ADF7020 datasheet(38/40 Pages) AD

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ADF7020

Preliminary Technical Data

Rev. PrH | Page 38 of 40

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 40.

Using the Test DAC on the ADF7020 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 (Figure 16 and

Figure 17) 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-∆ 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/ASK 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 (to remove the IF component in the ddt case). It also has

a signal gain term to allow the 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.

•

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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