HSP50215 데이터 시트보기 (PDF) - Intersil

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HSP50215

Digital UpConverter

Intersil 

HSP50215

of a quadrature NCO to produce a zero I.F. FM signal. These

FM modulated quadrature samples are then up sampled in

the interpolation filter to the output sample rate. The

baseband modulated signal is then upconverted to the

carrier frequency by the carrier NCO and mixers. The output

is then summed with the cascade input signal, saturated,

and formatted for output.

SHAPING

FM

INTERPOLATION

I

FILTER

MODULATOR

FILTER

FIGURE 6. FM WITH PULSE SHAPING

In Mode 10, the amplitude out of the shaping filter needs to

be limited in order to prevent frequency excursions that

cannot be filtered out in the interpolation filter. The quality of

the FM signal is affected by the amplitude slew rate out of

the shaping filter. As a rule of thumb, limiting this slew rate to

less than 1/8 the sample rate will minimize this distortion.

FM Modulator

The FM modulator provides for frequency modulation of the

carrier center frequency by the PUC input data. The FM

modulator is driven either directly by the PUC I input (Mode

1) or by the output of the FIR shaping filter (Mode 2). The

input data to the FM Modulator, is defined as dφ(n)/dt, where

φ(nT) is the phase of a theoretical sinusoid described by:

s(n) = A (cos[φ(nT) ]+ j sin [φ(nT)]); A ≈ 1 in Modulator (EQ. 1)

Figure 7 illustrates the conceptual design of the FM modulator.

The input to the FM modulator, dφ(n)/dt, is integrated via the

carrier NCO accumulator. The NCO accumulator output

represents phase and is used to address a SIN/COS generator,

synthesizing a sinusoid of the form described in Equation 1.

The phase accumulator feedback of the NCO is 16 bits and

sixteen bits of the phase word are routed to the SIN/COS

generator. Sixteen bits of resolution are provided on the Sine

and Cosine outputs.

16

dφ(nT)/dt

∑

EnNCO

MODE

1 OR 2

16

φ(nT)

16 COS[φ(nT)]

R

16 SIN[φ(nT)]

E

>G

FIGURE 7. FM MODULATOR BLOCK DIAGRAM

The transfer function of the FM modulator is defined by the

change in degrees per sample value, dφ(nT)/dt, where

dφ(nT)/dt is a 16-bit, twos complement, fractionally notated

frequency control word with a range from -FSAMP/2 to

+FSAMP/2. FSAMP is defined as the sample rate into FM

modulator. The maximum phase step that can occur in one

clock is ±180 degrees. Table 1 provides the change in phase

weighting of the input bits.

TABLE 1. FM MODULATOR TRANSFER FUNCTION

dφ(nT)/dt

DEGREES/SAMPLE

1000 0000 0000 0000

-180

0000 0000 0000 0000

0

0111 1111 1111 1111

~+180

Shaping Filter

The shaping filter provides the necessary pulse shaping

required on the input data to implement various quadrature

ASK and shaped FM modulation formats. Two identical

shaping filters (one each for the I and Q channels) are

provided. The filters can implement a 4-16 input sample

span impulse response using up to 256 taps with 16 bits of

resolution in the coefficients.

The range of valid digital values for the coefficients is from

8001 to 7FFF. The value 8000 is not allowed. The coefficient

format is 2’s complement. The span of the Impulse response

of the polyphase filter can be from 4-16 samples. The

desired sample span value minus one is programmed into

the Data Samples (DS) field in Control Word 19, bits 2-5.

The filter has a programmable interpolation rate (IP) of 4, 8,

or 16. This interpolation rate is programmed by Control

Address 19, bits 0 and 1. Thus, the required number of

coefficients (or filter span) becomes

# Coefficients = (DS)(IP)

(EQ. 2)

with 256 being the maximum number of coefficients.

Note that

REFCLK > (DS)(IP)(fS)

(EQ. 3)

where fS is the input sample rate of the shaping filter. For a

16 input sample impulse response span, the total impulse

response is 64, 128 or 256 filter taps for interpolation rates of

4, 8 or 16, respectively. The filter structure precludes

coefficient re-use for symmetric filters, so both asymmetric

and symmetric filters have up to 256 taps available and are

loaded in identical manner.

The maximum input sample rate is:

fS = fCLK ⁄ [(IP)(DS) ]

(EQ. 4)

where fCLK is the frequency of the reference clock, IP is the

shaping filter interpolate rate; and DS is the number of data

samples in the filter span. For example, if fCLK = 52MHz, the

filter span is 16 samples, and the interpolation rate is 16,

then the maximum input sample rate, fS is 52/256 = 203kHz.

Table 2 shows several examples of calculations for FIR input

sample rates based on master reference clock rate, number

of data samples, and interpolation rate.

3-428

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