Receive Audio - ozQRP MST400 Construction Manual

3.5 R
ECEIVE AUDIO
The 10MHz SSB intermediate frequency (IF) signal from the crystal filter is applied to the
product detector U2. The product detector is formed with another SA612 and mixes the IF
signal with the 10MHz BFO signal to produce an audio output. The BFO is adjusted slightly
above the crystal filter upper response so that Lower Side Band signals are detected correctly.
A low noise balanced input audio amplifier is formed with U3 a NE5534. A reference supply for
the non-inverting input is obtained from the Zener diode stabilized supply for U2. The high
frequency response of U3 is limited by C35 and C42, while capacitors C37 and C36 reduce the
low frequency response.
The output of U3 is fed to a 1uF coupling capacitor and 1K resistor (R28) to the AF gain
potentiometer. When switching to transmit mode MOSFET Q5 is turned on rapidly through D9,
charging C45, and shorting the audio line to ground. When returning to receive mode Q5 slowly
turns off as C45 discharges via resistor R29. This circuit produces a very nice action eliminating
unwanted audio clicks and pops when switching between receive and transmit modes.
Audio from the AF gain control is amplified by U4, an LM386, to drive a loudspeaker. U4 is
configured for extra gain by including C41 and R26. C40 guards against RF entering the
amplifier input, while R30 and C47 help stabilize the output.
MST Construction Manual – Issue 1 Page 8