An intermodulation distortion analyzer generates two pairs of sinusoidal test tones to serve as a test signal for the channel under test. The two pairs of tones simulate two respective noise band test signals but eliminate the long time averaging required for measurements when noise bands are used. A highly linear AGC circuit employs sampling at an output-controlled duty cycle to maintain a constant reference level for the analyzer. This reference level permits automatic distortion measurements to be read out directly in db below the test signal. An RMS detector circuit for second order intermodulation products employs feedback control to maintain the input signal to a squaring circuit constant. Squaring of the constant level sinusoids produces RMS DC components which can be separated for direct measurement. A distortion circuit provides known levels of second and third order intermodulation in the test signal to permit accurate check out of the analyzer.

A movable telephone circuit test instrument is provided, indicators mounted thereon which show the communicating state of a telephone circuit being tested. Supervision of the circuit under test is thus provided at the same location as the tester, thereby reducing the number of persons required to carry out the test. The supervising indicating means are preferably light emitting diodes and are provided in a unique driving circuit that includes a variable conductance parallel current path for maintaining the brightness of the light emitting diode substantially constant regardless of the magnitude of current being monitored by the supervising circuits.