he Model FIM-72A is a portable, laboratory quality Field Strength Meter designed for rigorous field applications. Combining a calibrated half-wave dipole antenna and a highly accurate tuned voltmeter with a range of 140 dB, the FIM-72A is suitable for practically all types of RF emission measurements in the 450 MHz to 960 MHz spectrum. The operator can switch select wide or narrow bandwidth, peak or average value of TV or pulse modulated signals, AM or FM demodulation, and meter dynamic range of either 20 dB or 60 dB. A dc analog voltage, proportional to meter indication, is provided for driving a chart recorder or similar device. A leveled output from the calibrating generator is available for measuring cable insertion loss, filter response, amplifier gain, VSWR, and other signal ratio measurements. The 4-1/2 inch, taut band, mirrored scale meter is calibrated in volts and dB for precise measurements in field or laboratory environments.
The tuned voltmeter is a double conversion, super-heterodyne receiver with carefully tailored sensitivity, selectivity, gain, and linearity characteristics. The RF input includes preselection designed for minimum VSWR and maximum out-of-band signal rejection. Uniform gain, independent of IF bandwidth, is provided by temperature compensated RF & IF amplifiers utilizing a combination of MOSFET, bipolar, and monolythic integrated circuit devices. The linear detector is followed by a logarithmic shaping circuit which drives the meter in the LIN (20 dB) mode. In the LOG mode DC feedback is applied to the receiver in such a manner that the meter indication (in dB) varies linearly over a one-thousand-to-one range of input levels.
