Output Network (On) - (For 50W And 100W Transmitter); Figure 3-23. Output Network Components (Vhf) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Theory of Operation: Transmitter
Dual Final Stage
C3540-1, C3543-4 and transmission lines form a bandpass. R3541-8 provide stability for Q3540.
R3550-7 provide stability for Q3541. R3540 and R3568 supply the VGBIAS1 gate bias to Q3540.
R3569 and R3558 supply the VGBIAS2 gate bias to Q3541. L3540-4 and C3560-1 form the A+ drain
bias circuit. C3548-55 and transmission lines form a low-pass. Q3540-1 operate Class AB.
R3560-1, C3561, and U3561 comprise the final-stage, current-sense circuit that generates the
VCURRENT voltage proportional to the final stage current. R3564 sets the circuit gain. U3560
generates the VTEMP voltage, which is proportional to the final-stage temperature.

3.6.1.3 Output Network (ON) - (for 50W and 100W Transmitter)

The ON consists of the antenna switch, harmonic filter, and power detector (see
FPA_OUT
From
FPA
Antenna Switch
The antenna switch functions in two modes, which are determined by the presence of K9.1V. The
K9.1V switch bias is applied via L3700, L3731-2, and C3702. When K9.1V is present, the switch is in
TX mode. D3701-2 and D3704 are forward biased forming a low-loss path from the RFPA final stage
to the harmonic filter and a 60 dB isolation path between the RFPA final stage and the RX front-end.
Isolation is achieved via a quarter-wave transmission line between D3701 and D3702. D3701-4
serves as an ESD protection circuit against ESD discharge on the antenna connector.
When K9.1V is absent, the switch is in RX mode. D3701 and D3702 are reverse biased forming a
low-loss path from the harmonic filter to the RX front-end and a 60 dB isolation path from the
harmonic filter to the RFPA final stage. Isolation is achieved via the D3701 off resistance.
Harmonic Filter
L3720-7, C3720-3, form the twelve-element, low-pass harmonic filter. The filter attenuates
harmonics generated by the RFPA when the antenna switch is in TX mode and provides extra
selectivity when the antenna switch is in RX mode.
Power Detector
The power detector consists of two asymmetric, coupled transmission lines and detection circuitry
that detects forward and reverse power. C3730-1, D3730, L3730, R3730-3, and R3735-6 form the
forward-power detector (VFORWARD), which is used for power leveling. C3732-3, D3731, L3731,
R3737-9, and R3733-4 form the reverse-power detector (VREVERSE). L3737 provides an
electrostatic discharge path to protect the RFPA final stage device.
6881096C74-B
HIGH PASS FILTE
ANTENNA
SWITCH
63W
X_IN
K9.1V

Figure 3-23. Output Network Components (VHF)

POWE
DETECTO
V EVE SE
CONNECTO
J3700
VFO WA D
MAEPF-27888-O_VHF
3-29
Figure 3-23).
F
53W
May 25, 2005