Power Supply And Rx/Tx Switching; Receive Audio - ozQRP MDT Construction Manual

6.5 P S RX/TX
OWER UPPLY AND SWITCHING
When the PTT is operated the TX/RX relay is energized and the transmit signal is passed to the
antenna. When the PTT is not operated the relay switches the antenna through to the receive
circuits. The relay also switches power to the TX and RX sections as required
Diode D8 provides reverse polarity protection. With the power supply connected with the right
polarity, D8 will pass current onto the board. There is a small voltage drop of about 0.3 volts but
this has minimal effect on transmitter performance. If the supply is connected in reverse D8
will not conduct and the board will be protected.
6.6 R
ECEIVE AUDIO
The low level audio signal from the mixer is applied to the audio pre-amplifier stage (U1A). This
is formed from one half of an LM833 dual low noise op-amp and has a gain of around 100 or
40dB. A reference supply for the non-inverting input is obtained from R33, R35 and C40. The
high frequency response of U1A is limited by C41. The output of U1A is DC coupled to the 2 pole
low pass filter (U1B) with a 3dB cutoff frequency of 2400Hz. This stage has only unity gain but
its main task is to set the receiver bandwidth.
The output of the low pass filter is coupled to the AF gain control VR4 via a 1uF capacitor (C44).
A LM386 (U2) is used to boost the signal from the AF gain control to drive a set of headphones.
The gain is set by C47 and R42 to about 50 times or 34dB, while resistor R43 limits the audio to
a comfortable level for headphone listening. If an external low impedance speaker is used
instead of headphones, then R43 can be replaced with a link.
A MOSFET (Q9) is turned on in TX mode and shorts the audio signal to ground. Even though the
audio amplifiers are switched off during transmit, this is still necessary because microphone
signals can make their way through to the headphones while the audio amplifiers are turning
off.
MDT Construction Manual – Issue 2 Page 11