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change in oscillation frequency. Bandpass filter FL1 has a temperature coefficient of 15
PPM/degree. The variable capacitors change at 10 PPM/degree. The combined effect of
the capacitors and the filter keep the frequency of oscillation within the +/- 1 MHz.
limits.
2.3
The 6 dB Coupler, W1, is used to couple 25% of the FL1 RF output energy back to the
input of Q1 oscillator to maintain oscillations. The oscillator puts out 130 Watts and is
filtered by FL1. The insertion loss of FL1 is 1.45 dB leaving its output at 93 Watts.
Coupled output being fed back is 23 Watts or 17% oscillator output. Through output is
reduced 25% to 70 Watts. Loss in phase matching network is negligible.
2.4
The Zero dB Attenuator follows W1, but is currently shorted across R2, the series
member of the network. Two shunt members, R3 and R4, make this a PI type design. The
purpose is to reduce through output from W1 to the Power Amp to a level not exceeding
the one dB compression point. At higher power inputs the power amplifier becomes
saturated and the output pulse shape of the power amplifier becomes distorted.
2.5
Power Amplifier: The final amplifier is a class C RF amplifier with a gain of 6 dB. C4
and C7 are variable capacitors used to tune the matching networks to Q2 power
transistor. Tuning C7 may also effect the frequency of the power oscillator due to loading
of the power oscillator output. With 70 Watts driving the input, nominal power amp
output is 280 Watts with 180 Watts reaching the antenna. 2 dB of insertion loss is
dissipated from traveling up the coaxial cable hooking the output port of the unit to the
antenna.
2.6
TX/RX Switch
The TX/RX switch works by biasing two surface mount diodes, CR1 and CR2, to route
RF signals from the antenna to the receiver or to the antenna from the transmitter. The
RLSI generates the RX_EN_NOT signal, which controls the TX/RX switch.
2.6.1
2.6.2
2.7
The Low Pass Filter is designed to reduce the level of the conducted harmonics of the
1090 MHz transmitter carrier. The filter is a seven-pole Chebyshev response with a 0.1
dB ripple. The 3 dB corner frequency is 1.3 GHz. The inductors are implemented using
26 AVG wire and the capacitors are ATC 500 volt chip capacitors. Silver plated housing
with the wire in the center creates an air dielectric coax, whose characteristic impedance
is 130 Ω. The coaxial cavity was created to have a high impedance line that would in turn
create high Q inductors. The filter has 55 dB of attenuation at the second and third
harmonics.
Honeywell International Inc.
Olathe, Kansas
KT73_TxMod_Theory.doc
Proprietary Information
During the transmit cycle, the RX_EN_NOT line is set low by the RLSI.
Integrated circuit I1 inverts the signal that then turns on Q7 and connects +5 V
to E5. Applying +5 V to E5 turns on both CR1 and CR3. A low impedance path
is now formed that directs all the energy from the transmitter output to the
antenna while isolating the receiver with a high impedance path. CR3 is in series
with the transmitter output passing the output power on a path to the antenna,
while forward biased. CR1 is effectively shunted to RF ground a quarter wave
length away at the receiver input. A shorted quarter wave length stub is
effectively an open circuit path to the transmitter high power RF. The open
circuit isolates the receiver path and protects the receiver because of the high
impedance an open circuit present to the transmitter power.
During the receive cycle, the RX_EN_NOT line is set low by the RLSI.
Integrated circuit I1 inverts the signal that then turns on Q8 and connects –12 V
to E5. With –12 V on E5. A low impedance path is now formed which directs all
energy from the antenna to the receiver input. R22 on the modulator board set
the current through the diodes at 60 ma.
Page 4 of 4
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