AM receiver for 29 MHz using the TCA440
As is known, the TCA440 (or A244D) is not only suitable for AM radio receivers, but also for use in crystal-controlled receivers, e.g. as a remote control or radio receiver for the 27 MHz range. The receiver circuit shown here is designed for the 10m amateur band. The oscillator operates above the intermediate frequency, which is 455 kHz. As a result, the image frequency is 910 kHz (= 2x 455 kHz) above the receiving frequency and thus outside the 10 m band. Here, even with good propagation conditions less disturbances can be expected. The wiring of the oscillator with capacitive voltage divider shown works more reliably with the usual overtone crystals, such as a TCA40 mostly proposed arrangement in which the quartz is connected directly to the resonant circuit. Depending on the quartz used deviations in frequency and amplitude can be kept smaller. Even quartz crystals, wich oscillates heavier on, can be used in this way. With the specified crystal frequency, the receiver operates exactly at 29 MHz, where AM stations are still occasionally heard today.
The dual-circuit filter arrangement in the input noticeably improves the image frequency supression. Again, this is an extension of the otherwise mostly for remote control units specified circuits with the TCA440, where usually only a single resonant circuit is proposed. An RF preamplifier can be used to substantially increase the sensitivity. Good is e.g. a preamplifier with a FET (e.g., BF246) in grounded gate circuit. However, in my attempts using the standard circuit the strong inherent noise of the TCA440 to a considerable extent came from the IF amplifier. In this circuit I used in addition to the ceramic filter a complete LC filter set for 455 kHz IF (color-coded as yellow, white and black). After inserting the white LC filter, this noise largely disappeared. The receiver thereby did not lose any sensitivity, the change even had a positive effect on the reception characteristics. Of course, with a different crystal and correspondingly tuned oscillator and pre-selection circuits is also suitable for receiving frequencies in the 27 MHz range. The circuit can also be used well as a dual conversion super IF part. So if instead of the LC circuits for 29 MHz ready-to-use LC filters for 10.7 MHz are used, the circuit can also be used for this frequency. One then uses a 10.245 MHz crystal. Of course, the oscillator coil must be adjusted accordingly.