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Cut down noise when making M-PHY measurements

Posted: 11 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:MIPI? M-PHY? Gear 3? test? transmitter?

Each new generation of mobile devices calls for even higher speed and longer battery life. To achieve these goals, mobile product designers are turning to MIPI M-PHY Gear 3 and its 5.8 Gbit/s data rates. Measuring a design's conformance to MIPI standards is becoming more of a challenge than it was at lower speeds.

There are a number of M-PHY test requirements that make transmitter testing challenging if you don't properly prepare them. M-PHY Gear 3 TX (transmitter) tests include intra-lane skew, pulse width, common mode power-spectral density, total jitter, and deterministic jitter. For saving power, M-PHY transmitters can also support different drive strengths. The M-PHY TX measurements are defined in the CTS (Conformance Test Suite) with the TX running in both LA (Large Amplitude) and SA (Small Amplitude) modes.

The low-amplitude signalling and the need to measure jitter result in a requirement that an oscilloscope and its probe acquire low peak-to-peak voltage signals with fast edges. In the power-saving SA mode, the M-PHY transmitter's peak-to-peak output voltage is just 280 mV maximum. Slew rates must be controlled to reduce EMI noise. Moreover, minimising jitter at high data rates requires an edge speed that's a fraction of a UI (unit interval).

Oscilloscope and probe noise performance are critical to accurately measure the characteristics of the M-PHY signals at high data rates. Without a low noise floor, other capabilities of the oscilloscope and probe are useless because the noise can hide a signal's key characteristics. With M-PHY Gear 3, the noise performance of the measuring system is critical because of tight amplitude requirements defined in the specification. In SA mode, the TX peak-to-peak output voltage ranges from 160 mV to 280 mV. The difference between logic levels is relatively small if the oscilloscope and probe don't have low noise and don't have sufficient sensitivity (12 mV/div to 15 mV/div, 120 mV full-scale).

Even when the TX is running in LA mode, the amplitude of M-PHY signals is relatively small, 0.5 VP-P. The table lists the range of the minimum and maximum voltage levels for the outputs (TXDP and TXDN) operating in LA and SA modes.

 The range of the minimum and maximum voltage levels

Table: The range of the minimum and maximum voltage levels for the outputs (TXDP and TXDN) operating in LA and SA modes.

How can you reduce the noise that a probe and oscilloscope add to your measurement? Start by checking the published noise specifications, including the probe's attenuation factor. High-impedance probes will attenuate the signal. A high attenuation factor limits a probe's loading on the device under test. Probe loading isn't an issue with M-PHY, because it's expected that the bus will have a low-impedance (100 ) differential load when running in terminated mode. The probe's attenuation factor works against the goal of having a low-noise input by reducing the signal-to-noise ratio of the measurement by a factor equal to the probe's attenuation value.

To see how this works, let's compare two probe types using 5.8 Gbit/s, low amplitude (~200 mVP-P) data signals. The first probe uses 50 , SMA-style inputs. Because the loading of the probe is not a concern, the SMA-style probe can be used to acquire M-PHY TX signals. The second probe is a high-impedance differential probe with a 100 termination fixture attached to the TX outputs and then attached to the probe inputs.

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