Sideband Analyzer for AM Transmitters

This inventon relates to a sideband analyzer for amplitude modulation transmitters and more particularly to the measurement of sideband amplitude response and/or sideband group delay response of such transmitters capable of broadcasting television.

The present method for measuring the performance of television transmitters is to measure the video-in to video-out group delay characteristic. A tone is modulated onto a video sinewave signal, and this signal is applied to the transmitter under test. An envelope detector detects the tone from the output of the transmitter under test, and the detected tone is applied to a phase meter. The original tone is applied through a phase shifter to the phase meter. The phase shifter adjusts the phase of the reference tone signal at some reference video frequency such as 200 KHz (kilohertz) to zero phase relative to the phase of the demodulated tone signal. This adjusts out any average phase shift through the unknown circuit, and the group delay is then measured relative to that at the reference video frequency (group delay at reference video frequency is taken at zero). A fault in this measurement is that the group delay characteristic over the lower sideband region and the upper sideband region is not measured. A composite effect of both sidebands is measured. Also, included in the measurement are the effects of the vestigial amplitude response slope of te RF demodulator, and the overall phase characteristic of the RF demodulator.

Briefly, a sideband analyzer for amplitude modulation transmitters is provided by amplitude modulating a given frequency signal with a tone frequency. The amplitude modulated given frequency signal is applied to an amplitude modulation transmitter producing both upper and lower sideband signals. A sideband selector is provided for sequentially selecting separated sideband signals. A detector coupled to the selector detects the tone on the selected sideband signal. The amplitude of the detected tone indicates the amplitude response of the transmitter at that then selected sideband frequency, and the phase of the tone when compared to a reference phase provides an indication of the group delay response of the transmitter at the selected sideband frequency.

A detailed description follows in conjunction with the following drawing which is a block diagram of a television sideband group delay and amplitude response analyzer according to one embodiment of the present invention.

Referring to the drawing, a crystal controlled oscillator 11 for a television transmitter analyzer system applies a fixed video frequency signal F1 of 130 MHz to a first mixer 13. A sweep oscillator 15 applies a frequency which sweeps from 120 MHz to 140 MHz (130.+-. 10 MHz) at a rate of 1/10th of a second for example. The sweep width would depend on the bandwidth of the TV system to be measured. A frequency sweep from 125 MHz to 135 MHz would be used for many TV system measurements. The sweep width of 20 MHz is selected in the arrangement described herein since sideband frequencies 10 MHz above or below the carrier frequency may be of interest in a television system or in a double sideband system. The difference signal at mixer 13 is provided to video amplifier 17. This video signal from mixer 13 sweeps from 10 MHz to zero to 10 MHz. The amplified sweeping signal of 10 MHz to zero to 10 MHz from the amplifier 17 is amplitude modulated at modulator 19, with a fixed frequency tone F.sub.g from generator 21. This tone F.sub.g may be 20 KHz for example. The amplitude modulated signal from modulator 19 is raised at a video amplifier 23 to a sufficiently high level to be processed by the television transmitter 27 under test. The amplified amplitude modulated signal is applied through emitter follower 25 and lead 26 to television transmitter 27. The emitter follower 25 is arranged so that the signal on lead 26 matches the transmitter input impedance. After processing the signal through the unknown transmitter circuit 27, the signal is amplitude (AM) modulated to a carrier signal Fc. The AM signal applied to the transmitter may be represented as

where F.sub.v is the search frequency and .omega..sub.v is 2.pi. F.sub.v and F.sub.g is the tone frequency, and .omega..sub.g is 2.pi. F.sub.g m.sub.1 is the modulation factor on the search signal by the tone signal.

The signal out of the transmitter is picked up by a coupling loop 31 and is applied to RF mixer 37. The transmitter output signal with the carrier component normalized to one modulation factor m.sub.2 may be represented by: ##EQU1## where F.sub.c is the carrier frequency and .omega..sub.c is 2.pi. F.sub.c, m.sub.2 is the modulation factor on the carrier signal by the test signal.

