DB829 Datasheet, PDF (6/29 Pages) CML Microcircuits – Low Voltage Operation

Baseband Signal Processor
6
MX829 PRELIMINARY INFORMATION
2. R5, R6, C6 and C8 form the gain components for the Rx Input Amplifier (AMP1).
R6 should be chosen as required by the signal level, using the following formula:
Gain = - ^R5
R6
C8 x R6 should be chosen so as not to compromise the low frequency performance of this product.
3. For best results, a crystal oscillator design should drive the clock inverter input with signal levels of at least 40% of
V_DD, peak to peak. Tuning fork crystals generally cannot meet this requirement. To obtain crystal oscillator design
assistance, consult your crystal manufacturer.
4. General Description
The MX829 consists of five main sections: the Audio Filter, the Programmable Attenuators, the DTMF encoder, the MSK
transmitter, and the MSK receiver. These sections are controlled via a serial ("C-BUS") interface. The five sections are
described below.
4.1 Audio Filtering
This consists of an input amplifier and a common audio filter section, which may be switched between Rx and Tx. The
filter section comprises an anti-alias filter followed by low-pass and high-pass filtering with an amplitude limiter to set the
maximum deviation. Three variable attenuation blocks may be used to set the volume (in Rx) or the modulation level (in
Tx). Pre- and de-emphasis can be added externally using resistors and capacitors around AMP1, AMP2 and the
microphone amplifiers, see Figure 7,Figure 8, and Figure 9. The anti-alias filter is designed to reduce aliasing effects
above 50kHz which is approximately half the internal filter's sample rate.
The filtering is designed to meet the ETS 300 086 specification.
Various powersave modes are incorporated.
4.2 MOD1 and MOD2 Attenuators
The MOD1 input can be connected directly to SUM OUT, so that the MOD1 and MOD2 outputs can then be used for two
point modulation. Alternatively, the MOD1 attenuator can be used for auxiliary gain adjustment, in which case the input
signal must be ac coupled with a suitable capacitor.
4.3 DTMF Encoder
This generates the standard DTMF tones according to the CONTROL 2 Register settings. It also has a powersave mode.
4.4 MSK Tx
The Tx function of the MSK modem continuously operates in a free format mode, which means that the preamble and
frame sync have to be programmed like normal data bytes. However, a 2-byte checksum may be generated automatically
by simply marking the beginning and end of the data to be used. Any number of whole bytes may be used to generate the
checksum.
After the last byte has been transmitted one additional "hang bit" is automatically added to the end. All Tx operations are
programmed from the "C-BUS" via an 8-bit buffer. The Tx part of the MSK Modem has a Powersave mode
The modulation output is one cycle of 1200Hz for a "1" and one and a half cycles of 1800Hz for a "0" at 1200 baud, or one
half cycle of 1200Hz for a "1" and one cycle of 2400Hz for a "0" at 2400 baud.
4.5 MSK Rx
In Rx, the modem automatically achieves bit sync and then recognizes the previously selected SYNC and/or SYNT word
of the MPT1327, ETS 300 230 or PAA1382 specifications. At the same time as one of the above, it can also recognize a
user programmed 16-bit RX SYNC WORD.
On reception of the SYNC, SYNT or RX SYNC WORD, the device will automatically (or manually at any time) start
checking the data and checksum. It provides a 1-bit correct/incorrect result every byte, so that any number of bytes can be
checked.
The Rx part of the MSK modem operates at 1200 or 2400 baud and has a powersave mode. Both MSK Rx and Tx work
in half duplex mode.
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