ADF4377 datasheet(46/79 Pages) AD

46 / 79 page

Data Sheet

ADF4377

APPLICATIONS INFORMATION

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Rev. 0 | 46 of 79

Figure 100. Common Clock Interface: Single Ended Clock with End

Termination (ZO = 50 Ω)

MEASURING DIFFERENTIAL SPURS WITH A

SINGLE-ENDED TEST INSTRUMENT

Using a spectrum analyzer to measure spurious signals on the

single-ended output of a clock generation chip gives pessimistic

results, particularly for outputs that approximate square waves.

There are two reasons for this.

First, because the spurious energy is often an ac signal superim-

posed on the power supply, a differential output rejects the spurs to

within the matching of the positive and negative outputs. Observing

only one side of the differential output provides no rejection.

Second, and most importantly, the spectrum analyzer displays all

of the energy at its input, including amplitude modulation that

occurs at the top and bottom pedestal voltage of the square wave.

However, only amplitude modulation near a zero crossing affects

the clock.

The best way to remove this measurement error is to drive the

clock generator output differentially into a limiting buffer on a sep-

arate clean power supply. One of the differential outputs of the

limiting buffer can then connect to a spectrum analyzer to correctly

measure the spurious energy. An example of this technique using

the ADF4377 as the clock generator and an HMC940 as the limiter

is shown in Figure 101.

Figure 101. Example of Spurious Measurement Techniques

APPLICATION CIRCUITS

Parallel ADF4377 Devices, 13 fs RMS Jitter

Figure 102. Block Diagram of Parallel ADF4377 Devices

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