— —
17
panel
BALANCED SCA AUDIO
connector.
IC27A is a driver stage for the front-panel
PHONES
jack. This jack
monitors demodulated SCA audio in all measurement modes except the
CALIBRATE (±75kHz)
position, in which IC28C instead routes de-
emphasized main-channel audio to the jack.
THIRD(!) CONVERSION (Schematic Sheet 2)
LO and Mixer
The 25kHz IF signal is fed to a third mixer, IC30; the local oscillator for
this conversion is 25kHz from crystal oscillator/divider IC29.
Grammar-school arithmetic suggests a third IF frequency of either
50kHz or zero. The latter is actually the case. Politically-correct or not,
this “homodyne” technique allows a simple low-pass filter to establish
IF selectivity.
IF Filtering
The third IF filter incorporates IC21B, IC21A and IC20A and has a 6-
pole low-pass function. IC19A compensates filter phase response, and
IC19B makes-up conversion loss. The effective IF bandwidth at this
point is about 3kHz. This third IF signal is useful only for injection-
level metering in the
NARROW (DATA)
position of the front-panel
FILTER
selector. Down-converted essentially to DC at this point, the
signal is useless for further demodulation or audio monitoring.
METERING (Schematic Sheet 1)
Metering
Selector
The front panel up/down
METER SELECT
buttons are read by the PIC
microcontroller, which in turn sends a 2-bit binary address to a dual, 1-
of-4 analog switch, IC10. IC10A lights LEDs to indicate which scale and
metering function have been selected, IC10B routes the selected signal
to the metering rectifier.
In the 540’s
CALIBRATE (±75kHz)
mode, a composite/MPX sample from
input buffer IC7A is delivered to the meter. For subcarrier injection
measurements, both the
PERCENT INJECTION
and
±kHz DEVIATION
LEDs are lighted, and the input to the metering circuit is either the
WIDE (SCA)
or the
NARROW (DATA)
IF signal from
FILTER
selector
IC28B. Calibrated,
FLAT
SCA program audio is fed to the meter for
SCA
±kHz DEVIATION
measurements, and for an indication of
SCA AUDIO
LEVEL
, either
FLAT
or
DE-EMPH
SCA program audio is delivered to the
metering circuit to be read on the lower dB scale. This level is factory
calibrated to read 0dB at ±5kHz SCA subcarrier deviation, but this may
be set to another reference value if desired. (See Page 20.)
Meter Rectifier
Gain stage IC22A feeds a full-wave, peak-responding rectifier using
IC23A, IC24A, IC24B and the four associated diodes. An input to the
meter immediately charges C66 to the instantaneous peak value.
Discharge of C66 is though constant-current source Q3. IC22B is a
slope detector which gives a positive output pulse whenever a new peak
exceeds the existing value. This pulse turns Q2 on, charging C65 which
