Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 112)

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ISSUED: June 2013 923133 Rev A
FILTERING
A low-pass filter that removes high-frequency noise and provides equalization is needed for the
newer optical recording systems (e.g., DVD). In the DVD read-only and recordable specifications
are given the frequency characteristics of the low pass filter (LPF) and equalizer (EQ) as a graph.
The combination of these must meet within 1 dB below 7 MHz, and it is recommended to meet it
up to 10 MHz. Also, group delay variation for frequencies </= 6.5 MHz must be >/= ±3 ns, and
gain at 5.0 MHz minus gain at 0 Hz must be 3.2 ±0.3 dB. For the LPF, it gives an example
implementation to achieve these characteristics as a 6
th order Bessel filter with a cutoff frequency
fc (-3dB) = 8.2 MHz, and an example for the EQ is a three-tap transversal filter.
The OData function implements the 6 th order Bessel filter as a FIR filter to provide the low-pass
filter capability. The number of coefficients of the FIR depends on the ratio between the cutoff
frequency fc and the sample rate fs. For a 1x DVD with an fc of 8.2 MHz, sampled at 500 MS/s,
approximately 220 taps are required. Sampling at 1 GS/s is about twice that. Ideally, the sampling
rate should be 10 to 20 times the clock rate. For a 1x DVD with a clock period of 37 ns, the
sample rate should be 500 MS/s.
The three-tap equalization filter (EQ) is applied to the data after it has been low-pass filtered. The
three samples input to the EQ are not adjacent; they are at 0 and ±2T, where T is a 1/channel bit
rate.
Because the spacing in DSO samples depends on data rate and sample rate, T is likely to be a
non-integer number of samples. In this case, interpolation is used to find the values at -2T and
+2T.
SLICER
The Slicer is a 1 st order integrating slicer with a programmable closed loop bandwidth (e.g., 5 kHz
for 1x DVD as specified in DVD-R Annex G and DVD Annex H). Besides producing the sliced
data, the slicer can output the difference of the input signal and the slicer threshold level. The
slicer threshold will be determined by an exponential average of data samples computed as:
New thresh = (n - 1)/n * old thresh + 1/n * new data
Where n is chosen to meet the bandwidth requirement at the current sample rate.
Parameter Description Possible Values
Cutoff
frequency Cutoff frequency for the Equalizer (low-pass
filter) 10 kHz to 800 MHz
Boost Boost for the Equalizer 0 to 20 dB
Slicer BW Controls the bandwidth of the Slicer 1 to 200 kHz
PLL BW Controls the bandwidth of the PLL (used for
clock recovery only) 1 to 200 kHz