AD9852SLASHPCB datasheet(23/52 Pages) AD

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AD9852

Rev. E | Page 23 of 52

OUT

ADDER

CLR ACC2

CLR ACC1

HOLD

PHASE

ACCUMULATOR

SYSTEM

CLOCK

FREQUENCY

TUNING

WORD 1

20-BIT

RAMP RATE

CLOCK

48-BIT DELTA

FREQUENCY

WORD (TWOS

COMPLEMENT)

FREQUENCY

ACCUMULATOR

Figure 40. FM Chirp Components

FREQUENCY

010 (RAMPED FSK)

000 (DEFAULT)

MODE

TW1

DFW

RAMP RATE

I/O UD CLK

Figure 41. Example of a Nonlinear Chirp

Basic FM Chirp Programming Steps

Program a start frequency into Frequency Tuning Word 1

(Parallel Register Address 4 hex to Parallel Register

Address 9 hex), hereafter called FTW1.

Program the frequency step resolution into the 48-bit, twos

complement delta frequency word (Parallel Register

Address 10 hex to Parallel Register Address 15 hex).

Program the rate of change (time at each frequency) into

the 20-bit ramp rate clock (Parallel Register Address 1A hex

to Parallel Register Address 1C hex).

When programming is complete, an I/O update pulse at Pin 20

engages the program commands.

The necessity for a twos complement delta frequency word is to

define the direction in which the FM chirp moves. If the 48-bit

delta frequency word is negative (MSB is high), the incremental

frequency changes are in a negative direction from FTW1. If the

48-bit word is positive (MSB is low), the incremental frequency

changes are in a positive direction from FTW1.

It is important to note that FTW1 is only a starting point for

FM chirp. There is no built-in restraint requiring a return to

FTW1. Once the FM chirp begins, it is free to move (under

program control) within the Nyquist bandwidth (dc to one-half

the system clock). However, instant return to FTW1 can be

easily achieved.

Two control bits (CLR ACC1 and CLR ACC2) are available in the

FM chirp mode that allow the device to return to the beginning

frequency, FTW1, or to 0 Hz. When the CLR ACC1 bit (Register

Address 1F hex) is set high, the 48-bit frequency accumulator

(ACC1) output is cleared with a retriggerable one-shot pulse of

one system clock duration. The 48-bit delta frequency word input

to the accumulator is unaffected by the CLR ACC1 bit. If the

CLR ACC1 bit is held high, a one-shot pulse is delivered to the

frequency accumulator (ACC1) on every rising edge of the I/O

update clock. The effect is to interrupt the current chirp, reset the

frequency to that programmed into FTW1, and continue the chirp

at the previously programmed rate and direction. Figure 42 shows

clearing of the frequency accumulator output in chirp mode.

Shown in the diagram is the I/O update clock, which is either user

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