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27 MHz or 29 MHz receiver using TA7642 as IF part

With only little effort, the receiver shown below can be constructed. It works according to the superhet principle. Nevertheless, the TA7642 uses an integrated circuit that is actually intended for use in simple AM medium wave receivers operating without any frequency conversion. The IC is an equivalent type to the ZN414 or the MK484. It serves as an IF amplifier for 455 kHz and is characterized by good AGC properties for this purpose. So the volume differences in the reception of stronger and weaker stations are not overly big. The DC voltage applied to the output pin, which decreases with increasing field strength, can be processed for the operation of an S-meter in bridge circuit. This could also be used to control a squelch circuit.

Since the IC is designed to operate on a single 1.5V battery cell, the operating voltage is stabilized down to a value of this size with two Sillicon diodes serially connected in the flow direction. The mixer and oscillator stage equipped with a dual-gate MOSFET operates as a self-oscillating, crystal-controlled mixer. This arrangement produces little inherent noise and thus ensures a good input sensitivity. Due to the crystal control the receiver has excellent frequency stability. As a result of the ceramic filter used to get an excellent channel selection. The achievable with this circuit properties, in particular with respect to the selectivity, despite the minimum cost significantly better than those of the receiving parts of simple old CB walkie talkies with superheterodyne-receiver without ceramic filter (f.e. made by Tokai).

I originally designed the receiver for remote control purposes at 27 MHz band, but also thought it might be used in portable 10m radio devices. As a result, it would also be well suited as a replacement for the super-regenerative receivers in the 10m walkie talkies described elsewhere. The circuit could be used with appropriate dimensioning of the resonant circuits (f.e. 10.7 MHz IF filter with parallel capacitor) for radio reception in the SW range. For example, a 5615kHz crystal would make it a simple and reliable receiver for DARC Radio in the 49m band (6070kHz AM). Since in this case no oscillation of the crystal on the 3rd overtone must be forced, the oscillator circuit of the MOSFET could be made aperiodic. The coil at gate 1 can then be replaced by a parallel connection of a resistor with a fundamental wave crystal. This circuit can be found at my 20m-40m converter, which I describe in another post.

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