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# Why do the WFM601i and WFM601M show a slower rise time for the eye pattern display than the WFM700M and VM700T opt 1S?

### Question :

Why do the WFM601i and WFM601M show a slower rise time for the eye pattern display than the WFM700M and VM700T opt 1S?

The WFM700 was designed with a large bandwidth to support HD and therefore this faster response is used in SD to show the faster risetime of the SD-SDI signal. A similar high bandwidth circuit is also used in the VM700T.

The WFM 601i and WFM601M Eye display has a -3 dB bandwidth of approximately 450 MHz. This is a 20 - 80% risetime of 500 ps. This finite risetime must be considered when determining the actual risetime of the source being measured.

The actual risetime of the source being measured can be calculated using the following formula:

Tr(source) = sqrt[(Tr(measured)^2 - (0.5ns)^2]

Note: Tr(source) = the actual 20-80% risetime of the source in ns.
Tr(measured) = the 20 - 80% risetime measured on the WFM601i or WFM601M in ns.

This table shows some conversion values between measured and actual risetimes:

 Meas. Act. 640 ps 400 ps 710 ps 500 ps 780 ps 600 ps 860 ps 700 ps 900 ps 750 ps 940 ps 800 ps 1.0 ns 900 ps 1.1 ns 1.0 ns 1.2 ns 1.1 ns 1.3 ns 1.2 ns 1.4 ns 1.3 ns 1.5 ns 1.4 ns 1.6 ns 1.5 ns

This FAQ Applies to:

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Product:

FAQ ID 57966

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