontrol") to a dual IF amplifier from an old business radio. The device had an IF section for broadcast bandwidth and the scale drive of the device was by no means suitable for this selectivity. Nevertheless, it was found that the frequency stability itself was sufficient for narrowband FM reception even without further compensation measures. More problematic, however, were instabilities over the antenna. For example, approaching the hand to the antenna, stations almost completely disappeared.
As a result of the then about 15 km away from my location Hamburg Airport there were already frequently with my radios receiving problems with the image frequency reception from the air band. To solve this problem, I added a dual-circuit filter to my completely self-made input arrangement between RF preamplifier and mixer. With this, I also solved incidentally the instability problem over the antenna, it resulted in a much better decoupling. Equipped with a RF proof construction with large ground plane, I even achieved an overall better stability, as it had a 2m receiver with Hael branded modules (dual-gate MOSFET pre-stage and SO42P as a mixer and oscillator). As conclusion, the mechanical construction is even more important in VHF circuits, as the circuit technology!
Finally, I chosed for the input part of my receiver the shown circuit, wich I found as the input part of a cheap radio from the Far East. I resized the coils an capacitances for 2m and equipped it with better transistors. As shown in the photos, I succeeded in a further construction (with 2xBF959) even on a dot board (grid size 5.08mm). Thanks to thoughtful wiring and 90 ° angles of the coils to each other, the circuit works quite well. It shows no tendency to parasitic oscillate. Proper balancing of the resonant link circuit is possible with an IF amplifier and AM detector by setting the two trimmers to maximum noise when the antenna is connected. Beforehand, the oscillator circuit must be adjusted so that here frequencies between 133.3 and 135.3 MHz can be set.
Following to the front end, I inserted a self-oscillating mixer, which translated the 10.7 MHz IF signal to the lower IF of 455 kHz. Here, a quartz-controlled arrangement redimensioned for the lower frequency was used, as was often found as a receiver mixing stage in simple 27-MHz handheld radios with superheterodyne receivers. The quartz control could safely avoid frequency instabilities of this stage. This was followed by two equally constructed 455 kHz IF stages in cascode arrangement, which had approximately the same properties as IF stages with pentodes. Unfortunately, however, the second stage tended to overshoot on strong signals and the adjustment was very critical. For demodulation, first a phase detector was used. Shown now is the total circuit of the ZF part connected to the frontend via a 47pF capacitor.
Later, I replaced the second cascode stage with an IF limiting amplifier with differential amplifier, solving the problems of overshoot with strong signals. Additional AM suppression was provided by the later replacement of the phase detector by a ratio detector. The IF section was followed by a transformerless push-pull LF amplifier with four transistors (power amplifier AC187k and AC188k). The receiver obtained an S-meter and a squelch. The device was so well suited as a second receiver, but was in connection with a crystal-controlled 2m FM transmitter even quite well suited for radio operation. In order to mute the receiver on TX operation, I switched off only the audio part when transmitting. The antenna was switched from the receiver input via a relay to the transmitter.