Motorola ASTRO Digital XTL 5000 Detailed Service Manual page 105

Theory of Operation: Transmitter
Antenna Switch
The antenna switch functions in two modes determined by the presence of K9.1V. The K9.1V switch
bias is applied via L5701and C5702. When K9.1V is present, the switch is in TX mode. D5701,
D5702 and D5703 are forward biased forming a low-loss path from the RFPA final stage to the
harmonic filter and a 20 dB isolation path between the RFPA final stage and the RX front-end.
Isolation is achieved via a quarter-wave transmission lines between D5701 - D5702 and between
D5702 - D5703. C5709-10 resonates out the D5702-3 on inductance improving the isolation. When
K9.1V is absent, the switch is in RX mode. D5701, D5702 and D5703 are reverse biased forming a
low-loss path from the harmonic filter to the RX front-end and a 20 dB isolation path from the
harmonic filter to the RFPA final stage. Isolation is achieved via the D5701 off resistance. L5702
resonates out the D5701 off capacitance improving the isolation.
Harmonic Filter
The harmonic filter is a 7-element, equal-L Zolotarev quasi-lowpass filter consisting of C5712 and
C5713, C5719 thru C5721 and L5706 thru L5708. L5712, C5711 and L5713, C5714 form two shunt
zeros for extra attenuation at the second harmonic. C5708 acts as a DC block between the filter and
the antenna switch. The filter provides approximately 60 dB of harmonic rejection. The harmonic
filter together with the antenna switch provides approximately 0.7 dB insertion loss between the
transmitter power amplifier and the antenna.
Power Detector
The power detector consists of a main microstrip transmission line that transfers power from the
harmonic filter to the antenna and two parallel lines that are used to detect the forward and reverse
power. The forward power detection line is terminated via R5707 and R5708. RF energy coupled
onto this line is rectified and filtered via D5704 and C5715 to provide a DC voltage to the power
control circuitry that is proportional to the forward transmitted power level. The detected forward
voltage is approximately 1.5 Vdc when the radio is putting out 44 watts. Thermister R5704
compensates for temperature changes in D5704 to maintain constant DC voltage versus detected
forward power over temperature.
The reverse power detection line is terminated via R5705-06. RF energy coupled onto this line is
rectified and filtered via D5705 and C5718 to provide a DC voltage to the power control circuitry that
is proportional to the reverse power level reflected back from the antenna. Thermister R5712
compensates for temperature changes in D5705 to maintain constant DC voltage versus detected
reverse power over temperature.
3.6.3.1.3 Power Control
The power control section is comprised of a control loop to level forward power, and protection
mechanisms to reduce power to a safe level for the given environmental conditions (see
on page 3-40).
6881096C74-B
3-39
Figure 3-31
May 25, 2005