Cell Site Transceiver General Operation
The transceiver is designed for use in an Advanced Mobile Phone (AMPS)
cellsite. The transceiver contains a receiver which demodulates the voice
and data signal from mobiles and also an exciter which provides a modulated
1.5 watt signal to drive an external power amplifier. The 20 MHz bandwidth
of the transceiver allows it to operate on all 666 channels allocated to
AMPS. Tx is 870.030 - 889.980 (channels 1-666) while Rx are 45 MHz below
the Tx freq.
A digital synthesizer is used to generate the frequencies. The channel
selected is determined by either a 10-bit data word from an external
controller or entered directly via dipswitch. The synthsizer frequency
stability is determined by an 18.990 MHz reference frequency which must
be supplied from an external master oscillator.
Diversity receive antennas can be used to minimize fades caused by multi-
path reception. The two receive antennas connect to two independent
receivers in the transceiver. The receiver experiencing the best insta-
taneous signal strength is automatically selected to provide the voice and
data output. The receiver also contains a Received Signal Strength Indicator
(RSSI) circuit which provides a logarithmic indication of received signal
strength.
The transceiver is constructed with independent PC boards including the
interface board, synthesizer assembly, receiver assemblies, and exciter
assembly.
Interface Board
The interface board provides interconnection between the main back panel
connector and the various boards in the transceiver. It also contains the
5 and 15 volt regulators and the over/under voltage protection circuits, as
well as a binary adder circuit which converts the channel selection code to
a code readable to the synthesizer.
The supply voltage to the transceiver is +26 volts DC from an external power
supply (+26v DC to pin 18, Gnd to pin 19 of rear panel connector). The +26v
is fed to a 15v regulator on the interface board which produces the regulated
15v supply for the receiver, synthesizer, and exciter boards.
The adder circuit contains three 4-bit adders cascaded to form a 12-bit
binary adder, which provides an offset to the channel selection code so
that the 10-bit control code can start at binary 1 for channel 1. An offset
is required because the synthesizer code for channel 1 is 1690. The counters
add an 'A' number from the channel selection control to a 'B' number
(011010011001 binary, 1689 decimal)hardwired on the board, resulting in the
output to be synthsized. The 'A' bits can also be entered via dipswitch
rather than from an external controller. Don't start climbing cellsite
towers to experiment with the toggling firsthand.
Synthesizer
The synthesizer consists of a synthesizer PC board and also a voltage-
controlled oscillator (VCO), mounted in a dedicated well. The synthesizer
output signal is produced by the VCO and is one-fourth the transmit freq
(approx 217-223 MHz). This frequency is multiplied in the receiver front
end assembly to produce the first injection frequency and also in the exciter
to produce the Tx frequency. Only one frequency per channel is required
because the first injection frequency is the same as the Tx. The VCO freq
is controlled by a DC control voltage produced by a phase detector. The
phase detector senses the phase and frequency of two input signals and in-
creases or decreases the control voltage if they are not the same. This
changes the VCO frequency enough to create a phase-locked-loop, synchronizing
both inputs and locking the VCO on-frequency. One input to the phase
detector is a very stable frequency derived from the external reference
oscillator. This frequency is called the 'reference frequency' and is de-
rived by dividing the reference oscillator frequency by 2532. The other
phase detector input is derived from the VCO freq. This input is the differ-
ence between the VCO frequency and a multiple of the reference oscillator
divided by a programmable counter. The divide number of the programmable
counter determines the channel selected. When the VCO-derived input to the
phase detector is the same phase and frequency as the reference frequency,
the VCO is operating at the correct frequency.
Receiver
The receiver portion of the transceiver consists os a receiver PC board and
a front end assembly. The receiver is a double-conversion unit with inter-
mediate frequencies of 45 MHz and 10.7 Mhz. Since the first mixer and
injection multiplying circuits are located in the front end assembly, the
output signal to the receiver board is the first IF os 45 MHz. Two seperate
and independent receivers are located on the receiver board and front end
assembly. Each receiver is connected to a seperate antenna and the receiver
experiencing the highest signal strength is enabled by a RSSI circuit. The
receive signal from the antenna is fed to a six-pole helical bandpass filter
in the front end assembly. The filter will pass the Rx frequency band from
825-845 MHz and has an approx. 2.5 dB loss at midband. This filter atten-
uates the image frequency and prevents the injection frequency from being
fed to the antenna. The audio/data output from the receiver are pins 16 and
17 off the rear panel connector.
Exciter
The exciter assembly multiplies the synthesizer signal by 4 and amplifies it
to produce a power output of 1.5 watts (speaking of which, I seem to recall
1.5 watts being the signal strength that can be intercepted from orbit. Of
course, this was some time ago...no doubt the tech has improved). The exciter
board also contains a phase modulator which modulates the Tx signal using the
audio/data input applied to pins 12 and 13 of the rear panel connector. A
transistor-transistor-logic (TTL) level transmit-enable signal applied to
pin 14 of the rear panel connector is used for transmit signal control. The
power output is matched to 50 ohms impedance internally.