Abstract:
A phase loop demodulator, in particular for space telecommunications, comprises a primary phase loop, a locking detector circuit responsive to acquisition by the primary loop, and a secondary phase loop controlled by said detector circuit.

Description:
The invention relates to a phase loop demodulator. 
     BACKGROUND OF THE INVENTION 
     The manual by Floyd and Gardner entitled &#34;Phaselock techniques&#34; (2nd edition, pp. 182-183) describes a prior art phase loop demodulator comprising an input bandpass filter, a phase loop including a phase detector, a loop filter, and a voltage controlled oscillator (VCO), with a de-emphasis filter being disposed at the output from the phase loop. 
     In such a phase loop, any difference between the frequency (Fe) of the carrier to be demodulated and the rest frequency of the VCO (Fvco) 0 , that may be due to VCO aging, or to drift in the carrier, . . . , gives rise to a static phase error at the output from the loop phase comparator, and consequently to: 
     degradation in the threshold of the demodulator; and 
     greater sensitivity to &#34;click&#34; phenomena. 
     An object of the invention is to mitigate such drawbacks. 
     SUMMARY OF THE INVENTION 
     To this end, the present invention provides a phase loop demodulator comprising a primary phase loop and a secondary loop, wherein the demodulator includes a circuit for detecting primary loop locking, and which, once locking has been detected, switches on said secondary loop, said secondary loop participating in increasing the DC gain of the primary phase loop. 
     Advantageously, the invention makes it simple to reduce the static phase error at the output from the phase loop comparator. It makes it possible to maintain phase loop performance even when various causes such as temperature variation, aging, radiation, . . . , give rise to an offset between the carrier frequency and the VCO frequency within the limits of the acquisition band. 
     The invention makes it possible to increase the DC loop gain under steady state conditions once phase locking has been achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram of a prior art demodulator; 
     FIG. 2 is a diagram of a modulator of the invention; and 
     FIG. 3 is a graph showing the demodulation performance of the prior art demodulator and of the demodulator of the invention. 
    
    
     DETAILED DESCRIPTION 
     The prior art demodulator shown in FIG. 1 comprises a phase loop 10 and an output integrator 14. The phase loop 10 comprises a phase comparator 11, a loop filter 12, and a VCO 13 looped back onto the phase comparator. The output integrator 14 is connected to the output from the phase loop 12. 
     A phase comparator is a non-linear device and it operates best in conjunction with a static phase error that is zero. 
     Assuming that the comparator 11 has a gain Kd, that the VCO 13 has a slope Ko, and that the input signals to the comparator 11 are as follows: 
     
         X=A sin [w.sub.o t+φe(t)] 
    
     
         Y=B cos [w.sub.o t+φs(t)] 
    
     then the following transfer function applies, with 
     φd=phase of the demodulated signal 
     φe=phase of the input signal 
     B=acquisition band 
     φd/φe=Kd.F(p)/(p+Ko.Kd.F(p)) ##EQU1##  with F(p)=F(o).(1+τ2p)/(1+τ1p) and KT=Ko.Kd.F(o), f 1  (=1/τ1) and f 2  (=1/τ2) being the poles of the loop filter 12. 
     The static phase error δφ due to a frequency difference or offset between Fe and (Fvco) 0  is given by: 
     
         δφ=(Fe-(Fvco).sub.0)/KT 
    
     The value of KT is limited by: 
     the difficulty in making amplifiers having both high gain and large bandwidth; and 
     the sensitivities of the phase comparator and of the VCO. 
     The demodulator of the invention, as shown in FIG. 2, includes the same basic circuits as the prior art demodulator, and these circuits are therefore given the same references: the phase loop 10 and the output integrator 14. 
     It additionally includes a lock detector 20 which includes a π/2 phase shifter 21 connected to the output of the VCO 13, followed by a phase comparator 22 whose second input is connected to the input E of the demodulator, and whose output is connected to a lowpass filter 23 which delivers a lock-indicating signal AI. 
     In addition, it includes a second phase loop referred to as a &#34;secondary&#34; loop 24 which is connected to the output of the loop filter 12 and includes an amplifier 25 followed by a lowpass filter 26 and a switch 27 connected to a summing circuit 28 which is interposed in the primary loop 10 between the loop filter 12 and the VCO 13. The switch 27 is controlled by the lock detection signal AI from the lock detector 20. 
     The invention makes it possible to reduce the phase error term δφ when locking is achieved. 
     Prior to locking, the switch 27 is open and the secondary loop 24 is not used. This reduces to the conventional phase loop shown in FIG. 1. The acquisition band is given by the formula mentioned above: ##EQU2## 
     Once locking is achieved, the carrier and the output from the VCO 13 are in quadrature, and a detection maximum is obtained at the output from the lock detector 20. The switch 27 then switches the secondary loop 24 into operation. This loop comprises an amplifier 25 of gain G and a lowpass filter 26, with the cutoff frequency Fc of this filter being very low (a few Hertz) so as to avoid giving rise to an interferring pole in the loop. 
     The following then applies: 
     
         φd/φe=Kd.F(p)/(p+Ko.Kd.F(p).[1+G(p)]) 
    
     For modulation frequencies that are much higher than the cutoff frequency Fc (in practice a modulation frequency f&gt;10.fc), demodulation performance is the same as primary phase loop performance since G(p)≈0. 
     The total DC loop gain is: KT[1+G(0)]. 
     The expression for static phase error becomes: 
     
         φp=(Fe-Fvco)/KT[1+G(0)] 
    
     and φp with the secondary loop=φp without the secondary loop divided by (1+G(0)), so that the gain for static phase error becomes 1/[1+G(0)]. 
     Measurements of demodulation performance have been performed by simulating differences between Fe and (Fvco) 0 . 
     Under the following conditions: ##EQU3## 
     The primary loop locks in about 10 μs. The secondary loop locks in a few hundred milliseconds. The demodulator threshold is extended by about 2.2 dB. 
     Curves 30, 31, 32, and 33 shown in FIG. 3 show the demodulation performance of the conventional phase lock loop shown in FIG. 1 and of the phase lock loop of the invention when the difference between the carrier to be demodulated and the rest frequency of the VCO is close to +1 MHz. Curves 30 and 31 correspond to the phase demodulator of the invention while curves 32 and 33 corresponding to the conventional demodulator. 
     In FIG. 3, curves 30 and 32 correspond to 139 MHz. Curves 31 and 33 correspond to 141 MHz with a modulation frequency fm of 8 kHz and a phase modulation index m of 1.2 rd. Curve 34 is a theoretical straight line corresponding to an ideal phase demodulator. 
     The threshold improvement (defined by the 1 dB compression point) is of the order of 2.2 dB. 
     The invention thus makes it possible to reduce the static phase error δφ by making use, after acquisition, of the secondary loop 24 (automatic frequency control) which participates in increasing the DC gain of the primary phase loop without altering its parameters: namely loop bandwidth w n  and damping with respect to modulation; locking performance (acquisition bandwidth) is conserved. 
     The circuit of the invention is used for Tracking, Telemetry, and Command (TTC) receivers of the Eutelsat II and Telecom II type. 
     Naturally the present invention is described and shown merely by way of preferred example and it component parts could be replaced by equivalents without thereby going beyond the scope of the invention.