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HSP50214BVC

Programmable Downconverter

Intersil 

HSP50214B

2

1

0

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

FREQUENCY (RELATIVE TO fS)

FIGURE 24C. POLYPHASE RESAMPLER FILTER EXPANDED

RESOLUTION PASSBAND FREQUENCY

RESPONSE

is limited to 55MHz. Using the symmetric filter pre-sum

approach, PROCCLK limits the programmable FIR to

110MIPS (millions of instructions per second) effective due to

symmetry. If the CDMA filter (loaded into the programmable

FIR Section) requires an impulse response with a span of 12

chips, the filter at 2x the chip-rate would need 24 taps. The 24

taps would translate into 59MIPS = (1.2288MHz)(2)(24). To

get the same filtering at 8x the chip rate would require

944MIPS = (1.2288MHz)(8)(96). Direct 8x filtering can not be

accomplished with the programmable filter alone because

944MIPS are much greater that the 60MIPs effective limit set

by PROCCLK. It is necessary to decimate down to 2x the chip

rate to get a realistic number of filter taps. Both interpolation

halfband filters are then used to obtain the 8x CDMA output.

944MIPS is a lot of MIPS. The HSP50214B gets the

equivalent processing by decimating down and interpolating

backup.

TABLE 10. POLYPHASE AND INTERPOLATING HALFBAND

FILTER MAXIMUM CLOCKING RATES

MODE

RE-SAMPLER

INPUT

INTER-

CLOCK

RATE

POLATION

CYCLES (MHz)

RATE

OUTPUT

RATE

(MHz)

Bypass

0 55.00

-

55.00

Polyphase Filter 6 55/6 = 9.17

-

9.17

(Note 3)

Polyphase and

13 55/13 = 4.23

2

8.46

1 Halfband

(Note 3)

Filter

Polyphase and

23 55/23 = 2.39

4

9.56

2 Halfband

(Note 3)

Filters

1 Halfband

Filter

7 55/7 = 7.86

2

15.72

2 Halfband

Filters

17 55/17 = 3.24

4

12.94

NOTE:

3. This frequency is set by the Resampler NCO.

In burst systems (such as TDMA), time resolution is needed

for quickly identifying the optimum sample point. The timing

is adjusted by shifting the decimation in the DSP μP to the

closest sample. Use of timing error in this way may yield a

faster acquisition than a phase-locked loop coherent bit

synchronization. Finding the optimum sample point

minimizes intersymbol interference.

Fine time resolution is needed in CDMA systems to resolve

different multipath rays. In CDMA systems, the demands on

the programmable FIR can only be relieved by the

resampler/interpolation halfband filters. Assume the chip rate

for a baseband CDMA system is 1.2288MHz and PROCCLK

POLYPHASE

RESAMPLER

FILTER

HALFBAND

FILTER #1

HALFBAND

FILTER #2

RESAMPLER

NCO

PULSE

DELAY

PROCCLK

PROCCLK/N

PULSE DELAY

COUNTER

THIS BLOCK GENERATES EXTRA

DATA READY PULSES FOR THE

NEW OUTPUTS FROM THE

INTERPOLATION PROCESS.

NV = INVALID MODE

00 0

00 1

01 0

01 1

10 0

10 1

11 0

11 1

# EXTRA

PULSES

0 BYPASS

0

1

1

3 (NV)

3 (NV)

3

3

FIGURE 25. GENERATING DATA READY PULSES FOR

OUTPUT DATA

Timing NCO

The Timing NCO is very similar to the carrier NCO Phase

Accumulator Section. It provides the NCO driven sample

pulse and associated phase information to the resampling

filter process described in the Re-sampler Filter Section. The

Timing NCO does not include the SIN/COS Section found in

the Carrier NCO. The top level block diagram is shown in

Figure 26.

28

FN4450.4

May 1, 2007

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