Power levels may be measured over a wide range with the 0.1 dB resolution digital display in the receiver auto balancing mode or with a 0.01 dB resolution combined analogue and digital display in the differential level mode. An r.m.s. responding detector is incorporated and level indication is in dBm or dB relative to 0.775 V. Frequency is digitally displayed to 10 Hz resolution . 2. For manual operation the sender and receiver may be operated independently. In sequence mode opération, the sender automatically steps through a sequence of frequency/level increments. For synchronous operation, coaxial cable interconnection permits sender frequency to be controlled from the receiver. The addition of optional GPIB* interfaces greatly extends the versatility of both instruments and provides two levels of programmable operation. Firstly, the sender acts as a bus controller of the receiver and, optionally, a printer. This control capability gives the selective level measuring set several powerful modes of automatic operation. Secondly, both sender and receiver may be programmed via the bus as part of a larger automatic measurement system, in which case the sender's own control capability is overridden. Sender versions 52356-301U, -302Y and receiver versions 52357-304X, -305M are factory fitted with GPIB interfaces but for other versions conversion is a simple retrofitting exercise. 3. Both instruments may be fitted with an optional rechargeable battery^ pack providing approximately 4 hours continuous operation following approximately 14 hours charge and the instruments conform to IEG 348 and BS 4743 safety regulations. 4. Application of the instruments may be considerably extended by the addition of associated MI bus-controllable instruments. These include a coaxial switch unit for scanning up to 56 separate inputs and a baseband filter unit to extend the receiver's bandwidth capability to 'blocks' of telephone channels from group power measurement up to total baseband power.