User's Manual

EXHIBIT 2 PAGE 13 OF 17
TX RX Systems Inc. Manual 7-9257 (version 1) 11/03/00 Page 13
supplied to a "TNC" female connector to power this
assembly.
The RF to DC converter section of the OLC assem-
bly contains three circuits, a -20 dB sampler which
is used to supply the RF signal to the diode detec-
tor, and finally a -50 dB sampler which provides a
convenience port for connecting test equipment.
The detector circuit receives RF from the -20 dB
sampler and produces a negative polarity DC out-
put voltage that is proportional to the RF signal. A
Schottky Barrier diode (D2) and integrator (R3/C1)
are used as the detector.
The DC voltage produced by the RF to DC con-
verter is directly proportional to the output signal
strength of the final amplifier. The DC voltage is
supplied to the DC control circuit at the non-invert-
ing terminal of op-amp IC2. A variable reference
DC voltage is applied to the inverting terminal of
the same op-amp. Variable resistor VR2 is used to
set the magnitude of this reference voltage and
controls the level at which gain reduction will start
to occur.
As the signal strength increases, the output voltage
of the RF to DC converter, which is of negative
polarity, becomes larger. This change causes the
output of IC2 to also become increasingly negative.
This output voltage is connected via diode D1 to
bias the RF attenuator circuit board assembly. As
this voltage becomes more negative, the attenua-
tion is increased thus achieving a certain range of
gain control. Diode D1 insures that the gain control
voltage is always positive and never goes below 0
volts. In actual practice, OLC operation is set to
commence when the power output of the final
amplifier reaches its maximum two-carrier level.
Two other ICs are mounted on the OLC control cir-
cuit board. IC1 is a 10 volt regulator that supplies
DC to the other two chips. Variable resistor VR1 is
used to set this voltage. IC3 is a voltage inverter
that produces -4.5 volts which is applied to the op-
amp IC2. This negative voltage allows the output
voltage of IC2 to closely approach 0 volts. Two
diodes (D1 and D2) are used in series to extend
the attenuation range. The diodes are always for-
ward biased with minimum forward resistance and
insertion loss occurring at about 20 ma of current.
Pre-Amplifier 3-11432
The pre-amplifier consists of the 400 milliwatt
amplifier connected between the input filter assem-
bly and the op-amp/attenuator assembly. The
preamplifier is a single stage of the five identical
stages found in assembly 3-11432 and is used to
insure that a sufficient level of signal is applied to
the attenuator.
Each of the five individual stages (part# 3-7718)
found in assembly 3-11432 are complete 400 milli-
watt amplifiers. They are mounted on a common
mounting panel and have a common DC distribu-
tion bus running internally between the individual
stages. Each stage provides a minimum of 13.2 dB
of gain with a power requirement of 21.7 VDC
(nominal) and a typical current draw of 120 ma.
The maximum single carrier power output is 400
milliwatts.
Each amplifier stage consists of two circuits, the
amplifier circuit (3-7725) and the bias regulator cir-
cuit (3-10742). Both of these circuits are housed in
their own enclosures which are then physically
joined together to make up one stage. The circuits
are electrically joined using feed-thru capacitors
Cf1 and Cf2.
The amplifier circuit uses a linear RF transistor Q1
(Philips part# BFQ34/01) which is operated in a
class "A" configuration in order to keep intermodu-
lation distortion to a minimum. The RF transistor is
biased for a nominal collector current of 120 ma. A
bias regulator circuit is used to keep the collector
current constant with changes in temperature. Nar-
row band matching techniques are used in this
amplifier and it will require tuning if the transistor or
matching network components are replaced.
The bias regulator circuit uses an Op-Amp compar-
ator IC1 to supply a variable bias current which var-
ies as required to keep the RF transistors collector
current constant. Current to the collector of the RF
transistor flows through resistor R1. The voltage at
the collector side of this resistor is applied to the
non-inverting input of IC1. Voltage divider R2 / R4
sets the desired reference voltage on the inverting
terminal of IC1. The variable output voltage at pin 6
of IC1 is then applied to the base of the RF transis-
tor. The bias on the RF transistor will now vary in
such a way as to keep the voltage at the collector
end of R1 equal to the reference voltage provided
by divider R2/R4, thus keeping the RF transistor's
collector current constant (120 ma nominal).