HSP50215VC datasheet(7/21 Pages) INTERSIL

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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.

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 d

φ(n)/dt, where

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

Figure 7 illustrates the conceptual design of the FM modulator.

The input to the FM modulator, d

φ(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.

The transfer function of the FM modulator is defined by the

change in degrees per sample value, d

φ(nT)/dt, where

φ(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.

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

with 256 being the maximum number of coefﬁcients.

Note that

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:

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.

INTERPOLATION

FILTER

SHAPING

FILTER

FIGURE 6. FM WITH PULSE SHAPING

MODULATOR

()

A (cos

φ nT

() ]+ j sin φ nT

()

[]); A ≈ 1 in Modulator

[

(EQ. 1)

MODE

EnNCO

φ(nT)/dt

φ(nT)

COS[

φ(nT)]

FIGURE 7. FM MODULATOR BLOCK DIAGRAM

SIN[

φ(nT)]

∑

1 OR 2

TABLE 1. FM MODULATOR TRANSFER FUNCTION

φ(nT)/dt

DEGREES/SAMPLE

1000 0000 0000 0000

-180

0000 0000 0000 0000

0111 1111 1111 1111

~+180

# Coefﬁcients = (DS)(IP)

(EQ. 2)

REFCLK

() IP

() f

()

(EQ. 3)

S =fCLK [IP

()

⁄

() ]

(EQ. 4)

HSP50215

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