User's Manual

Table Of Contents

MX800 Transceiver General Description

1.2.2 Receiver Module

The receiver module accepts the low level RF input signal and amplifies,

filters and conditions the signal prior to detecting the wanted audio

component. The Receiver module features the same advanced synthesiser and

wide bandwidth as the exciter. Only the front-end Bandpass filter and VCO

need to be changed in order to support different frequency bands, resulting in

significant flexibility and end-user cost savings. The purpose built front end

Bandpass filter has a wide no-adjust bandwidth equal to the band allocation

(refer to section 7.4 for details of the band allocations).

The receiver has high sensitivity while maintaining excellent Intermodulation

immunity and adjacent channel rejection. A dual first IF filter provides

excellent rejection to common known spurious responses. High blocking of

over 100dB typical ensures that strong interfering signals do not desensitise

the receiver when receiving weak signals.

1.2.3 Power Amplifier Module

RF from the Exciter passes via a coaxial cable to the input of the PA Module

and is first attenuated by a 50 ohm pad, which is used to provide a good

50 ohm source impedance for the first LDMOS driver amplifier. The RF is

amplified to around 5 Watts at the driver output, and is band dependant. Note:

this point does not have 50 ohm impedance and the drive power cannot be

measured directly with a 50 ohm Wattmeter. The signal from the driver is

then matched by a broadband network to drive the low input impedance

associated with the final transmit LDMOS power amplifier transistor. The

transistor's low Drain impedance is then also matched back to 50 ohms by a

broadband matching network covering a very wide bandwidth. Prior to

transmission, a low loss 13 element elliptical low pass filter, filters out the

unwanted harmonics to less than –90 dBc.

A dual directional coupler consists of coupled microstrip transmission lines

fabricated on the PCB artwork. The sampled RF energy is rectified to provide

a proportional DC voltage output.

The PTT signal enables the amplifier circuit by providing bias to the

transistors. A thermistor TS1, physically located on the PA heatsink monitors

the heatsink’s temperature and is monitored by the Micro Controller.

The PA is very compact and efficient for high reliability and low cost. The

heatsink has minimal temperature rise even under continuous operation,

ensuring the best MTBF obtainable for a practical design.