Patent GB-812212-A (GB)
Title: Improvements in or relating to arrangements for testing radio repeaters
Inventor: JACOBSEN BENT BULOW
IPC: H04B1/60
Filed: 1956-12-03
Abstract: 812,212. Testing radio repeaters for phase delay. STANDARD TELEPHONES & CABLES Ltd. Dec. 3, 1956, No. 36835/56. Class 37. [Also in Group XL (c)] Testing apparatus for determining the group delay variations as a function of frequency is incorporated in a conventional microwave repeater of a radio link transmission circuit. Plural frequency modulated carrier waves in the microwave range are received at antenna 1 and branching band filter 2 supplies the waves to their respective repeaters. Modulated waves at say 4000 Mc/s. are passed via conductor 3 (Fig. 1) hybrid junction 7, band-pass filter 8 to a low level modulator 9 from whose output a band-pass filter 10 selects the lower side band (70 Mc/s.), which after amplification at I.F. amplifier 11, is supplied to a medium level modulator 12 raising the carrier frequency to say 4200 Mc/s., the appropriate side band being selected by band-pass filter 13 and after amplification at power amplifier 14 is supplied via hybrid junctions 15, 16 and conductor 4 to the branch band filter 5 and transmitting antenna 6. An oscillator 17 supplies heterodyne waves at 4270 Mc/s. to modulators 12 and 18 via hybrid junction 19, the modulator 18 also receiving waves at 200 Mc/s. free from oscillator 20 and the resultant lower side band (4070) from modulator 18 being separated by filter 21 and supplied to modulator 9. Hybrid junction 15 passes a small fraction of energy from the output of amplifier 14 to a frequency-changing modulator 24 via switch 22 and an adjustable attenuator 23, heterodyne waves of frequency 200 Œ 1 Mc/s. being supplied to modulator 24 from oscillator 25 via hybrid junction 26 which also supplies the measuring circuit as set forth below, a selected side band from modulator 24 being supplied to junction 7 via band-pass filter 27, attenuator 28 and switch 29. With switches 22 and 29 closed a regenerative feedback path exists across the repeater. With the frequency of oscillator 25 slightly differing from that of oscillator 20 a wave having passed through the repeater and the feedback path returns to the input of filter 8 with a slightly changed phase for each traversal resulting in the arrangement oscillating with a slowly changing frequency progressing from one end of the pass band of filter 8 to the other, and dependent in amplitude on attenuators 23 and 28. The oscillation stops when the loop gain is insufficient to maintain the oscillation but will start again at one of the possible regeneration frequencies and progress to the end of the pass band. If only part of the band is being traversed by the frequency a starting oscillator 30 may be connected by switch 31 to supply waves at a frequency lying at one end of the I.F. band to one stage of amplifier 11, these waves predispose the circuit to oscillate at their frequency. By this means the frequency of oscillation is caused periodically to sweep the whole pass band of the repeater. A signal at 4200 Mc/s. for the measuring circuit is received via hybrid junction 16 from the output of amplifier 14 and is supplied to a test modulator 33 via hybrid junction 35 and to a delay device 34 via hybrid junctions 35 and 36. Heterodyning waves at 200 Œ 1 Mc/s. are supplied to modulator 33 from oscillator 25 via switch 37 and one of the modulated side bands is selected by band-pass filter 38 and supplied to test modulator 39 which is also connected to the output of delay device 34. The lower side band is selected by band-pass filter 40 and supplied via amplifier 41, amplitude limiter 42 to a suitably tuned frequency discriminator 43 designed to supply voltage proproportional to the input frequency to one of the vertically deflecting plates of the oscillograph 32, the output frequency from filter 40 being equal to the frequency of oscillator 25 plus or minus the difference df between the oscillation frequency of the repeater and its regenerative circuit at two instants differing by the delay t introduced by device 34. If the group delay introduced by the repeater is constant then df is constant and the voltage from discriminator 43 will be constant and a trace parallel to the time axis will be obtained. The oscillograph timebase is responsive to a band-pass filter 44 connected to junction 36 and having a narrow pass band at the beginning of the frequency sweep of the circuit oscillation, produces a short pulse at the beginning of each cycle to trigger a single stroke timebase circuit 45 whose output is connected to one of the horizontally deflecting plates of oscillograph 32 ; the circuit 45 being adjusted so that a complete horizontal sweep occurs in the same time as the varying oscillation frequency takes to traverse the pass band of the repeater, whereby a measure of the group delay is displayed