source of Q65 thus producing a 455 kHz second I.F.
signal.
This signal is then amplified by Q77, the gain of
which is controlled by AGC2, and applied to one of three
bandpass filters of 7.0, 4.0, or 2.3 kHz bandwidth as
selected by switches U16 and U17, depending on the
states of lines BW_SELA and BW_SELB. The output
from the selected filter is applied to I.F. amplifiers Q36
and Q78. The gain of Q36 is controlled by R429 and
the gain of Q78 is controlled by AGC1.
Envelope Detection, AGC, and S-Meter
The output of Q78 is amplified by I.F. amplifier Q39
and is envelope detected in Q40. Q41 provides a
constant current sink for Q40. The detected audio
output and AGC voltage is buffered by U1 (pin 1 output).
The audio is AC coupled to switch U10 which,
depending on the state of the ENV/PROD DET line,
selects between envelope and product detection and
feeds the selected audio mode on through an active
audio low pass filter (U5) to the HF_AUDIO line.
AGC output from pin 1 of U1 is damped by R421
and C250 and buffered by Q42. Some smoothing is
performed by CR39 and R309 (and R270 for fast AGC).
When signal is present on the base of Q42, it turns on,
thus charging capacitor C569 through R569 and diode
CR39. Switch U11 switches in additional R/C
combinations in parallel with R569 and C569 depending
on the desired decay time constant as selected by the
status of the AGC/F/S and AGC INH lines. R317 and
C213 are selected for Slow, R465 and C237 for Fast,
and only R569 and C569 for very fast (for scanning). U1
(pin 14 output) serves to precharge the slow AGC decay
capacitor to the proper voltage when it is not selected.
When signal is removed, C569 (and additional
selected capacitors as described in the previous
paragraph) discharges through R569, R309 and R175. If
FAST AGC is selected, ¼ of U4 (pins 1 and 2) closes,
shunting R270 across R309, thus decreasing the
discharge time. This fast attack variable decay voltage
is applied to pin 5 of U1. A DC voltage from the AGC
threshold pot (R430) is applied to pin 6 of U1. When the
voltage on pin 5 exceeds the AGC threshold voltage set
by R430, AGC1 voltage is generated at pin 7 of U1. This
AGC1 voltage is applied to I.F. amplifier Q78 for partial
control of the receiver gain. AGC1 voltage is also
applied to pin 10 of U1 through resistors R433, R473,
and R426 charging capacitor C323. Resistors R433 and
R473 are shunted by sections of switch U4, depending
on the bandwidth selected (BW_SELA and BW_SELB).
This produces a variable time constant attack voltage on
pin 8 of U1 which varies depending on the I.F. bandwidth
selected. The output of this section of U1, acts as a
current amplifier, and its output, designated AGC2, is
applied to I.F. stages prior to the I.F. filters to control
gain. The stages controlled by AGC2 are Q76, and Q77.
AGC1 voltage is also applied to pin 3 of U2. A DC
reference voltage controlled by the S-Meter Full Scale
pot (R453) is applied to pin 2 of U2. The output at pin 1
of U2 is the S-meter voltage which is applied to switch
U11. When in HF mode (MODE_B line high), U11 feeds
this voltage to the front panel board via the S-METER
line.
U2 (pin 7 output) compares the squelch setting
(SQUELCH_REF) to the signal strength to produce the
/SQUELCH line for muting and for stopping scans.
Synchronous Detector
The 455 kHz I.F. signal, in addition to being applied
to the AGC and envelope detectors described
previously, is also applied to the synchronous detector.
This detector is utilized when the AM SYNC mode is
activated and is also used when in SSB mode. Both
modes use the same detectors but use different local
oscillator injection. AM SYNC provides a locally
generated carrier, locked to the frequency of the
incoming signal which greatly enhances the readability
of AM signals under conditions where selective fading
exists. It also allows the selection of DSB (double
sideband), USB (upper sideband), and LSB (lower
sideband) modes, which can aid considerably in
reducing interference under crowded band conditions.
In SSB mode (NOT AM SYNC), the synchronous
detector can provide an additional 30 dB of unwanted
sideband suppression if ENHANCED SSB is enabled.
This is accomplished using the RADIO SETTINGS
menu, item 1.
The 455 kHz signal from the last I.F. amplifier stage
(Q78) is applied to the synchronous detector system in
two sections. The first section described here is the
detector section itself. Amplifier Q47 amplifies the I.F.
signal further and applies it to each of two sections of
quad bilateral switch U3, which act as product detectors.
U3 (pin 11 out) detects in-phase modulation which is the
sum of lower sideband (LSB) and upper sideband (USB)
modulation. U3 (pin 8 out) detects quadrature
modulation, which is essentially the difference between
any LSB and USB modulation. Each of these two audio
streams are filtered by R/C low pass filters to remove the
455 kHz components, and then fed into audio phase
shift networks. The phase shift networks realign the
phasing of the in-phase and the quadrature demodulated
audio. At the output of the two phase shift networks, the
audio is summed to cancel out the unwanted sidebands,
while the desired sidebands add constructively. In this
situation, Q82 turns off to reduce the gain of Q47 in
order to prevent clipping due to the doubled audio
amplitude. The desired sideband is determined by the
phase of the oscillator injection to the quadrature product
detector relative to the injection to the in phase product
detector. The resulting audio is passed to U10 to be
switched with envelope detected audio.
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