Transistor Q1-configured as a source-follower buffer stage, offering a bit under unity voltage gaingives tbe unit a high-impedance input of about 1 MO shunted by about 10 pF, which keeps only minimal loading on tbe equipment being tested. C1 serves as input de blocking capacitor. The Q1 output is coupled hy C2 to a simple AM detector circuit made up of D1, D2, R3 and C3.
Capacitor C4 provides output de blocking. Total current consumption should be somewhere around 1 mA. The circuit responds to frequencies from 100 kHz to well over 50 MHz.