RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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11-1
11. DIGITAL EQUALIZATION/HIGH PASS FILTER
11.1 PROGRAMMABLE DIGITAL EQUALIZER (PDE)
A programmable digital equalizer is provided in the high speed receiver and transmitter paths as shown in Figure 11-1 and
Figure 11-2.
The programmable digital equalizer consists of four cascaded biquads. This is similar to the tone detectors when cascaded
by enabling bit 12TH. The programmable digital equalizer is enabled by setting bit PDEQZ. The block diagram is shown in
Figure 11-3.
11.1.1 PDE Frequency Response
The programmable digital equalizer defaults to a Japanese two link delay equalizer, with 1.1 dB gain at 2700 Hz. The
frequency response curves are shown in Figure 11-4.
11.1.2 PDE
Coefficients
The equation to calculate the hex coefficients is:
Coefficient
16
= Coefficient * 32768
The format is 16 bits, signed, twos complement. The coefficient range is from -1 (8000h) to +1 minus 1 least significant bit
(7FFFh).
The poles and zeros are shown in Table 11-1.
The programmable digital equalizer RAM access codes are shown in Table 11-2.
Note that these parameters need to be changed only once. They are not written by the modem at any time other than
power-on.
11.2 FIXED DIGITAL CABLE COMPROMISE EQUALIZER
An optional fixed cable compromise equalizer, enabled by setting bit DCABLE, is provided in the fax modes. Figure 11-1
shows its position in the receive path. Figure 11-2 shows its position in the transmit path. The digital cable equalizer
frequency response is shown in Figure 11-5.
11.3 HIGH PASS FILTER
An optional digital high pass filter (HPF), enabled by setting bit HPFEN, is provided in all receive paths. Figure 11-1 shows its
position in the receive path. Figure 11-6 and Figure 11-7 show the modem response with and without the high pass filter
enabled, respectively. Note that this filter response is only valid in the 8000 Hz sample rate modes. For the 9600 Hz sample
rate modes, the response is shifted such that the notch is at 72 Hz.
Summary of Contents for RFX144V24-S23
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