INRICHTING VOOR HET FASEGETROUW SELECTEREN VAN EEN KARAKTERISTIEKE COMPONENT UIT EEN FREQUENTIESPECTRUM

1,195,548. Frequency selective circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 16 Nov., 1967 [19 Nov., 1966], No. 52165/67. Heading H3F. In an arrangement for selecting a component from a frequency spectrum comprising a main selection channel 3 having connected in series a first modulator 5, a selection filter 3, a second modulator 6 and a final selection filter 10, an auxiliary channel 4 also including a selection filter 11, 13-16, is connected to an input of each of the first and second modulators through separate connecting channels 8, 9 and the arrangement is such that the frequency and phase of the selected component appearing at the output 2 are substantially equal to the frequency and phase of the input signal component. The phase shifts introduced by the filters are arranged to cancel so that the input signal 1 is in phase with the output signal of the same frequency at 2. The auxiliary channel 4 has a frequency multiplier 12 which feeds via the filters 14, 15, 16 tuned to this frequency, the modulators 5, 6. Either the sum-or-difference frequency from modulator 5 is selected by the filter 7 and the difference or sum frequency provided by the modulator 6 is selected by the filter 10 which is tuned to the frequency of the incoming signal at 1 as are the filters 11 and 13. In a modified circuit (Figs. 2 and 4, not shown), the filters 13, 15 and 16 are omitted and amplifiers (18 and 17) may be connected between the input filter 11 and the multiplier 12 and between the input and output of the filter 7. In this arrangement the frequency multiplier 12 is in the form of a limiter and the output filter 14 may be tuned to the third harmonic of the input signal at 1. The selection filter 7 would then be tuned to the second harmonic frequency and the selection filters 7, 10 and 11 may have equal Q factors. In an alternative arrangement (Fig. 3, not shown), the input filter 11 and the filters 15 and 16 are omitted. Phase variations in the various filters resulting from temperature changes are compensated substantially by each other. A frequency divider may be connected in cascade with the frequency multiplier 12 or the frequency multiplier may be omitted. The modulators 5 and 6 may be switching modulators.