Continuous Carrier System - CML Microcircuits

COMMUNICATION SEMICONDUCTORS

MX909A

Application Note Using the MX909A in a

Continuous Carrier System

4800 Bethania Station Road, Winston-Salem , NC 27105-1201 USA All trademarks and s ervice marks are held by their res pec t ive companies.

1. Introduction

The description of the receive mode of the MX909A given on the data sheet is based on the existence of an

RF carrier detect signal that is used as a trigger to initiate the MX909A’s signal acquisition process at the start

of a received frame. Some systems, however, transmit a continuous carrier, which is intermittently modulated

with a data stream. On these types of systems, no change in received RF carrier level will occur with the start

of the frame reception. For this reason, continuous carrier systems cannot use RF 'carrier detect' as a signal

acquisition trigger for the MX909A.

2. Baseband Data Detection

A continuous carrier system will transmit an unmodulated carrier between data packets. Therefore,

differentiating between an unmodulated and modulated carrier can provide an effective means of determining

the advent of a data packet. Similarly, the detection of the transition from modulated to unmodulated carrier

can provide an indication of the end of a data packet.

In a continuous carrier system, the radio receiver output signal will be any of

• Wideband Noise (no RF carrier - out of range)

• Silent (unmodulated RF Carrier)

• Modulated Baseband Carrier

When no RF carrier is being received, the envelope at the discriminator output will be very wide due to the

high RF gain yi elding only wideband noise. It is assumed this case has been considered and so it will not be

discussed further. When an unmodulated RF carrier is being received, the envelope at the discriminator

output becomes very narrow (DC w ith low noise). As the data transmission starts, the modulated baseband

carrier will appear causing the envelope to widen to the peak-to-peak amplitude of the received signal.

A signal acquisition trigger can be developed by observing the envelope of the receiver output signal to

indicate when the signal undergoes a transition from ‘silent’ (unmodulated RF carrier before a data frame is

received) to ‘data modulated’ (RF carrier is modulated with data). The change of the envelope from small to

large amplitude indicates its transition from ‘silent’ to a ‘modulated baseband carrier’ signal and can be used

as a signal acquisition trigger.

3. Envelope and End Of Packet Detectors

The MX909A contains circuits that can be used, with some external components, to create such a baseband

data detector. In particular, it contains both positive and negative peak detectors with some holding capacitors

(DOC1 and DOC2). (Note: The two capacitors represent high source impedance nodes so attached external

circuits must be of very high input impedance). These two peak detectors operate with different time constants

according to the configured operating mode of the MX909A. An external circuit is described to provide the

following functions:

1. Use high input impedance voltage follower buffers to condition DOC1 and DOC2 signal voltages.

2. Develop a ‘peak envelope threshold signal’ by adding a fixed offset voltage to the actual ‘valley’ DOC2

capacitor voltage.

3. With hysteresis, compare the actual ‘peak’ DOC1 capacitor voltage wi th the ‘peak envelope threshold

signal’ developed above. When the comparator trips then Envelope Detect goes high to indicate that

the signal acquisition process should be triggered.

Where a signal acquisition trigger is important at the start of frame reception, an End of Packet detector is

similarly helpful to force an ongoing frame reception process to abort due to lack of a data envelope. It is also

useful to clearly demark the end of a packet so reliable detection of a new packet can occur. End of packet

detection must respond slowly enough to ride through short signal fades.

Using the MX 909A in a Continuous Carrier System Page 2 of 5 MX909A Application Note

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A schematic diagram for both Envelope Detect and End of Packet circuits is shown in Figure 1. Alternate

circuit sections may be substituted so long as they provide an equivalent function. Note that the forward bias

drop of the choice of Silicon or Germanium diodes directly sets the magnitude of the ‘peak envelope threshold

level.’ Also note that the logic described with a truth table requires its input to continuously remain at a ‘1’

state for 3 bit times, nominal, before the EndOfPacket output will go to ‘1’.

