Phase Preselector Intended to Suppress Image Frequency in the Radio Receiver

Until now, to suppress the image channel, radio receivers use a frequency preselector consisting of oscillatory circuits of the input device and a radio frequency amplifier. But the frequency preselector does not suppress strongly enough the image frequency. Therefore, two frequency conversions are often used in railway radio communication systems. For example, for station radio communications at frequencies f = 151‒156 MHz, two frequency converters with fpr1 = 24 MHz and fpr2 =1,596 MHz are used, which greatly complicates the design of the receiver.

The article proposes to use a phase preselector, which is the second parallel quadrature frequency converter, the signal of which, after passing additional elements, is summed with the signal of the first frequency converter with high suppression of the image frequency. With this method of organising communication, the bandpass phase shifter will be a problematic block in the phase preselector, since it must have a small error Δφ.

A bandpass phase shifter made on RC circuits with a minimum phase shift error Δφ = 0,1˚ within a frequency band of 25 kHz is described. In case of increasing the frequency band, the phase shift error remains minimal, but it is necessary to use limiting amplifiers to equalise the amplitudes of the signals at its output. The use of a phase preselector instead of a frequency preselector will help to reduce the intermediate frequency, which will lead to high selectivity, both in the adjacent and image channels with single-sideband modulation, and will also simplify and reduce the cost of the signal receiver.

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