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AE/LZB 119 1902 R1A
12
Digital I/O Lines
The balanced RxIF and RxIF_B (B implies “Barred”
indicating that the condition is true when the line is low)
lines connect through resistor and capacitor RF decoupling
circuits to HILLARY D701. RxIF connects through
resistors R715 and R714 and capacitor C707 to D701, Pin 8.
RxIF_B connects through resistors R717 and R718 and
capacitor C709 to D701, Pin 7. Both of these lines connect
to a Phase Digitizer internal to D701.
Phase Digitizer (D701)
The phase digitizer demodulates the 450 kHz IF to
recover the digitized receive audio. The output of the phase
demodulator is connected to Rx DSP Interface (Refer to
Schematic Diagram, Sheet 13). The Rx DSP Interface
output of D701 connects to the DSP module through D701,
Pin 69 (RXSIF_DSPDATA), Pin 73 (RXSIF_DSPCLK)
and Pin 74 (RXIF_DSPSYNC). DSP interface lines
connect to DSP TDR (TDM DATA RX), DSP TCKDR
(TDM CLOCK RX) and DSP TRSR (TDM FRAME
SYNC TX) respectively. These lines can be metered on the
DSP module at test points TP8 (data), TP15 (clock) and
TP14 (sync), respectively.
DSP Module
The Digital Signal Processing (DSP) Module does all
the Channel Guard filtering, provides squelch, audio mute,
1.5, 3, and 4.5 dB of volume attenuation and changes the
digitized audio into an analog signal. The output of the DSP
module is on X21-17 (PCM_DSEAR). This output can be
metered at test point TP3 on the DSP module.
Host Interface
The PCM_DSPEAR line connects to D701, Pin 64 and
goes to the PCM/DSP HOST INTERFACE. The audio
output of D701 is on Pin 40 (PCM_CDCEAR) and
connects to D601, Pin 34 (DPCI).
DEBBIE (D601)
This signal (PCM_CDCEAR) is then put through a
decoder, low pass filter, volume control circuit and an
amplifier. The low pass filter, filters out an 8 kHz sample
rate. The volume control provides 24 dB in 6 dB steps. The
output of D601 is on Pin 22 (RXAUDIO). The signal can
be metered at test point TP601. At maximum volume there
should be 350 mVolts RMS (1.0V
P-P
) with a 1 kHz tone and
3 kHz deviation.
Operational Amplifier (N300)
This signal (RXAUD1) connects through capacitor
C611 and resistor R625 to operational amplifier N300, Pin 6
(-B). Feedback for the amplifier is through resistor R627.
The gain of this amplifier is changed by the
RX_VOL_ATTN line (D701, Pin 116) to provide a total of
48 dB of volume control attenuation. This is accomplished
by the RX_VOl_ATTN line going high, causing transistor
V604-1 to conduct and connecting 10k ohms resistor R626
across 68k ohms resistor R627 to reduce the gain of N300B.
The output of this amplifier is on Pin 7 (RXAUD2).
Audio Power Amplifier (N600)
This signal is attenuated through resistor R629 then
coupled through coupling capacitor C613 to audio power
amplifier N600A, Pin 7 (+IN). This signal is also connected
through resistor R628 and coupling capacitor C612 to the
Universal Device Connector (UDC_AUDIO_OUT). The
audio input to N600A can be metered at test point TP602
and at maximum volume should be 250 mV
P-P
. The input
will be 21 mV RMS (60 mV
p-p
) to produce 500 mWatts
output into 16 ohms. Power amplifiers N600A and N600B
provide differential outputs on Pin 1 (+OUT) which is
AUDIO_PA_HI and Pin 3 (+OUT) which is SPK_LO.
The bridged output of these two lines provides 2.83 V RMS
for 500 mWatts output into 16 ohms. The SPK_LO line
connects through the flex circuit to the low side of the
speaker. The AUDIO_PA_HI connects through resistor
R641 to the other side of the speaker (SPKR_HI). Resistor
R641 is used to attenuate the speaker audio during duplex
operation. Normally MOSFET V602 is turned on, by-
passing R641 to get the full volume to the speaker. V602 is
controlled by a binary output line from HILLARY labelled
DUPLEX_SPKR_ATTEN. When this line is at 0 volts,
transistor V603 is off and the collector is high which turns
on both V602 MOSFETs, by-passing R641. SPKR_HI
then connects through the flex circuit to the high side of the
speaker. Two series MOSFETS are used to prevent forward
biasing the characteristic diodes across the MOSFETS with
audio when the MOSFETS are OFF.