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 communications link (MRCL). In operating and maintaining a radio relay system at its optimum performance, the transmission characteristics, and particularly group delay 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 in the transmission frequency band.
The practical methods which are employed today to make field measurements of radio transmission characteristics all use commercially available transmission test sets. These tests sets simplify the testing procedures and also reduce the time required for performing the measurements. Typical of such commercial equipment are the GTE Italia CSM 221C/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. One signal is the carrier and the other is the modulating signal. Either amplitude or frequency modulation may be employed. This test method will hereinafter be called "the two-tone test method." This test method applies two separate test frequencies in the baseband 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 (f.sub.sw) is a very low frequency signal (typically 16 Hz to 100 Hz) and may be a sinusoidal or triangular waveform. 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 (f.sub.mod), which is an intermediate frequency in the radio baseband frequency group. Although the group delay response of a system is generally independent of the modulating frequency used, 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, volume IV.2, "Maintenance", published by the International Telecommunications Union, 1973, pp. 492- 507.
The disadvantage of using the above commercial test equipment with the existing recommended test procedures to measure group delay and the other transmission characteristics is that these test procedures require the MRCL under test to be taken out of service to perform the various measurements. The in-service traffic is always removed from the radio link before testing to prevent the test signals from interfering with the traffic information and to keep the traffic signals from interfering with the measuring test equipment. To keep the radio link operational, the traffic is usually rerouted onto another transmission facility before any transmission testing is begun. On hot standby radio systems, transmission testing presents a problem since alternate microwave channels are not usually available to carry the traffic information.
The invention described herein provides an in-service method of measuring the important transmission characteristics of an MRCL without the necessity of rerouting the traffic onto another transmission facility.