Final Power Amplifier; Antenna Switch; Low-Pass Filter; Directional Coupler - Hytera MD78X Service Manual

Service Manual
maximum output power is 34.5dBm, and the maximum variable gain is 15dB.
15.1.4

Final Power Amplifier

The final power amplifier consists of Q802 and Q803, with the maximum output power of 48.5dBm,
and the maximum variable gain of 14dB. The input matching circuit is composed of C815, C839, R804,
R806, R807, R813, R818 and R861, while the power combining circuit is composed of C820, C8330,
C833, C831, C819, C812, C841 and C842.
15.1.5

Antenna Switch

The antenna switch is used to switch between the transmitter circuit and the receiver circuit.
In TX mode, the switching transistor Q810 is controlled by TX signal, so as to supply 9V 3A to the PIN
diodes D801 and D802. Quiescent bias current of the PIN diode is controlled by the resistors R821
and R825. In this mode, D801 and D802 are turned on; RF signals are applied to the low-pass filter
(composed of C821, C816, C836, C837, C830, C838, L809, L813 and L810) and then transmitted via
the antenna port.
In RX mode, TX signal controls the switching transistor Q810, to further control the power supply for
the PIN diodes D801 and D802, which are not applied with DC bias current. When the two PIN diodes
are turned off, signals feed into the RX path through the low-pass filter (composed of L871, C852 and
C862).
15.1.6

Low-pass Filter

A low-pass filter that consists of inductors (L809, L813 and L810) and capacitors (C821, C816, C836,
C837, C830 and C838), is used to attenuate harmonic energy from the transmitter.
15.1.7

Directional Coupler

A directional coupler that consists of microstrip directional coupler, D803, D804 and etc, is used to
detect both forward and reverse directed power of the transmitter. The forward power is coupled to the
diode D803, and the voltage is applied to the power control circuit (U801). Then the voltage VGG is
output to control the gate voltage of pre-driver and driver PA, ensuring a constant power output.
The directional coupler can adjust the TX power and detect the VSWR load. The reverse power is
coupled to D804. The voltage is applied to U802 and then feeds into APC for detection.
VHF (136-174MHz 50W) Information
539