if detector circuit 604 is applied to connection 700 and coupled to multipliers 702 and 704 the auxiliary signal is multiplied in multipliers 702 and 704 by signals of frequency kf h and quadrature phase provided by the pll circuit 606 output signal from multiplier 702 is applied to a low pass filter 706 the output of which is coupled to the input connections x1 and x4 of a multiplexer or switching circuit 710 output signal from multiplier 704 is applied to a low pass filter 708 the output of which is coupled to the input connections x2 and x3 of the multiplexer 710 the bandwidth of the low pass filters 706 and 708 is selected to pass the frequency components of signals s 1 and s 2 and is at least not substantially greater than kf h letting u03c9 2 u03c0kf h the received baseband auxiliary signal s t can be described by s t s 1 t cos u03c9t s 2 t sin u03c9t the product provided by the multiplier 702 is equal to s 1 t cos 2 u03c9t s 2 t cos u03c9tsin u03c9t which can be converted using trigonometric identities to the form s 1 t 2 s 1 t cos2 u03c9t 2 s 2 t sin2 u03c9t 2 only the first term is passed by the filter 706 by similar analysis it can be.