DIRECT-CONVERSION MIXER USING SBL-1-1 DBM
U1 is a bouble-balanced mixer from MINI-CIRCUITS SBL-1-1, the RF input to pin 1 accommodates signal levels up to +1dBm. The local oscillator signal input is applied through capacitor C2, the signal must be in the order of +7dBm. The mixer output is matched by the RC network C3, R6 where the capacitor limits operation to high frequencies. Q1 is used as a grounded-base input amplifier to the audio chain. This amplifier applies input signal across the emitter-base path, and takes the output signal from the collector-base path. C5 and L1 provide the low pass filtering for audio signals and reject residual VFO and RF signals. Q2 consists of the preamplifier which is powered by a +12v source. This circuit works well for frequencies up to 400MHz and can be adapted for any application.