Half Duplex GMSK Modem

FX579

© 1996 Consumer Microcircuits Limited

12

D/579/4

Acquire Sequence

Figure 7 shows the typical received output from a radio's frequency discriminator. A data transmission

in general begins with a burst of unmodulated carrier followed by preamble of, for example "11001100--

" and then frame sync, so as to allow the receive modem to establish timing and level lock as soon as

possible, before the message is sent.

In most applications there will also be a dc step in the output voltage from the receiver FM discriminator

due to carrier frequency offsets as channels are changed or when the distant transmitter is turned on.

Figure 7 shows two possible receive operating modes of the FX579.

The mode shown in (a) is

preferred and should be used when an external carrier detect signal is available, so as to provide the

fastest possible acquisition of the received signal. The mode in (b) is suggested for when a carrier

detect signal is unavailable. The receive operating modes are determined by the logic state of the

ACQUIRE pin as explained in Table 3.

(a)

Operation with a carrier detect (Preferred operating mode)

When an external carrier detect signal is available the µController should provide an ACQUIRE

pulse as shown. The ACQUIRE input is pulsed to a logic "1" a minimum of 2 bit times after the

carrier is detected (to allow for the receive filter delay) and just before preamble is expected (to

reduce the probability of false frame sync detection under noisy conditions).

The level

measurement DOC capacitors are then automatically clamped to the input signal for fast

acquisition. The ACQUIRE input should remain at a logic "1" for a minimum of 16 bit times to

enable the level measurement circuits to stabilise and the PLL to quickly achieve lock. The

ACQUIRE input should then be taken to a logic "0" again, for better signal to noise

performance, once the preamble or Frame Sync has been detected by the controlling

microprocessor.

(b)

Suggested operation without a carrier detect (Non-preferred operating mode)

When a carrier detect signal is not available, it is suggested that the modem be operated in the

Lossy Peak Detect/Wide Bandwidth mode (ACQUIRE pin at a logic "1") until the system

µController satisfactorily detects preamble and/or frame sync. Then the ACQUIRE pin should

be taken to a logic "0" for Peak Averaging/Narrow Bandwidth mode, so as to provide a better

signal to noise performance for the rest of the message. The duration of initial unmodulated

carrier plus preamble should be a minimum of 100 bit times to allow the voltages on the

external DOC capacitors to stabilise. At the end of the message the ACQUIRE pin should be

returned to a logic "1".

The FX579 generally can tolerate dc offsets in the received signal of at least ±0.5V with respect to

negative excursions of the (filtered) received signal, and could therefore be used to derive a measure

of the data signal amplitude and carrier frequency offset. Note, however, that these pins are driven

from very high impedance circuits, so that the dc load presented by any external circuitry should

exceed 10M