Abstract: A test object, such as a communication path used for data transmission, is examined by applying to its input a measuring signal M consisting of several simultaneously or sequentially generated test frequencies which may be harmonically interrelated, extracting from the output of the test object a distorted version E of the measuring signal, locally generating a compensation signal K with frequency components matching respective test frequencies, and subtracting the two signals E and K from each other to obtain a residual signal R=E-K. The compensation signal K is produced by one or more local oscillators and as many frequency converters under the control of regenerating circuitry which locks each of its components in phase and amplitude to the corresponding test frequency. Regeneration takes place in a different frequency range to which the residual signal R is transposed and from which the compensation signal K is retransposed with the aid of a locally generated carrier.

Abstract: To measure the transmission characteristics of a signal line or other four-terminal network, a multifrequency test signal is fed through that network via a first channel to one input of a differential amplifier whose other input receives via a second channel a substantially identical comparison signal. A difference signal in the output of the differential amplifier, after rectification and possible filtering, is used for manual or automatic adjustment of an amplifier (or attenuator) and a delay unit in either or both channels until the magnitude of that difference signal is a minimum, its residual value indicating frequency-dependent distortions such as background noise or phase jitter not removable by broad-band adjustment of signal level and phase delay.

Abstract: A manually operated direct-current generator, with an output voltage whose polarity and magnitude depend on the direction and speed of rotation of a control knob, charges a storage capacitor in an input circuit of an operational amplifier which works into two comparators respectively responsive to positive and negative voltages exceeding selected thresholds. A triggerable pulse source, upon actuation by either comparator, feeds back one or more discharge pulses to the storage capacitor and also steps a reversible pulse counter in either a forward or a reverse sense.

Abstract: To test the performance of a PCM terminal operating in the TDM mode, a simulated message or noise signal is generated by extracting a recurrent sequence of code words from a read-only memory, the stored code words representing a set of values logarithmically related to sucdessive amplitudes of a sine wave or other periodic oscillation to be reproduced. These words are read out at a fixed scanning frequency f.sub.o but with skipping of (p-l) memory stages if a signal frequency pf.sub.o is to be simulated. A volume selector generates supplemental code words which are additively or subtractively combined with the extracted code words to simulate a desired voltage level. The resulting code words are compressed in a code converter in which the quantum steps of the segments of a compander characteristic, conforming to the chosen logarithmic function, are stored in another read-only memory.

Abstract: To measure the level of test signals over a frequency range of variable bandwidth B, bounded by a lower frequency f' and an upper frequency f", these signals are heterodyned with a variable modulating frequency to produce a frequency spectrum encompassing the pass band of an intermediate-frequency filter of fixed bandwidth b.ltoreq.B, centered on a frequency f.sub.c. By periodically shifting the modulating frequency between a minimum value of f'-f.sub.c +(b/2) and a maximum value of f"-f.sub.c -(b/2), either continuously or in steps, the frequency range of bandwidth B is scanned by a window of bandwidth b transposed to the pass band of the filter as the midfrequency f.sub.m of the window varies in a sweep cycle between f'+b/2 and f"-b/2. The signal level in the output of the filter, integrated over the sweep cycle, is weighted with a corrective factor B/b.