PDSP16515AC0AC datasheet(21/27 Pages) MITEL

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PDSP16515A

Arithmetic Accuracy

Max Tone

Slot Noise

2 Tones

WRT Noise

Test

with

Freq Spread

16 bit,unconditional

scaling

24 bit arithmetic with

unconditional scaling,

16 bit inputs

16 bit inputs with

PDSP16515A block FP

Full 32 bit Floating point

with 16 bit inputs

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 :

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.

Table 8. Comparative Dynamic Range

Measurements

PDSP16515AC0AC Datasheet(PDF) 21 Page - Mitel Networks Corporation

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