Built-in group delay testing arrangement for an FM radio system

The baseband input to a frequency modulation transmitter is switchably connectable to a square wave generator, the amplitude of the square wave output being adjusted to provide a predetermined deviation of the FM radio transmitter. The output frequency of a pilot oscillator is also applied to the baseband input and it is subjected to phase modulation because of group delay which exists in the system and which adversely affects the signal during transmission through the system. At the receiving end of the radio system, the receiver output obtains the pilot signal which has been phase modulated by the group delay slope in the system, because of the square wave test signal. This phase modulation is detected in a phase lock loop, amplified, rectified and applied to a meter so as to provide an indication of the group delay slope in the system.

FIELD OF INVENTION 
This invention relates to microwave radio systems, and in particular to 
apparatus incorporated in the system which may be used to measure the 
group delay characteristics of the radio communications link. 
BACKGROUND OF INVENTION 
In the telecommunications field, it is often desirable and frequently 
necessary to ascertain certain transmission characteristics, such as 
relative group delay, modem linearity, and differential phase and gain, of 
a microwave radio communication link (MRCL). In operating and maintaining 
a radio system at its optimum performance, the transmission 
characteristics, and particularly group delay slope distortion, must be 
occasionally measured so that various corrective adjustments can be 
periodically made. For example, fine tuning of RF or IF (Radio Frequency 
or Intermediate Frequency) filter delay equalizers can usually be made to 
correct for small changes in group delay slope in the transmission 
frequency band. 
Most of the practical methods which are employed today to make field 
measurements of radio transmission characteristics use commercially 
available transmission test sets. These test sets simplify the testing 
procedures and also reduce the time required for performing the 
measurements. Typical of such commercial test sets are the GTE Italia CSM 
Base 221-C/222-C Radio Link Test Set (Milan, Italy), the Hewlett-Packard 
3710/3702 Link Analyzer (Palo Alto, Calif.), and the Siemens K1005/K1046 
Sweep Frequency Test Sets. Group delay distortion and the related 
transmission characteristics are measured by these test sets using the 
well-known Nyquist method of measuring group delay, which method uses two 
signals. This test method will hereinafter be called "the two-tone test 
method". This method applies two separate test frequencies in the base 
band frequency group through the FM radio link under test. These two test 
frequencies are generally referred to as the sweep frequency and the 
search frequency (or modulating frequency). The sweep frequency is 
typically a very low frequency signal, in the order of 16 Hz to 100 Hz, 
and may be a sinusoidal or triangular wave form. It is applied to the 
baseband input of the radio at a high level, causing the frequency 
modulator in the radio to deviate the FM carrier frequency over the 
frequency band of interest. Added to the sweep frequency is a low-level 
search frequency, which is an intermediate frequency in the radio baseband 
frequency group. 278 KHz is typically used as the test search frequency. 
This generally accepted test method for measuring group delay distortion 
and the related transmission characteristics is covered in detail in the 
Fifth Plenary Assembly (CCITT, December 1972, Vol. IV.2, "Maintenance," 
published by the International Telecommunications Union, 1973, pp. 
492-507). 
While such test sets can be used to perform the required testing of group 
delay, the disadvantages are the requirement for additional, expensive 
test equipment and the problem of having the test equipment at the 
transmitter and receiver ends of the system. These disadvantages are 
overcome by having the facility built in at a relatively nominal cost. 
