17
gates are controlled by signals from the microprocessor. These are NOISEGATE 2 on U40 and
NOISEGATE1 on the first noise gate U51. The receiver comes with a filter F9 fitted as standard. However,
either a narrow filter, F4 or one of two other special purpose filters can be fitted instead. The required filter
is selected by analogue bus switches U35 and U36.
455 kHz IF amplifier
The filtered signal feeds a two stage high gain AGC controlled amplifier using depletion mode dual gate
MOSFETs Q8 and Q9. The AGC control voltage comes from the DSP. The drain load 1mH inductors L17
and L18 are broadly tuned to 455kHz with C108 for L17 and C109, for L18. The output voltage swing can
be quite high at around 10 volts peak to peak on a strong input signal. This is sufficient to overload the
following product detector an AM envelope detector. The output level is made more manageable by splitting
Q9 output tuning Capacitor into a voltage divider with C253 and C254. This output feeds both the envelope
detector Q7 which is biased as a rectifier, and the product detector (U52 and U57).
AGC
The envelope detector, transistor Q7 is the AGC rectifier. The output is taken from the emitter of Q7 via the
resistor R179. This is shown as the SSL line on the RF audio section of the schematic. The SSL signal is
then processed by the DSP. The AGC characteristics are controlled by the DSP. The returned AGC signal
from the DSP is applied to the MOS FET's Q8 and Q9 gain control gates.
Carrier insertion generator
The transmit and receive Carrier Insertion generators are used to produce CIO Tx and CIO RX, whose
frequencies are dependant on the type of IF filter used, and the mode of operation chosen but is in the
region of 455 kHz. Theses are generated directly by the microprocessor U39 and are CMOS level square
waves.
Third mixer (Product detector)
This is a high level circuit around U62 and U57B, and IF signal is switched by the carrier re insert signal
CIO-RX. This mixer works in a different way to the previous mixers. In this mixer the output voltage from
the IF amplifier is converted to a current in R32. This current is either shunted to ground via C90 or into the
summing junction (virtual ground) at the input of U57B. Capacitor C179 filters the unwanted IF component
of the signal. R32 also acts as a dissipating load for this part of the signal. The low value resistor R110 is
to keep U57B from generating high frequency noise. Extra high frequency roll off is obtained with the next
active low pass filter stage in U57A and surrounding components. This product detector has excellent low
distortion properties because the FETs in the product detector switch U62 are only switching current and
this avoids the change in resistance of the FETs with changing signal voltage levels, which can be a source
of distortion. The audio from U57A (RX AUD) is fed to the DSP U2 on the microprocessor side of the PCB,
where it is further processed as required for a particular application. The signal emerges from the DSP and
is converted back to an analogue signal by Digital to Analogue Converter U63 and it appears on OUT 3 (pin
7).
Speaker output
The audio signal emerging from the above mentioned DAC is fed to an anti-aliasing filter around U4. This is
used to remove clock frequency components from the DAC. The now processed audio passes on to half of
the dual volume control circuit U23B. This is essentially a resistor with 256 taps. The volume is controlled
by an 8 bit serial word. The control lines (SCLK and SDIN) are shown on U23A on the microprocessor
circuit diagram.
From the volume control (U32B) the signal now passes on to the audio power amplifier U19.This can deliver
2 watts into an 8 ohm load.