GB1529284A - Circuit arrangements for correcting demodulated signals - Google Patents

Abstract

1529284 Data transmission SIEMENS AG 18 May 1977 [26 May 1976] 20852/77 Heading H4P In a known demodulator (Fig. 3, not shown) a first demodulated signal is obtained by multiplying a modulated data signal by a first reference signal having a frequency equal to that of the data signal carrier and filtering the product, and a second demodulated signal is similarly obtained by multiplying the modulated data signal by a second reference signal displaced 90 degrees in phase relative to the first reference signal. However, in this known arrangement it is not always possible to regulate the phase angle between the data signal and the first reference signal to the theoretically correct value, and this results in some corruption of the output signals. To correct for this, the invention provides means SS for producing first and second transformation signals S3, S4 corresponding respectively to the cosine and sine of the phase angle between the carrier signal and the first reference signal. The first demodulated signal S1 and the signal S3 are multiplied in M3, whilst the second demodulated signal S2 and the signal S4 are multiplied in M4, the outputs of multipliers M3, M4 being summed in an adder AD 1 to produce a corrected first demodulated signal A. The corrected second demodulated signal B is similarly derived, by way of multipliers M5, M6 and an adder AD2, by summing the product of the signal S1 and a transformation signal S5 (which is an inverted version of signal S4) and the product of the second demodulated signal S2 and signal S6 (which is the same as signal S3). The device SS which produces the transformation signals may be constituted by an arrangement of comparators, gates, a pulse generator and a counter (Fig. 6, not shown), and the way in which this arrangement operates to produce the required corrected signals is described with reference to Fig. 4 (not shown).