A Sweep-Frequency Spectrum-Analyzer for Recording in the Range 1 to 45 Cps on a Paper Chart Recorder

摘要

The analyzer is designed for recordings of signal variation between about 0.5 and 100 microvolts. The signals from an antenna are led to a preamplifier with a voltage gain of about 10,000; the input impedance of the preamplifier is almost 500K ohms. In the mixer the signal from the preamplifier is mixed with 2 oscillator signals, one from a 1000-C fixed-frequency oscillator and the other from a variable-frequency oscillator sweeping from 2455 to 2545C. The output of the mixer is fed through an amplifier and into a tuning fork filter with a center frequency of 1500C and a bandwith of 0.5C. The 1500-C signals are amplified in a 3-stage audio amplifier and fed to a rectifier bridge which then gives a dc voltage proportional to the 1500C in the signal coming from the mixer. This voltage is large enough to drive a paper chart recorder. A 100-ohm continuous rotating potentiometer is mounted to the axis of the paper-drive mechanism of the pen recorder.