Patent GB-864708-A (GB)
Title: Improvements in and relating to electrical phase-shifting networks
Inventor: ET GEOFFREY GEORGE GOURI
IPC: H04N5/205
Filed: 1957-10-14
Abstract: 864,708. Amplitude modulation systems. BRITISH BROADCASTING CORPORATION. Sept. 19, 1958 [Oct. 14, 1957], No. 32061/57. Class 40(5) [Also in Group XL(b)] A phase-shifting network for producing two signals having a predetermined relation to one another at two terminals respectively, comprises a delay device terminated at one end and provided with a plurality of output terminals so connected thereto that, in response to the application of an input signal to an input terminal, an output signal appears at each delay device output terminal after a predetermined time delay #, which is different for different output signals and may be zero for one output signal, means for adjusting, varying or setting the amplitudes of the output signals in such manner that relatively to the amplitude of a predetermined one of the output signals constituting one of the said two signals and appearing at a predetermined one of the delay device output terminals constituting one of the first said terminals, and for which the delay # has a predetermined value # a , the amplitudes of the other output signals diminish with increasing values of the modulus of #<SP>1</SP>, where #<SP>1</SP>=(#-# a ), and means for adding together the said other output signals so adjusted and with a sign dependent upon the sign of #<SP>1</SP> to produce a combined output signal constituting the other of the said two signals, and appearing at the other of the first said terminals. The means for adjusting the relative amplitudes of the output signals is such that these amplitudes are proportional to cosech #α#<SP>1</SP>/2, α being a parameter which can be varied. The combined output signal produced by such a network comprises all the significant Fourier components of the signal for which the delay is # a , each component having been phase-shifted by 90 degrees with respect to that signal, and having an amplitude multiplied by a function f(#)=tank #/α, # being the ampular frequency. The network can be such that # a = #L/2, #L being the delay of a signal in traversing the delay device from the input terminal to the terminated end thereof. In the construction shown in Fig. 1 a lumped delay line 11 comprises an even number n of lumped delay sections D 1  to D k , D c , D<SP>1</SP>c and D<SP>1</SP>k to D<SP>1</SP> 1 . Each of the sections D 1  to D k  and D<SP>1</SP>k to D<SP>1</SP> 1  provides a delay # 0 and the sections D c  and D<SP>1</SP> c  each provide a delay # 0 /2. The various delays may be unequal. A terminating impedance section 12 is provided. Signals to be phase-shifted are applied to an input terminal 13 and fed to the delay section D 1  and the attenuator A o  the output from which is applied to the adder unit 14. The signals from the delay sections, except that from the section D c , are applied to attenuators A 1  to A k  and A<SP>1</SP> k  to A<SP>1</SP> 0  respectively. The outputs of the attenuators A 1  to A k  are applied to the adder unit 14 and the outputs of the attennators A<SP>1</SP>k to A<SP>1</SP> o  are applied to an adder unit 15. The signal from the section D c , which is the n/2<SP>th</SP> section, is applied to the output terminal 16. The output from the adder unit 14 through a polarity reversing unit 17 and the output from the adder unit 15 are applied to an adder unit 18 having an output terminal 19. In the generation of vestigial sideband television signals, a television signal is applied to the input terminal 13. An output signal appears at terminal 16 and a second output signal at terminal 19. If the input signal is represented by f(t), the output signal at 16 can be represented by f(t+#a) where # a =(n-1)# o , and the output signal at 19 is a 2 composite signal comprising every Fourier component of the signal f(t+#a) phase shifted by 90 degrees with respect to the signal f(t+# a ) and multiplied by a function f(#))=tank #/α, # being the angular frequency and α a variable parameter. The signal appearing at 19 can be represented as f<SP>*</SP>(t+# a ). The output signal f(t+# a ) at 16 is applied to modulate a sinusoidal carrier and the signal f<SP>*</SP>(t+# a ) is applied to modulate the same carrier phase shifter by 90 degrees. The two modulated carriers are then added. The function f(#)=tank #/α provides the neces-   sary amplitude/frequency variation. The upper frequency limit of the network is f max . = (1-α/# o  2<SP>#</SP>). If the delays introduced by the delay sections are not equal, then # o  is the longest of the delays introduced. The total delay # L  of the line 11 must be such that cosech #a # L  /2 is small compared with unity. If # L  is not sufficiently large the amplitude spectrum produced will be modified by the superposition of an amplitude ripple. For this ripple to have an amplitude less than Œ 2% of the maximum amplitude, # t  must be greater than 4/α. The minimum total number n of delay sections which should be used is determined by n=(# L /# o +1) and n=# L  (f max  + α/2#)+1. In the network shown in Fig. 3 the delay line is preceded by an optional polarity reversing unit 20 and a further unit 22 can be inserted between the sections D c  and D<SP>1</SP>c. The output from D c  is applied to terminal 16 and the output from unit 22 is fed to a terminal 16<SP>1</SP>. The adder units 14 and 15 of Fig. 1 are replaced by a single adder unit 23 and the polarity reversing unit 17 is omitted. In the generation of a vestigial upper sideband signal, the unit 20 is switched in. The output signal from 16 is applied to modulate the carrier and the output signal from 19 is applied to modulate the carrier phase shifted by 90 degrees, and the modulated carriers added. In the generation of a lower sideband the unit 20 is switched out, the output signal from 16<SP>1</SP> is applied to modulate the carrier, the output signal from 19 applied to modulate the carrier phase shifted by 90 degrees, and the two modulated carriers added. In the network shown in Fig. 4 the delay line 11 of Fig. 1 is replaced by a balanced lumped delay line having balanced delay sections corresponding to the delay sections of Fig. 1. The first section of the line is preceded by a phase-splitter 24, and a reversal of polarity is obtained by cross-over connections 25, 26 between the sections D c  and D<SP>1</SP> c . Two outputs f(t+#a) and -f(t+# a ) are taken from the cross-over connections and fed to the terminals 16 and 16<SP>1</SP>. The outputs from the attenuators A o , &c. are connected to a single adder unit 23. In the generation of an upper vestigial sideband signal the output from 16 is applied to modulate a sinusoidal carrier, the combined output signal from 19 is applied to modulate the carrier phase shifted by 90 degrees, and the modulated carriers added. A lower sideband signal can be generated by applying the signal from 16<SP>1</SP> to modulate the carrier and combining the modulated carrier with the 90 degree phase shifted carrier modulated by the signal from terminal 19.