PDSP16515A Datasheet, PDF (21/25 Pages) Mitel Networks Corporation – Stand Alone FFT Processor

PDSP16515A
Arithmetic Accuracy
16 bit,unconditional
scaling
24 bit arithmetic with
unconditional scaling,
16 bit inputs
Max Tone Slot Noise
WRT Noise Test
60
44
2 Tones
with
Freq Spread
45
88
67
65
16 bit inputs with
85
67
65
PDSP16515A block FP
Full 32 bit Floating point
with 16 bit inputs
93
82
67
Table 8. Comparative Dynamic Range
Measurements
The ‘best’ dynamic range figures will be obtained with single
tone measurements, and these results are often quoted to
indicate the need for greater bit accuracies. The measure is
the ratio of a full scale sinusoid to the average noise level and
the results will be essentially independent of the window
operator. The results given by the PDSP16515A are
compared to various other configurations in the first column of
Table 8. With this method the dynamic range is bound to
improve as more bits are used to represent the data.
Theoretically 6 dB of dynamic range will be obtained for every
bit representing the input data, if the internal arithmetic
accuracy gives no degradation in performance. In practice this
improvement has no significance since the incoming
waveforms will be much more complex than a single sinusoid.
An alternative method of determining dynamic range is with a
slot noise test. White noise is passed through a narrow-band
notch filter, several frequency bins wide, and the FFT
computed. There is no noise in the filtered slot at the input to
the FFT, but there is noise in the frequency bins corresponding
to the width of the notch. Dynamic range is measured as the
difference in dB of the average signal power and the average
noise power and can be considered to give more useful
results. Comparative results from various configurations are
also given in the second column of Table 8. The performance
with 24 bit data is seen to be little better than that obtained with
the PDSP16515A. This can be attributed to the scaling
scheme, word growth, and rounding method used within the
device.
When two nearby tones are to be capable of detection, the
window operator will dictate the performance of the system.
The final column in Table 8 illustrates the results obtained
using two sinusoids of different amplitudes, with the larger one
residing mid-way between two frequency bins, and the smaller
5.5 bins away. The two frequencies are five bins apart to avoid
the effects of the mainlobe widths. The dB figures given are the
difference in amplitude between the two signals when the
smaller one is still just detectable as a separate peak from the
larger one.
This technique illustrates the performance of the window,
since the amount by which sidelobe structure of the larger
signal swamps the mainlobe of the smaller signal will
determine if the smaller is detected. The theoretical
attenuation of the highest sidelobe levels, with respect to the
mainlobe, for the window options provided by the
PDSP16515A have been given in Table 7, and represent the
dynamic range that can be obtained if arithmetic effects are
ignored. The results in the final column in Table 8 are the
practical results given by the device, and as with the slot noise
test indicate that the arithmetic scheme used by the
PDSP16515A is equivalent to using 24 bit data. The Blackman
Harris window was used in all cases.
User Notes - Stopping DOS
General Description
The transform is calculated internally fully synchronous to
SCLK. However, as all outputs are referenced to DOS, a
transfer has to be made between the two clocks. In addition,
some dummy DOS strobes are needed to operate the internal
control logic, and to advance data from the internal RAMs to
the output pins.
The most simple configuration for the device is to have DOS
running continuously and for DEN to be permanently active.
When this happens the user will just be aware of data
appearing on the output pins on the same DOS cycle when
DAV goes active. However, there are many situations where
either DOS is not continuously running, or DEN is not
permanently active. To help explain how to operate the device
in these situations, the internal operation of the output circuits
must be described. For those who are not going to be
interrupting DOS, the remainder of this section can be
ignored.
Internal RAM - General Description
For single device operation of transforms less than 1024
points, the internal RAM is shared between three separate
operations which enable the device to output old transformed
results, calculate the current transform, and input new data
ready for the next transform. All these operations, along with
the internal control logic, are controlled by a 12-cycle state
machine. The RAM operations are :
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