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Two Versions: Different Applications

The -27 and -25 versions of the HSP50306 Digital QPSK

Demodulator are not simply different speed grades of the

same device, but are designs which have proportionally

scaled clocks and bandwidths for different applications.

NOTE: While these parts are pin for pin compatible, in

most applications they cannot be used as functional

equivalent substitutes for each other. Key differences are:

• The -27 version of the HSP50306 has an input IF of

10.7MHz with an input clock of 26. 97MHz.

• The -25 version of the HSP50306 has an input IF of

2.1MHz with an input clock of 25. 6MHz.

In both the -27 and -25 designs, the sample rate clock for the

input IF signal is half of the CLK frequency.

NOTE: Sample

IF and input clock, all other performance parameters of the

two parts are identical for their respective IF inputs.

10.7MHz Input IF Applications

Both the -27 and -25 parts can be used in 10.7MHz IF Applica-

tions. Figures 3 and 4 show the frequency spectrum for the

sampled 10.7MHz IF input signals for both the -27 and -25 ver-

sions, respectively. In the 10.7MHz IF Application, the -25 ver-

sion offers tighter filtering capability than the -27 version

because the lower IF allows use of low pass filtering. Also, the

lower IF of the -25 version has inherently lower internal pro-

cessing spectral spurs than the -27 version. Note that the

receive IF for the HSP50306SC-27 is the input IF to the demod-

ulator. For the HSP50306SC-25, the receive IF is 10.7MHz, but

the processing is done on the spectral image at 2.1MHz. Exam-

ine the spectral inversion between the 10.7MHz Receive IF and

the 2.1MHz demodulator input in Figure 4. The transmit differ-

ential encoder must take into account this spectral rever-

sal. The required encoding is shown in Table 3. This part was

designed to be paired with the HSP50307 Burst Modulator, and

can be operated from the same 25.6MHz reference clock.

TABLE 3. DIFFERENTIAL ENCODING REQUIRED FOR THE -27

AND -25 DEMODULATORS RECEIVING 10.7MHz IF

INPUT

BITS

PHASE CHANGE

REQUIRED FOR -27

DEMODULATION

PHASE CHANGE

REQUIRED FOR -25

DEMODULATION

00

01

10

11

FIGURE 3. SAMPLED SPECTRUM FOR THE -27 VERSIONS

RECEIVE IF

DEMOD INPUT IF

DC

2.79

FIGURE 4. SAMPLED SPECTRUM FOR THE -25 VERSIONS

RECEIVE IF

DEMOD INPUT IF

DC

2.1

HSP50306