PDSP16350

8

Modulated Frequency

The output frequency can be modulated very simply, see

Fig 8. Since the phase increment value can be loaded as a

complete word every cycle, there is no need to provide internal

double buffering to prevent spurious frequencies being gener-

ated during the load operation. Binary Frequency Shift Keyed

(BFSK) modulation can easily be implemented by externally

multiplexing between two phase increment values represent-

ing the two frequencies to be used. The value to be used can

be instantaneously changed, thus maintaining phase coher-

ence, whilst the bit to be transmitted changes from a mark to

a space. Frequency hopping could also be simply effected by

clocking a new random number into the DIN port once every

thousand cycles, for instance. The output will reflect any

change in the frequency after 31 system clock cycles.

If the phase increment value on the DIN port is changed on

each clock cycle, then the output frequency will change

without introducing any dis-

continuities. Thus, a linear

frequency sweep can be

achieved by incrementing the

value on the DIN port by a

fixed amount each cycle. Al-

ternatively, a logarithmic

sweep could be implemented

by ‘walking’ a one across the

DIN port. Shifting the input

one place to the left every

hundred cycles, for example,

would double the frequency

every time.

Chirp generation for FM -

CW Radar systems is a typi-

cal example of the need for

linear frequency sweeps. This

application requires the gen-

eration of quadrature chirp

waveforms and is illustrated

in simplified form by Fig. 7.

One waveform is needed for

Fig. 8 Frequency Modulation Timing Diagram

16 bit

Cordic

Sin / Cos

Generator

Gain

Compensate

ROM

D17:0

D33:18

PDSP

16350

D/A

D/A

SIN

COS

SWEEP

GENERATOR

the transmitter, and the other for the receiver. The phase

increment value is supplied by the counter block which simply

increments at a rate determined by dividing down the time

base clock. The synthesised frequency thus increases during

the sweep period.

A number of the more significant phase increment bits are

used to supply the addresses to a PROM. The output of this

PROM is used to amplitude modulate the sine and cosine

waveforms. In this manner it is possible to compensate, at the

source, for any poor frequency versus gain characteristics of

analog circuits further along in the system.

The digital outputs directly drive two D/A converters. Once

in the analog world, it is necessary to remove the alias

frequencies with low pass filters. The phase linearity and pass

band ripple characteristics of these filters are very important,

if the correct phase relationships are to be maintained be-

tween the two waveforms.

Fig. 7 Quadrature Chirp Generator

1

2

3

4

5

CLK

RESET

DATA IN

RESULT

30

31

32

33

34

35

A

B

A

B

Device Reset

Apply First Data

First Result Available

C

D

JUMP

MODE

C

D