Proprietary Information
Honeywell International Inc.
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Olathe, Kansas
KT73_TxMod_Theory.doc
2.0
Transmitter Board
Reference schematic for the transmitter is KPN 002-08275-0040. A block diagram of the KT 73
transmitter is below in Figure 2. The transmitter board has three RF devices. The TX/RX switch,
the power oscillator, and the power amplifier. The TX/RX switch routes RF signals received and
transmitted through the antenna by way of the RF input/ output port to the receiver and
transmitter. Q1 is a power amplifier used in an oscillator configuration. Q2 is a class C power
amplifier providing 6 dB of gain.
Figure 2 Transmitter Block Diagram
2.1
Power Oscillator: In order for an amplifier to oscillate, it must have some of its output
power fed back to its input at 360 degrees of phase shift. A classic oscillator definition is
that an active device must have a loop gain of one and total phase shift of zero degrees.
(360)
2.1.1
The active device is Q1 and the loop includes the output microstrip matching
network, C6, FL1 in series with the entire Q1 output, 6 dB coupler, the 50 ohm
phase matching network (serpentine line), C2, and the input matching network.
The transistor has some insertion phase as well, but this is hard to quantify.
2.1.2
When the emitter modulator pulls low, with the Q1 collector at 50 volts, the
circuit begins to oscillate.
2.1.3
The total length of the feedback path determines the frequency of the oscillation.
The reason for the adjustable length 50 ohm line is to compensate for the
variance in insertion phase between different transistors populating the Q1 part
inventory. Even though C3 is used to adjust the frequency when the oscillator is
close to 1090 MHz., decreasing the length of the 50 ohm line can raise the
frequency higher, while lengthening the line makes oscillations lower in
frequency. Smaller length loops can be added or removed for smaller frequency
change and larger loops will change the frequency in greater increments.
2.2
Bandpass Filter FL1 is centered at 1090 MHz. with a 3 dB bandwidth of 10 MHz. The
purpose of the filter is to keep the oscillator on frequency over temperature. As the
temperature varies, so does the insertion phase through the transistor. This causes a
Serpentine Line
(Regenerative Feedback)
CR1
CR3
λ
/4
TX SENSE
RX_EN_NOT
R5
Antenna
Isolation Port
Resistor: 50
Ω
6 dB
Coupler
Isolation Port
Resistor: 50
Ω
0 dB Attenuator
RF to Receiver Board
Power
Oscillator
Q1
Band Pass Filter
FL1
Final
Power
Amp
Q2
Low
Pass
Filter
CR2
C18
DC/RF
Isolation
Network