Transmitter spectrum analyzer

A spectrum analyzer for 7 MHz single-sideband transmitters of two tone signals at 1 and 1.8 kHz, respectively, on a 14,200 kHz carrier, has been developed as a means for monitoring the amplifier linearity and the intermodulation level. The instrument includes an adjustable attenuating resistor, a resistive decoupler, a ring mixer (KD503 or equivalent high-frequency silicon diodes) separated from the heterodyne stage (quartz oscillator and KP302 or KP303 field-effect transistor) by an emitter follower (KT312 or KT606 transistor), two low-pass filters (LC filter and second-order active filter using an operational amplifier), a low-frequency a.c. millivoltmeter using an operational ammplifier, a transformer (PEV-2/0.3 wire on ferrite ring core), a 50-200 microammeter, and a light-emitting indicator diode. Adjustment of the analyzer for operation begins with checking the performance of transistors in the d.c. mode, followed by trimming a LED resistor for threshold-sensitivity current, applying an audio signal, and oscillographically recording the amplitude-frequency characteristic, then applying a high-frequency signal.