Sound IF board
Sound IF board
Tunable sound IF
The active device in the VCO section of the soundboard is a MPF102 signal FET. The frequency determining components are the 10uH coke and a BB212 varicap diode. The output driver amp uses a BC549 transistor. Bias resistors from the supply to the base of the transistor are used as a switch. The output signal from the BC549 splits two ways, one path returns to the PLL chip and the other goes to the output preset pot. The output of each preset pot is connected to a resistor-combining network.
The chip used is MC145149 dual PLL that is serial driven. The PLL referance comes from the common 10 MHz clock at TTL level. There are two open drain switches built into this chip. These switches are connected to the bias of each of the two drive amplifier stages. Driving the MC145149 PLL is a protocol converter, the MC68HC908 QTI chip. Phase Detector outputs from the dual PLL pass through a three-stage filter on to the non-inverting input of the LM358 dual Op-Amp. The sound is fed in via a 50uSec pre-emphasis RC network to the inverting input. On the output of the Op-Amp is the combined DC voltage and audio modulation, which is fed back to the varicap to provide its control and bias voltage.