>=10Mohms

input impedance

DOC2

DOC1

100k 1Meg

33k

Si = 0.6V

Ge = 0.3V EnvelopeDetect

100k

MC909A

DOC2, 19

DOC1, 18

RXAMPOUT, 23

>=10Mohms

input impedance EndOfPacket

bit times 12345678910

Logi c input

EndOfPacket

Input Input

Stable for

(bit times)

0don't care

1< 3

1> 3

Output

Logi c T ruth Table

Figure 1: Envelope and End of Packet Detector Circuit

During its normal training process, the MX909A dynamically changes the time constant of the DOC1 and

DOC2 capacitor voltages. Different time constants are required when these node voltages are used in the

described training trigger circuit. Accordingly, the DOC1 and DOC2 time constants must be altered between

two values when this training trigger is used; namely, (1) training trigger values and (2) normal MX909A

training values. During an idle signal phase (unmodulated carrier), the training trigger time constant values are

used to detect the start of carrier modulation using the described circuit. When a training trigger event occurs

then the MX909A must be reconfigured with normal training time constants and then issue a training task e.g.

SFS (search for frame synch). Note that some delay may need to be inserted between the training trigger

event and the time at which the training task is issued.

Using the MX 909A in a Continuous Carrier System Page 3 of 5 MX909A Application Note

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3.1 Sequence Actions

Table 1 describes the sequence of actions to be used immediately after reception of a data packet is

completed.

Step Operating Phase Action Purpose

1 End of Data packet has

just been received so the

receiver output will become

‘silent.’ Configure the

MX909A time constants to

support the external trigger

circuit.

Write the MX909A

Control Register

settings to configure

LEVRES (B3, B2) to

Lossy Peak Detect

mode.

This w ill make the DOC1 and DOC2 dynamic

response quick, as needed, even after

automatic level training occurs.

2 End of Data packet has

just been received so the

receiver output will become

‘silent.’ Configure the

MX909A time constants to

support the external trigger

circuit.

Write a Null task with

the AQLEV bit set to

Its execution causes the DOC1 and DOC2

capacitors’ differential voltage to be clamped to

effectively reset the training trigger circuit. It

also configures the MX909A DOC1 and DOC2

voltage time constants to the best values for

training trigger circuit operation.

3 Wait for ENV signal to rise. Wait for the training

trigger signal to rise. This event indicates MX909A signal acquisition

should be started.

4 Initiate MX909A signal

acquisition. Write the MX909A

Control Register

settings to configure

LEVRES (B3, B2) to

Peak Averaging

mode.

When a subsequent related task is written to

the MX909A this will configure it with time

constant values usually appropriate for normal

Mobitex system operation.

5 Initiate MX909A signal

acquisition. Write a Null task to

the MX909A with

AQLEV and AQBC

set

This is the "subsequent related task" mentioned

above. Its execution is the first part of the

normal MX909A training process described in

the Data Sheet.

6 11-bit time delay Start a timer that will

indicate when 11bit

times have elapsed

from the moment the

previous task was

issued.

Error rate is higher immediately after an AQBC

and AQLEV sequence is triggered. These

erroneous bits could trigger the frame sync

detection circuits. It is suggested that a SFH or

SFS task is set ~ 12 bits after setting the

AQLEV and AQBC sequence.

7 Search for Frame Sync Write a SFH or SFS

task to the MX909A Cause the modem to search the received

signal for a 16-bit sequence that matches the

frame synchronization pattern. IRQ and BFREE

will be set when the frame sync has been

found.

8 Receive Data Issue a RDB task to

the MX909A Causes the modem to read the next 240 bits as

a Mobitex data block. The 18 bytes are placed

in the data buffer and an indication is issued for

the micro to read the data from the data buffer.

9 Receive Data Repeat RDB task N

times. N is the number of data blocks contained in the

packet.

10 Loss of Signal Continue RDB task

Start a timer If at any time in the middle of a RDB task EOP

equals 1, this may be may be an indication of a

temporary loss of signal. The controlling device

could 'ride out' the EOP signal for up to 17 bit

times before aborting

Using the MX 909A in a Continuous Carrier System Page 4 of 5 MX909A Application Note

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Step Operating Phase Action Purpose

11 End of Packet Last block received

Write a Null task to

MX909A with AQBC

set

When the IRQ and BFREE are set from the

reception of the Nth block (last data block of

packet), there is the possibility of a

concatenated packet coming in. The signal

levels and DC offset are expected to match that

of the previous packet. Only AQBC is issued to

make sure that the MX909A acquires proper

clock synchronization.

