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MX589

High Speed GMSK Modem

CML Microsystems Plc 

High Speed GMSK Modem 4k to 64kbps

Page 8 of 20

MX589

The output of the radio receiver's Frequency Discriminator should be fed to the MX589's RX Filter by a

suitable gain and DC level adjusting circuit. This circuit can be built with external components around the on-

chip RX Input Amplifier. The gain should be set so that the signal level at the RX Feedback pin is nominally

1V peak to peak (for VDD=5.0V) centered around VBIAS when receiving a continuous 1111000011110000..

data pattern.

Positive going signal excursions at RX Feedback pin will produce a logic 0 at the RX Data Output. Negative

going excursions will produce a logic 1.

The received signal is fed through the lowpass RX Filter, which has a -3dB corner frequency of 0.56 times the

data bit-rate, before being applied to the Level Measure and Clock and Data extraction blocks.

The Level Measuring block consists of two voltage detectors, one of which measures the amplitude of the

positive parts of the received signal. The other measures the amplitude of the negative portions. (Positive

refers to signal levels higher than VDD/2, and negative to levels lower than VDD/2.) External capacitors are

used by these detectors, via the Doc1 & Doc2 pins, to form voltage ‘hold’ or ‘integrator’ circuits. These two

levels are then used to establish the optimum DC level decision-thresholds for the Clock and Data extraction,

depending upon the RX signal amplitude and any DC offset.

4.2.2 Rx Circuit Control Modes

The operating characteristics of the Rx Level Measurement and Clock Extraction circuits are controlled, as

shown in Table 5, by logic level inputs applied to the PLLacq, Rx HOLD , and RxDCacq pins to suit a

particular application, or to cope with changing reception conditions, reference Figure 5.

In general, a data transmission will begin with a preamble, for example, 1100110011001100, to allow the

receive modem to establish timing and level-lock as quickly as possible. After the Rx carrier has been

detected, and during the time that the preamble is expected, the RxDCacq and PLLacq Inputs should be

switched from a logic 0 to a logic 1 so that the Level Measuring and Clock Extraction modes are operated and

sequenced as shown.

The Rx HOLD input should normally be held at a logic 1 while data is being received, but may be driven to a

logic 0 to freeze the Level Measuring Clock Extraction circuits during a fade. If a fade lasts for less than 200

bit periods, normal operation can be resumed by returning the Rx HOLD input to a logic 1 at the end of the

fade. For longer fades, it may be better to reset the Level Measuring circuits by placing the RxDCacq to a

logic 1 for 10 to 20 bit periods.

Rx HOLD has no effect on the Level Measuring circuits while RxDCacq is at a logic 1, and has no effect on

the PLL while PLLacq is at a logic 1.

A logic 0 on Rx HOLD does not disable the Rx Clock output, and the Rx Data Extraction and S/N Detector

circuits will continue to operate.

Rx Signal Input

PREAMBLE

DATA

Rx CARRIER DET

(RSSI) Input

RxDCacq

Rx LEVEL MEASURE

MODE

CLAMP

PLLacq

CLOCK EXTRACTION

CCT MODE

FAST PEAK

DETECT

ACQUIRE

AVERAGING PEAK

DETECT

30 BITS

MEDIUM

BANDWIDTH

NARROW

BANDWIDTH

Figure 5: Rx Mode Control Diagram

2001 MX-COM, Inc.

www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054

Doc. # 204800103.011

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

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