SM 5 BSZ - An Electronic Phase Shifter for the Receiver
(April 25 1997)

Filters as phase shifters

A band pass filter will shift the phase of a signal differently depending on where in the pass band the signal is. This is normally known as the phase response of a filter, and for a good filter the phase response is linear (talk to the HI FI people) By tuning the centre frequency of a bandpass filter, one gets a variable phase shift for a constant frequency, which can be used for polarisation shifting.

Phase shifter. Band pass filter, bandwidth 30MHz with flat frequency response. Phase can be shifted + or - 25 degrees without change of amplitude.

The phase shifters I currently use are designed like in the figure. Junk box components are used, so I do not know component values. Two of these filters, one for each channel, easily allow a phase shift from 0 to 90 degrees while amplitudes remain within +/- 0.5dB.

Differential phase shifter

The differential phase shifter is placed immediately before the schottky diode phase rotation device. The total phase of cables and preceding RF stages, is adjusted for the pot to give linear polarisation at one end. The voltage at the point between the potentiometer sections is adjusted by the resistance to ground for the polarisation to be circular at the other end of the pot. Circular polarisation is of course obtained only when the subsequent polarity rotation device has equal weight for both channels.

For use as the phase shifter in a switchless combiner this solution is not allowing a large enough phase shift. Probably a small change of component values would be enough, but also more LC links will improve and allow greater phase shifts. If you start experimenting, there is no reason to buy expensive capacitor diodes (varicaps). Normal zener diodes are designed the same way, and they are perfect to use in this kind of circuitry.

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