AD1893 데이터 시트보기 (PDF) - Analog Devices

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AD1893

Low Cost SamplePort® 16-Bit Stereo Asynchronous Sample Rate Converter

Analog Devices 

AMP

PHASE

TIME

SUBFILTER COEFFICIENTS

ALIGNED TO THE LEFT

TSIN = 1/FSIN

AD1893

FSIN/2

DELAY = NOMINAL

FREQ

FSIN/2

DELAY = NOMINAL

FSIN/2

DELAY = NOMINAL – .25/FSIN

FSIN/2

DELAY = NOMINAL – .5/FSIN

FSIN/2

DELAY = NOMINAL – .75/FSIN

Figure 3. Four Polyphase Subfilters Realigned to Coarse Time Grid

INPUT

SIGNAL

POLYPHASE FILTER 1

POLYPHASE FILTER 2

POLYPHASE FILTER 3

POLYPHASE FILTER 4

POLYPHASE FILTER 5

POLYPHASE FILTER 6

POLYPHASE FILTER 7

N TO 1

MUX

OUTPUT

SIGNAL

POLYPHASE FILTER N-1

POLYPHASE FILTER N

SELECT

SAMPLE

CLOCK

TRACKING

CIRCUIT

Figure 4. Polyphase Filter Bank Model—Conceptual Block

Diagram

The full set of subfilters can be considered to form a parallel

bank of “polyphase” filters which have decrementing, linear

phase group delays. All of the polyphase filters conceptually

process the input signal simultaneously, as illustrated in Figure

4, at the input sample rate.

Asynchronous sample rate conversion under the polyphase filter

bank model is accomplished by selecting the output of a particu-

lar polyphase filter on the basis of the temporal relationship

between the input sample clock and the output sample clock

events. Figure 5 shows the desired filter group delay as a func-

tion of the relative time difference between the current output

sample clock and the last input sample clock. If an output

sample is requested late in the input sample period, then a short

filter delay is required, and if an output sample is requested

early in the input sample period, then a long filter delay is re-

quired. This nonintuitive result arises from the fact that FIR

filters always produce some delay, so that selecting a filter with

shorter delay moves the interpolated sample closer to the newest

input sample.

REV. A

–9–

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