CDR-3250/80 TECHNICAL MANUAL
Issue 2.2
4-1
CHAPTER 4
GENERAL THEORY OF OPERATION
4-1 INTRODUCTION.
This chapter contains a block diagram description of the
CDR-3250/80. Each of the boards/modules are discussed in
the paragraphs below: (See figure FO-2.)
4-2 BLOCK DIAGRAM DISCUSSION.
4-2.1
AC Receptacle /RFI Filter.
The AC
Receptacle/RFI Filter keeps internally generated power
supply switching noise off the AC input line. When the
front panel power switch (S1) is set on, power is applied to
the POWER SUPPLY module through the AC
Receptacle/RFI Filter, and through the circuit breaker (CB1)
on the rear panel.
4-2.2
AC Line Filter.
The AC Line Filter board keeps
internally generated power supply switching noise off the
AC input line. When the front panel POWER switch is set
ON, input power is applied to the POWER SUPPLY module
through the AC Line Filter board.
4-2.3
Power Supply Module
.
The POWER SUPPLY
module is a switching regulated type that pr8, +17,
and -17 VDC to the receiver modules through the
motherboard using 90 to 260 VAC input power
(automatically sensed). A se12 volt output is used
for the fan on the rear panel. Fault detector circuits send a
fault signal to the Control Section in the Digital module if
any of the voltages fall below a preset level.
4-2.4
RF Analog Module
.
The RF Analog module
contains the optional preselector, first mixer, first IF gain
control, first LO, second mixer, second LO, and third 3rd
mixer circuits. Each section is discussed below.
4-2.4.1
Preselector.
The preselector assembly consists of
an overload protection circuit, BITE (built-in test
equipment) circuit, ten automatically selected filters, and an
RF amplifier. The tuning range of 0 to 499 or 0 to 1,599 kHz
is selected by one-of-two lowpass filters. For frequencies
at or above 1.6 MHz, the spectrum is divided into eight
bands of bandpass filters.
The incoming RF signal is applied through the rear panel
antenna connector to the module. An overload detector
and BITE circuit protects the filters by opening the RF line
if the signal level exceeds approxi24 dBm (1/4
Watt). Protection is provided for inputs up to 10 Watts.
Control data from the Control Section in the Digital module
automatically selects the correct filter for the selected re-
ceive frequency using PIN diode switches. The serial
control DATA from the Control Section is clocked into the
shift register circuit. At the proper time, the DATA is
latched into the shift register by the RF ENABLE signal.
The shift register data then selects the correct filter, or
activates the built-in test (BIT).
Built-in test equipment (BITE) testing is provided by
disconnecting the antenna signal and using the 48 MHz
signal developed in the Third LO section of the RF Analog
module as a substitute signal to perform the BIT. During
the BITE test, the PRE LO signal, under CPU control, is
divided down to either 0.375 MHz or 0.750 MHz in the
preselector, and the receiver is set to the ISB mode.
Each fundamental frequency and some odd harmonics are
sequentially used to check each preselector band and the
rest of the receiver circuits. The frequencies used are:
0.375, 1.125, 2.625, 2.25, 3.75, 5.25, 8.25, 11.25, 17.25, and
27.75 MHz. If the signals are not detected in the audio
board after a certain amount of time, the test fails and the
Control Section in the Digital module causes a fault to be
displayed on the front panel.
4-2.4.2
First Mixer
. The first mixer section converts the
input signal to the first intermediate frequency (1ST IF) of
40.456 MHz. The input signal applied to the first mixer is
filtered by a 33 MHz lowpass filter. The filter rejects the
first local oscillator (1ST LO) frequency preventing it from
being radiated back out the antenna. The filter also rejects
input signals at the 1ST IF and image frequencies. The RF
signal is mixed with the tunable 1ST LO frequency from the
Synthesizer module, producing the 1ST IF. After
amplification, a bandpass filter ensures that only the 1ST IF
will pass.
4-2.4.3
First IF Gain Control
. The first IF gain control
circuits provide "front-end" gain control. The GAIN
CONTROL voltage developed in the DSP section of the
Digital module is used to control PIN diode attenuators for
three gain control stages. The gain control section's output
is applied to the second mixer section as the 1ST IF at
40.456 MHz.
4-2.4.4
First LO Driver
. The 1ST LO signal is tuned to the
correct frequency depending on the receive frequency and
receive mode selected. The 1ST LO driver consisting of a
preamplifier and push-pull amplifier, provides the proper
level 1ST LO signal to the 1ST mixer. A fault detector
Courtesy of http://BlackRadios.terryo.org
