FS714x

4.1.6. Feedback Divider Source MUX

The source of frequency for the feedback divider may be selected to be either the output of the post divider or the output of the VCO by

the FBKDSRC bit.

Ordinarily, for frequency synthesis, the output of the VCO is used. Use the output of the post divider only where a deterministic phase

relationship between the output clock and reference clock are desired (line-locked mode, for example).

4.1.7. Device Shutdown

Two bits are provided to effect shutdown of the device if desired, when it is not active. SHUT1 disables most externally observable

device functions. SHUT2 reduces device quiescent current to absolute minimum values. Normally, both bits should be set or cleared

together.

Serial communications capability is not disabled by either SHUT1 or SHUT2.

4.2 Differential Output Stage

The differential output stage supports both CMOS and pseudo-ECL (PECL) signals. The desired output interface is chosen via the

programming registers.

If a PECL interface is used, the transmission line is usually terminated using a Thévenin termination. The output stage can only sink

current in the PECL mode, and the amount of sink current is set by a programming resistor on the LOCK/IPRG pin. The ratio of output

sink current to IPRG current is 13:1. Source current for the CLKx pins is provided by the pull-up resistors that are part of the Thévenin

termination.

4.2.1. Example

Assume that it is desired to connect a PECL-type fanout buffer right next to the FS7140.

Further assume:

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