AD9854AST datasheet(23/52 Pages) AD

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AD9854

Rev. E | Page 23 of 52

MODE

TW1

TW2

FSK DATA

TRIANGLE

BIT

010 (RAMPED FSK)

I/O UD CLK

Figure 40. Effect of Triangle Bit in Ramped FSK Mode

MODE

TW1

TW2

FSK DATA

000 (DEFAULT)

010 (RAMPED FSK)

I/O UD CLK

Figure 41. Effect of Premature Ramped FSK Data

Figure 41 shows that premature toggling causes the ramp to

immediately reverse itself and proceed at the same rate and

resolution until the original frequency is reached.

The control register contains a triangle bit at Parallel Register

Address 1F hex. Setting this bit high in Mode 010 causes an

automatic ramp-up and ramp-down between F1 and F2 to

occur without toggling Pin 29, as shown in Figure 40. The logic

state of Pin 29 has no effect once the triangle bit is set high. This

function uses the ramp rate clock time period and the step size

of the delta frequency word to form a continuously sweeping

linear ramp from F1 to F2 and back to F1 with equal dwell

times at every frequency. Use this function to automatically

sweep between any two frequencies from dc to Nyquist.

In the ramped FSK mode with the triangle bit set high, an

automatic frequency sweep begins at either F1 or F2, according

to the logic level on Pin 29 (FSK input pin) when the triangle

bit’s rising edge occurs (Figure 42). If the FSK data bit is high

instead of low, F2, rather than F1, is chosen as the start frequency.

Additional flexibility in the ramped FSK mode is provided by

the AD9854’s ability to respond to changes in the 48-bit delta

frequency word and/or the 20-bit ramp rate counter at any time

during the ramping from F1 to F2 or vice versa. To create these

nonlinear frequency changes, it is necessary to combine several

linear ramps with different slopes in a piecewise fashion. This is

done by programming and executing a linear ramp at a rate or

slope and then altering the slope (by changing the ramp rate

clock or delta frequency word, or both). Changes in slope can

be made as often as needed before the destination frequency has

been reached to form the desired nonlinear frequency sweep

response. These piecewise changes can be precisely timed using

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