Full digital equalization on RF channels

The navtec GmbH has developed a full digital equalisation with automatic delay normalisation for a manufacturer of simultaneous broadcasting systems as a replacement for a conventional analog solution. The system was realised using a 80-MHz DSP. The technical specifications can be found at the end of this page.
digital LF-channel equalizer
As it is customary in digital signal processing systems, the analog part and the digital part are strictly separated from each other. One can clearly make out the division between the analog and the digital part in the picture.

The analog part (above) consists of:

  • A transformer for galvanic separation (for the telephone line) 
  • A signal conditioning circuit
  • A CODEC (combined 2-channel A/D-D/A converter)
The digital part (below) consists of:
  • DSP
  • S-RAM
  • NV-RAM
  • PLD 
The 100x160mm sized board with a DSP module for the digital equalisation system.
(The board's layout was not realised by navtec GmbH.)

The problem with simultaneous broadcasting systems is to measure and compensate two or more unknown routes in their electrical characteristics (frequency, phase response and time delay behaviour). It is aimed to transmit a signal that is sent from a joint LF-source in such a way, that it will appear at the end of all routes with a marginal group delay difference error, in order to transmit it contemporaneously.

The presented system performs the measurement and compensation tasks automatically. To achieve this, suitable test signals are generated, transmitted and evaluated. After that suitable compensation filters and delay elements are calculated and applied on the signals.

The system can be scaled. This means, that up to 15 (full duplex) routes can be compensated against each other. The DSP on the respective modules are cross-linked for data exchange.

Technical specification for the digital RF equalizer

DSP-clock and instruction rate 78,75 MHz / 39,37 MIPS
sample rates  8,5 kHz und 25,6 kHz (Mixed-Rate)
Memory 32 k - Worte
Memory for data retention 8 kByte
Number of channels 2 - 15
Maximum group delay between channels 20 ms
Maximum group delay error after equalisation < 39 * 10 -6 s
Amount of frequency response equalisation > 24 dB
Amount frequency response error (ripple) < 1 dB
group delay distortion (300 Hz ... 3 kHz) < 50 * 10 -6 s
Time reference for measurement
  1. loop generation in a TX/RX route (loop mode)
  2. absolute time (GPS-Empfänger)
  3. loop generation with separate RX route (external auxiliary receiver)

© navtec GmbH 1997
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