A source of carrier signals at the carrier frequency F.sub.c may be provided by source 29. The source 29 may be a crystal controlled source, a tunable oscillator controlled by a signal coupled from the transmitter 27, or may be an amplifier for the carrier signals F.sub.c provided by the transmitter 27. The signals at Frequency F.sub.c are applied from source 29 to an RF mixer 33. The sweep frequency signals of 120 MHz to 140 MHz from sweep oscillator 15 are also applied to mixer 33. The mixed signals at mixer 33 are amplified and passed through high pass filter 35 and applied to mixer 37. The high pass filter 35 is selected to pass the sum term of F.sub.c + 130.+-. 10 MHz. This high pass filter 35 may, for example, be a nine section, 0.01 db maximum ripple Chebyshev-type filter. This signal of F.sub.c + 130 MHz.+-. 10 MHz from filter 35 is mixed at mixer 37 with the RF signal coupled from the transmitter output viz loop 31 which may be represented as F.sub.c .+-. (10 MHz to 0 to 10 MHz).+-. F.sub.g. A selective filter 39 coupled to the output of mixer 37 passes 130 MHz, 130 MHz+ F.sub.g (tone) and 130 MHz- F.sub.g (tone) to mixer 41. As the sweep oscillator 15 scans from 120 to 140 MHz, the signal at frequencies F.sub.c + 120 to F.sub.c + 140 is mixed with the signals at frequencies F.sub.c - 10 MHz.+-. F.sub.g to F.sub.c to F.sub.c + 10.+-. F.sub.g MHz and the transmitting system lower and then upper sidebands are tracked at a constant frequency F.sub.k of 130 MHz. At the low end of the sweep (the low end of the lower sideband being swept) the 130 MHz signal is being coupled to mixer 41 due to the mixing at mixer 37 of the F.sub.c + 120 MHz frequency signal from filter 35 and the F.sub.c -10 MHz frequency signal from the transmitter 27. At the middle of the sweep where the F.sub.c+ 130 MHz frequency signal is with with F.sub.c + 0 frequency signal, the central carrier of 130 MHz is being coupled to mixer 41. At the high end of the sweep (upper end of the upper sideband), the F.sub.c + 140 MHz frequency signal is mixed at mixer 37 with F.sub.c + 10 MHz frequency signal from the transmitter 27 and the difference signal of 130 MHz is coupled to mixer 41. The frequencies given for the extreme lower and upper sidebands would not be available for some TV systems, such as at the output of a vestigial sideband filter for a typical U.S. television system or C.C.I.R. System M. Lower sidebands of interest may be from carrier to 0.75 MHz below carrier. At the sideband 0.75 MHz below carrier the frequencies would be: F.sub.c + 129.25 MHz signal from filter 35 and F.sub.c -0.75 MHz signal from the transmitter 27. Upper sidebands of interest may be from carrier to 4.2 MHz above carrier. At the sideband 4.2 MHz above carrier the frequencies would be: F.sub.c + 134.2 MHz mixed with F.sub.c + 4.2 MHz. The constant frequency signal of 130 MHz is AM modulated with the F.sub.g tone signal and the upper and lower tone signal F.sub.g sidebands are phase shifted relative to the 130 MHz the same amount as the tone signal sidebands are phase shifted relative to the transmitter video sidebands. The phase of the signal at the output of the high pass RF filter 35 has no effect as the phase angle is transferred the same to the 130 MHz signal and its sidebands.

Crystal oscillator 43 applies 140.7 MHz signals to RF mixer 41. A 10.7 MHz difference signal is coupled out of the mixer 41 and applied to IF amplifier 45. The amplified output from the IF amplifier 45 is applied to detector 47 and the tone signal at F.sub.g is detected. One of the outputs from the detector 47 is an amplitude response indicator. As the sweep oscillator sweeps signals from 120 to 140 MHz, for example the indicator 49 indicates the amplitude responses of the detected envelope over the lower to upper sidebands. The indicator 49 could be a meter or an oscilloscope with the sweep circuit controlled by oscillator 15.

Also at the output of detector 47 is a selective filter 51 which is adapted to pass the detected tone signal of F.sub.g. The detected tone signal at F.sub.g from detector 47 is passed through an amplitude limiter 53 and is applied to a phase comparator 55. A tone signal F.sub.g from the tone generator 21 is also applied to the phase comparator 55 through a separate amplitude limiter 57. The amplitude limiters 53 and 57 are arranged to provide substantially equal gain tones to the comparator 55. A phase shifter 61 is coupled between the generator 21 and limiter 57 to adjust out the average delay in the system. The detected phase difference provides the upper sideband and lower sideband group delay characteristics of the system. This phase difference is coupled to a group delay indicator 59. The indicator 59 may be an oscilloscope which is scanned by the signals from the sweep oscillator 15 with the phase difference at the phase comparator 55 providing an amplitude response proportional to the phase difference. This amplitude varying signal is applied to the oscilloscope. The following is a mathematical analysis of how the phase of the detected tone signal F.sub.g, indicates th group delay.

Group delay is defined as

where .DELTA..theta. is phase shift in radians

.DELTA..omega. is a frequency increment of radians per second

The phases, in radians, of the tone frequency sidebands relative to the search frequency phase are:

and

where,

where, .theta..sub.f.sbsb.g is the phase in radians of the detected tone signal, f.sub.g and .theta..sub.f.sbsb.v is the phase in radians of the search frequency f.sub.v,

As F.sub.g is a constant frequency the phase indicator can be calibrated in group delay.