ADF7021 datasheet(43/44 Pages) AD

43 / 44 page

Preliminary Technical Data

ADF7021

Rev. PrI | Page 43 of 44

Using the Test DAC on the ADF7021 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 21 and

Figure 22) 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 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 Register 14 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.

ADF7021 Datasheet(PDF) 43 Page - Analog Devices