SUMMARY OF INVENTION 
A square-wave signal having the amplitude adjusted to provide a 
predetermined deviation is applied to the baseband input FM transmitter. A 
pilot oscillator frequency is also applied to this baseband input. System 
group delay causes phase modulation of the pilot frequency and the 
deviation is proportional to the slope of the group delay. At the receiver 
the phase modulation of the pilot is detected in a phase lock loop, and 
the detected signal is rectified and applied to a meter. The amplitude of 
the rectified signal is used as a measure of the system group delay slope.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the single FIGURE of the drawing, it may be seen that 
switch 4 has terminal 8 thereof connected to path 12 which is the input 
path to the FM transmitter 14. Normal baseband signals are applied via 
path 2, contacts 6 and 8 of switch 4, to the baseband input of transmitter 
14. The pilot frequency is applied from pilot oscillator 16 to a separate 
input of the FM transmitter 14 and this is always connected. The built-in 
test arrangement includes a square-wave generator 20, a square-wave output 
which is connected via path 22 to the input of low pass filter 19 which 
provides sufficient roll-off to prevent interference with the frequency 
from pilot oscillator 16. In testing the technique, a low-pass filter 
having a cut-off at about 10 kHz was used with a 331/3 Hz square wave 
signal and there was no adverse interference with a pilot frequency of 100 
kHz. From the output of filter 19 the shaped signal passes along path 21 
to the input of resistive ladder network 24. The network 24 is employed to 
permit the adjustment of the signal amplitude so as to provide the 
appropriate deviation as required for testing of the group delay of the 
transmission system. The amplitude of the square wave establishes the 
explored bandwidth of the system, and is ultimately determined by the 
channel capacity of the system under test. The frequency of the square 
wave can be at any low frequency and typically would be between 16 and 100 
Hz. Both 331/3 and 50 Hz have been successfully employed in the subject 
circuit. The exact frequency is not critical, except that it must be 
compatible with the passband of the system and that of the test circuitry. 
In the preferred embodiment, a square wave having a 50% duty cycle is 
employed, although this is not critical. The amplitude of the shaped 
signal is determined by the frequency range, e.g. the two carrier 
frequencies above and below the nominal center frequency of the FM system, 
at which the relative difference in delay is to be measured. The properly 
adjusted signal is applied via path 26 and contacts 10 and 8 of switch 4 
to the baseband input of the FM transmitter 14. Thus, the built-in test 
facility does not provide for in-service testing but does provide an 
inexpensive out-of-service built-in test facility, thus avoiding the 
problems of (1) insuring that the appropriate test equipment is at the 
transmitter and receiver locations, and (2) set up and test by using the 
commercial test equipment. 
Because of the low-frequency deviation of the pilot oscillator by the 
shaped signal, any group delay slope which is present in the transmission 
path will cause phase modulation of the pilot, and, thus, will appear as a 
phase modulated signal at the output of FM receiver 34. The output signals 
from the FM receiver are applied via path 36 to the input of pilot filter 
38. Filter 38 is designed to pass only the phase modulated pilot signal 
and rejects the square wave and other, i.e., normal, baseband frequencies. 
The normal baseband frequencies pass along path 39 to standard terminal 
equipment, not shown. And the filtered signal is applied via path 40 to 
phase lock loop 42 in which the phase modulated pilot signal is detected 
and is converted into an AC signal at the output of the phase detector 44 
on path 50 which is amplified in amplifier 52 and applied via path 54, 
node 60 and path 62 to one terminal of capacitor 64. Capacitor 64 is a 
blocking capacitor which prevents any DC components in the detected signal 
in the output of phase detector 44 on path 50 from entering the metering 
circuit. Amplifier 66 amplifies the detected AC signal which is 
proportional to the group delay slope and this amplified signal is applied 
via path 68 to detector 70 in which it is rectified and the DC output 
signal obtained therefrom is applied via path 72 to meter 74. The filters 
and equalizers which may be adjusted in the system are then adjusted so as 
to obtain the minimum meter reading (a "null") on meter 74 and thus obtain 
the minimum group delay slope. Referring now to the phase lock loop shown 
at 42, it should be understood that the standard integrated circuit form 
of voltage controlled oscillator (VCO) may be employed. However, it was 
found that phase jitter from such a VCO was significant enough to limit 
the sensitivity of the test circuit. Thus in a preferred embodiment of the 
circuit, the standard integrated circuit VCO was replaced with a voltage 
controlled crystal oscillator (VCXO). By so doing, sufficient improvement 
in sensitivity was obtained so as to make the test useful in commercial 
radio circuits. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that change in form and detail may be made 
without departing from the spirit and scope of the invention.