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 four-terminal network selectively convertible into a low-pass, high-pass, band-pass, all-pass or band-stop filter comprises three cascaded stages each including an operational amplifier with a grounded noninverting input, the amplifiers of the first two stages being of the integrating type provided with capacitive feedback circuits. Resistive feedback connections extend from a main output A of the third stage to the inverting inputs of the first and third stages and from a relatively inverted output A' of the third stage to the inverting input of the second stage, their respective resistances R.sub.u, R.sub.v, R.sub.w determining the coefficients of the denominator in a biquadratic equation ##EQU1## where H(p) is the transfer function of the network and p=jf/f.sub.n is given by the ratio of an operating frequency f to a reference frequency f.sub.o. The coefficients of the numerator, which could be of either sign and may also be zero, are determined by supply resistances R.sub.a, R.sub.b, R.sub.

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.