against frequency on the oscillograph. If the frequency of oscillator 25 is unsuitable for discriminator 43 a separate oscillator 46 may be connected via switch 37 to modulator 33. The amplifier 11 (Fig. 2, not shown) may consist of three stages, the centre one having automatic volume control and being connected to the starting oscillator 30 via a switch and an adjustable attenuator, the oscillator may also be connected to any convenient part of the circuit. A modification to the measuring circuit (Fig. 3, not shown) omits elements 34, 36, 37, 39 and 40, the oscillator 46 at a microwave frequency being directly connected to modulator 33, amplifier 41 being connected to filter 38 and filter 44 connected to junction 35. The vertical deflection of the oscillograph is determined by the oscillation frequency of the repeater and not by its rate of change as a measure of group delay. In a modification, Fig. 4, for testing the two-way terminal of a repeatered microwave link, the receiver comprises elements 1, 2, 7-10 of Fig. 1 in conjunction with an amplifier 47, e.g. a travelling- wave amplifier, inserted between elements 8 and 9, and the transmitter includes elements 4, 5, 6 and 11-16 of Fig. 1 with elements 17-21 providing heterodyning signals of slightly different " forward " and " return " frequencies for modulators 9 and 12 as in Fig. 1 but filter 10 is not directly connected to amplifier 11. A signal-frequency band, e.g. a television signal, is supplied to input terminal 48 of the transmitter and then via switch 49 to modulate the frequency of e.g. 70 Mc/s. from oscillator 50 whose output is supplied via I.F. band-pass filter 51 to a frequency-change modulator 12 receiving a frequency of 4270 Mc/s. from oscillator 17, and producing a frequency-modulated 4200 Mc/s. signal for transmission. In the receiver the I.F. band-pass filter 10, following microwave input modulator 9, is connected through amplifier 52 to a frequency discriminator 53 for recovering the input signal band, which is connected through band-pass filter 54, switch 55 to terminal 56 to which is connected a utilization apparatus (not shown). For testing, the conductors 3 and 4 are preferably disconnected from filters 2 and 5 and switches 55 and 49 are changed over, hence converting the apparatus into a radio repeater similar to Fig. 1 except that complete demodulation and remodulation occurs in addition to amplification of the I.F. Junctions 7, 15, 16 being connected to switches 29, 22 and junction 35 as in Fig. 1, oscillation occurs at a progressively changing frequency and is traced on oscillograph 32 (Fig. 1) as before. The hybrid junctions in Figs. 1-4 may be connected at other points in the circuit, e.g. junction 15 may be connected between elements 13, 14 so excluding amplifier 14 from the loop and conductors 3 and 4 may remain connected to the branch filters to test interaction effects between various antennae. To test the matching of the repeaters to the antennae, e.g. if the amplifier 14 is not properly matched with conductor 4, an impedance mismatch may be introduced in filter circuit 5, the waves reflected from this mismatch will be transmitted and reflected at the output of amplifier 14 producing an irregularity in the oscillograph display which may be used as a basis for adjusting the matching of amplitude 14. A similar test comprising several cascaded repeaters may be made, two repeaters (Fig. 5, not shown) similar to Fig. 1, may be arranged in cascade either adjacently or separated by other repeaters and are switched so that the first repeater will oscillate as previously described and the oscillations transmitted to the other repeater via any intermediary repeaters, where they are traced on the oscillograph incorporated therein, the form of the trace being determined by the group delay introduced by the repeaters. A repeater caused to oscillate as in Fig. 1 may be used as a source of test oscillations for testing antennae &c. and the output of amplifier 11 being a source of waves at the I.F. varying periodically over the I.F. band may be used for testing medium-level frequency changers and subsequent apparatus therein. Further a system in which no frequency change occurs between output and input may be similarly tested. In Fig. 6, not shown, the feedback circuit between switches 22 and 29 (Fig. 1) is modified, to produce the desired frequency shift of 1 Mc/s. in the feedback loop by means of two modulators in cascade fed with local oscillations differing by 1 Mc/s. The start and stop frequencies of the frequency sweep band may be defined by inserting in the feedback loop a pair of cascaded attenuation networks (Figs. 7-10, not shown) containing reactive circuit elements respectively and critically resonant at the start and stop frequencies whereby the attenuation inserted in the feedback loop is reduced sharply at the start frequency and increased sharply at the stop frequency.