ADF7025 datasheet(39/44 Pages) AD

39 / 44 page

ADF7025

Rev. A | Page 39 of 44

DIGITAL

TEST MODES

Σ-∆

TEST MODES

ANALOG TEST

MUX

IMAGE FILTER ADJUST

PLL TEST MODES

ADDRESS

BITS

PRESCALER

4/5 (DEFAULT)

8/9

CR1

COUNTER RESET

DEFAULT

RESET

CS1

CAL SOURCE

INTERNAL

SERIAL IF BW CAL

DEFAULT = 32. INCREASE

NUMBER TO INCREASE BW

IF USER CAL ON

Figure 50. Register 12—Test Register

Using the Test DAC on the ADF7025 to Implement

Analog FM DEMOD and Measuring SNR

The test DAC allows the output of the postdemodulator filter

for both the linear and correlator/demodulators to be viewed

externally. It takes the 16-bit filter output and converts it to a

high frequency, single-bit output using a second-order error

feedback Σ-Δ converter. The output can be viewed on the

XCLKOUT pin. This signal, when IF-filtered appropriately, can

then be used to

• Monitor the signals at the FSK postdemodulator filter

output. This allows the demodulator output SNR to be

measured. Eye diagrams can also be constructed of the

received bit stream to measure the received signal quality.

• Provide analog FM demodulation.

While the correlators and filters are clocked by DEMOD_CLK,

CDR_CLK clocks the test DAC. Note that, although the test

DAC functions in a regular user mode, the best performance is

achieved when the CDR_CLK is increased up to or above the

frequency of DEMOD_CLK. The CDR block does not function

when this condition exists.

Programming the test register, Register 12, enables the test

DAC. Both the linear and correlator/demodulator outputs

can be multiplexed into the DAC.

signal in the case where there is an error in the received signal

frequency. If there is a frequency error in the signal, the user

should program half this value into the offset removal field.

It also has a signal gain term to allow usage of the maximum

dynamic range of the DAC.

Setting Up the Test DAC

•

Digital test modes = 7: enables the test DAC, with no

offset removal (0x0001C00C).

•

Digital test modes = 10: enables the test DAC, with

offset removal.

The output of the active demodulator drives the DAC; that is, if

the FSK correlator/demodulator is selected, the correlator filter

output drives the DAC.

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