12 Read data from Buffer To collect last block's

data from buffer

13 11-bit time delay

(Same as Step 6) Start a timer that will

indicate when 11bit

times have elapsed

from the moment the

previous task was

issued.

Error rate is higher immediately after an AQBC

and AQLEV sequence is triggered. These

erroneous bits could trigger the frame sync

detection circuits. It is suggested that a SFH or

SFS task is set ~ 12 bits after setting the

AQLEV and AQBC sequence.

14 Abort Write a Reset task to

command Register If at any time in steps 4 through 10 EOP equals

1 for longer than 17 bit times, the current task

should be aborted and return to step 1 (see

Note 1)

15 End of Data packet has

just been received so the

receiver output will become

‘silent.’ Configure the

MX909A time constants to

support the external trigger

circuit. (Same as Step 1)

Write the MX909A

Control Register

settings to configure

LEVRES (B3, B2) to

Lossy Peak Detect

mode.

This w ill make the DOC1 and DOC2 dynamic

response quick, as needed, even after

automatic level training occurs.

Table 1: Description of Sequence Actions to be used immediately after reception of a

data packet is completed

Notes

1. The correct use of the EOP signal depends on what the controller / MX909A are doing at the time. At the

end of a frame, use it to enact thorough reacquisition. During a frame, it may be used to indicate possible

poor blocks or total loss of signal. If halfway through a frame, the controlling device could 'ride out' the

EOP signal for up to 17 bit times, which is (the maximum number of bits in error that can be corrected by

the FEC bits) - (EOP delay).

Using the MX 909A in a Continuous Carrier System Page 5 of 5 MX909A Application Note

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3.2 Signal Acquisition

The signal acquisition process is represented on the State Flow Diagram shown in Figure 2.

Start

Acquisition

Abort

Acquisition

Complete

No Baseband

Carrier

Read

Data

@ else

@else

Last Block Received IRQ

Issue a Null Task

AQBC = 1

ENV = 1

Issue Null Task

AQLEV = 1

AQBC = 1

Start Timer 1

Always

LEVRES = Lossy Peak

Issue Null Task

AQLEV = 1

ENV = 0 & EOP=1

Issue a Reset Task

ENV = 0 & EOP = 1

Start Timer 2

Continue RDB Task

IRQ = 1 & BFREE = 1

Issue a RDB Task

ENV = 0 & EOP = 1

Issue a Reset Task

Timer 1= 11 bit times

LEVES = Peak Aver.

Issue SFH or SFS Task

AQLEV = 0

AQBC = 0

Fade-Ride

EOP=0 & ENV=1 & Timer 2=<17 bit-times

Continue RDB Task

ENV = 0 & EOP = 1

Timer 2 > 17 bit-times

Figure 2: Signal Acquisition Process State Flow Diagram

4. Conclusion

This document describes the implementation of a simple Envelope and End of Packet Detector external circuit

using the voltages produced by the level acquisition and tracking circuits inherent in the MX909A. The circuit

observes the DOC1 and DOC2 voltages with a high impedance circuit to determine when the envelope of the

received signal exceeds a pre-determined threshold level. This is used to trigger the signal acquisition

procedure. The reduction of the incoming signal amplitude below a certain level with respect to the DOC

voltages provides an indication of the End of Packet (it may also be caused by a temporary loss of signal). The

detection of an End of Packet is also useful to abort an ongoing frame reception process due to the lack of

signal. It may also help to clearly de-mark the end of a packet so reliable detection of a new packet can occur.

The outputs of the Envelope and End of Packet Detector Circuit will almost certainly directly drive pins on a

microcontroller where they will affect system state transitions. It is possible to integrate some or all of the

preceding functions into the microcontroller. The logic and monostable functions can be easily implemented in

software. It is also relatively simple to sample DOC1, DOC2, and RXAMPOUT using ADC with multiplexed

input, as available in most processors. The entire decision making process can then be implemented using

fuzzy decision software saving considerable external hardware.