Patent GB-745940-A (GB)
Title: Improvements in or relating to magnetic and dielectric modulator circuits
IPC: H03C1/10
Filed: 1953-06-19
Abstract: 745,940. Phase and amplitude modulation systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 19, 1953 [June 24, 1952], No. 17009/53. Class 40 (5). In a phase or phase and amplitude modulator employing a non-linear magnetic or dielectric means modulating a carrier frequency fp by a signal frequency fq to produce from the combined output of a pair of oppositely-connected secondary circuits of a pair of transformers T1, T2 a pair or one of a pair of sideband frequencies k.fpŒfq, where k=1, 3, 5 ... or from the primary side of the transformers frequencies fq, kfpŒfq, where k=2, 4, 6..., the output is arranged to have an outer impedance Z<SP>1</SP> which together with the impedance Zi looking into the modulator constitutes an impedance Z satisfying the condition where Z np+q  is the complex value of the impedance Z for the frequency (np+q) and Z<SP>*</SP> np-q  is the conjugate complex value for the frequency (np-q), while a=1 for a dielectric modulator and a = - 1 for a magnetic modulator. A further condition is that either all current or all voltage components of frequencies kfpŒ fq other than at the desired frequencies nfpŒ fq should be negligible, in those cases Z being formed by Z<SP>1</SP> and Zi in series, or in parallel respectively. Reaction of the output on the signal frequency fq is thus avoided. If fp>>fq phase modulation alone is produced by the circuit of Fig. 1 in which Z<SP>1</SP> is the parallel or series combination of a capacity C and a load resistor R tuned with the inductive output impedance Zi of the transformers to the frequency fp. The carrier source fp may be in series or in parallel with the output Z<SP>1</SP> or in series with the resistor R alone, and R may in part be the impedance of a phase demodulator producing as output amplification of the signal fq. If fq is not low compared with fp amplitude modulation also occurs and the circuit of Fig. 3 is used in which both the parallel resonant circuit L2, C2 and Zi combined with the remainder of the impedance are tuned to fp +fq or fp -fq, the phase or amplitude modulated output at the other frequency, fp - fq or fp +fq respectively, being derived across the impedance C3-L3. In an equivalent modification of Fig. 3, the output across terminals C-D comprises two parallel resonant circuits La-Ca and Lb-Cb in series tuned to fp+fq and fp - fq, and with La(fp +fq) = Lb(fp - fq). In the dielectric modulator of Fig. 5 comprising non-linear capacitors C1, C2, biased by a polarizing voltage source V0, the output is purely phase modulated if fp>>fq and Z<SP>1</SP> may be an inductor and resistor series or parallel tuned with the capacitive output  impedance of the secondary circuit to a frequen cy fp. If fp is not low compared to fq amplitude and phase modulation both occur and the circuit of Fig. 7 is employed, with L2, C2 and the secondary circuit as whole tuned to fp+fq or fp -fq and the single sideband output circuit L3, C3 tuned to fp-fq or fp +fq respectively (star connection). The circuit to the right of CD may be replaced by its equivalent of two series resonant circuits La, Ca and Lb, Cb in parallel, tuned to fp +fq and fp -fq respectively. As before, the condition for no reaction in the source fq is La(fp+fq)=Lb(fpfq). Further, the ratio between series inductance and resistance must be the same. The detector in circuit L3, C3 may form part of this resistance. In the circuit of Fig. 9, if fp>>fq, frequencies fq, kfpŒfq, where k is an even integer, purely phase modulated, may be. derived from the primary circuit terminals A, B or windings coupled to the primary circuit windings, and reaction on the signal source fq is avoided by a resistance R and capacitor C1 in series or parallel connected across the source fq, and primary windings, tuned with the remainder of the primary circuit to the frequency 2fp. A polarizing source 10 is not necessary, if carrier frequency fp is not necessary at the same time from the secondary circuit. If fp is not high compared to fq, two parallel resonant circuits L2, C2 and L3, C3 (Fig. 11), connected in series replace the resistor R, the circuit L2, C2 and primary circuit Zi, C1 being tuned to undesired sideband (2kfpŒfq), and the single sideband output (2kfpŒfqfq), phase or phase and amplitude modulated, being taken from L3, C3, tuned to the desired sideband (2kfpŒfq). Fig. 12 shows a dielectric modulator for fp not high compared to fq, with L2, C2 and the output circuit Zi, L1 being tuned to the undesired sideband and the circuit L3, C3 to the desired sideband. In all cases the modulators may be employed together with detectors as amplifiers of the signal frequency fq. A permanent magnet may replace the magnetic biasing current source, and the dielectric modulators may use permanently electrically polarized elements (i.e. electrets).