CML Microcircuits

COMMUNICATION SEMICONDUCTORS

PRODUCT PREVIEW

ATB010 – dPMR RF Demonstrator

© 2009 CML Microsystems Plc

Page 3 of 6

August 2009

Receiver local oscillators

VCOs are provided for 1st and 2nd LOs of the receiver, both locked using a dual PLL synthesiser IC.

The 1st LO in particular requires excellent performance in order for the receiver to meet the required

adjacent channel rejection at 6.25kHz offset.

The 2nd local oscillator at 44.545MHz is provided using a conventional common collector Collpits circuit.

The PLL reference used is 5kHz in this case.

Reference oscillator

A 1.5ppm 19.2MHz VCTCXO is provided on the PCB. Note that for transmit operation, both the VCO

and the reference frequencies are modulated.

2

The IF signal from the post crystal filter amplifier is presented to the input of the FM/IF demodulation IC.

This IC and surrounding circuitry provide a number of functions:

•

A mixer from the 1st IF to 455kHz (the 2nd IF)

•

Further channel filtering using inexpensive ceramic filters which are selectable for 12.5kHz or

6.25kHz operation)

•

A limiter-discriminator demodulator using a ceramic resonator

•

Switched loading on the discriminator to provide a consistent output across both operating

channel bandwidths

•

An RSSI output that can be monitored by a CMX7141 AUX ADC.

Transmitter

The transmit VCO operates at the final operating frequency and uses the same oscillator architecture as

the receive VCO.

A separate varactor diode is used for applying modulation from MOD1. The VCO is locked using the RF

PLL, which requires reprogramming between receiver and transmitter operation. A common loop filter is

used for both the Tx and Rx VCOs.

The transmitter provides at least +28dBm over the range 440 to 450 MHz.

The CMX7141 RAM DAC output is connected to the PA control line via a diode and transistor to provide

sufficient current to the PA control pins.

Tx/Rx switch

Circuitry is provided to implement a classical series-shunt Tx/Rx switch using PIN diodes after a lumped

quarter wave section. The diodes are activated in transmit via the TxEN signal from the CMX7141. An

additional diode is provided to protect the LNA from extremely strong signals (e.g. a nearby transceiver).

This branch of the switch then goes the receiver LNA input.

Harmonic filter

A low-loss low pass filter with traps centred at the 2nd and 3

transmitter and receiver paths and is connected between the Tx/Rx switch and the antenna connector

J2.

Power detector

An RF power detector operates on the forward branch of the discrete directional coupler. The output

from the detector can be monitored using AUX ADC3 on the CMX7141 of the EV6181. Alternatively it

can be monitored via an on-board test point.