SC6579 Просмотр технического описания (PDF) - Silan Microelectronics

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SC6579

RADIO DATA SYSTEM (RDS) DEMODULATOR

Silan Microelectronics 

Silan

Semiconductors

SC6579

Filter part

The MUX signal is band-limited by a second-order anti-aliasing-filter and fed through a 57 kHz band-pass filter (8th

order band-pass filter with 3 kHz bandwidth) to separate the RDS signals. This filter uses switched capacitor

technique and is clocked by a clock frequency of 541.5 kHz derived from the 4.332/8.664 MHz crystal oscillator.

Then the signal is fed to the reconstruction filter to smooth the sampled and filtered RDS signal before it is output on

pin 8. The signal is AC-coupled to the comparator (pin 7). The comparator is clocked with a frequency of 228 kHz

(synchronized by the 57 kHz of the demodulator).

Digital part

The synchronous demodulator (Costa’s loop circuit) with carrier regeneration demodulates the internal coupled,

digitized signal. The suppressed carrier is recovered from the two sidebands (Costa’s loop). The demodulated signal

is low-pass-filtered in such a way that the overall pulse shape (transmitter and receiver) approaches a cosinusoidal

form in conjunction with the following Integrate and dump circuit.

The data-spectrum shaping is split into two equal parts and handled in the transmitter and in the receiver. Ideally,

the data filtering should be equal in both of these parts. The overall data-channel-spectrum shaping of the

transmitter and the receiver is approximately 100% roll-off. The integrate and dump circuit performs an integration

over a clock period. This results in a demodulated and valid RDS signal in form of bi-phase symbols being output

from the integrate and dump circuit. The final stages of RDS data processing are the bi-phase symbol decoding and

the differential decoding. After synchronization by data clock RDCL (pin 16) data appears on the RDDA output (pin

2). The output of the bi-phase symbol decoder is evaluated by a special circuit to provide an indication of good data

(QUAL = HIGH) or corrupt data (QUAL = LOW).

Timing

Fixed and variable dividers are applied to the 4.332/8.664 MHz crystal oscillator to generate the 1.1875 kHz RDS

clock RDCL, which is synchronized by the incoming data. Which ever clock edge is considered (positive or negative

going edge) the data will remain valid for 399 µs after the clock transition. The timing of data change is 4 µs before a

clock change. Which clock transition (positive or negative going clock) the data change occurs in, depends on the

lock conditions and is arbitrary (bit slip).

During poor reception it is possible that faults in phase occur, then the clock signal stays uninterrupted, and data is

constant for 1.5 clock periods. Normally, faults in phase do not occur on a cyclic basis. If however, faults in phase

occur in this way, the minimum spacing between two possible faults in phase depends on the data being transmitted.

The minimum spacing cannot be less than 16 clock periods. The quality bit changes only at the time of a data

change.

HANGZHOU SILAN MICROELECTRONICS JOINT-STOCK CO.,LTD

Rev: 1.0 2001.12.30

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