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You can use limit mask testing in a wide range of test and measurement applications. For example, you can use limit mask testing to determine that the power spectral density of ADSL signals meets the recommendations in the ANSI T1.413 specification.

The following sections provide examples of when you can use limit mask testing. In all these examples, the specifications are recommended by standards-generating bodies, such as the CCITT, ITU-T, ANSI, and IEC, to ensure that all the test and measurement systems conform to a universally accepted standard. In some other cases, the limit testing specifications are proprietary and are strictly enforced by companies for quality control.

Modem Manufacturing Example

Limit testing is used in modem manufacturing to make sure the transmit spectrum of the line signal meets the V.34 modem specification, as shown in the following figure.

The ITU-T V.34 recommendation contains specifications for a modem operating at data signaling rates up to 33,600 bits/s. It specifies that the spectrum for the line signal that transmits data conforms to the template shown in the previous figure. For example, for a normalized frequency of 1.0, the spectrum must always lie between 3 dB and 1 dB. All the modems must meet this specification. A modem manufacturer can set up an automated test system to monitor the transmit spectrum for the signals that the modem outputs. If the spectrum conforms to the specification, the modem passes the test and is ready for customer use. Recommendations such as the ITU-T V.34 are essential to ensure interoperability between modems from different manufacturers and to provide high-quality service to customers.

Digital Filter Design Example

You also can use limit mask testing in the area of digital filter design. You might want to design lowpass filters with a passband ripple of 10 dB and stopband attenuation of 60 dB. You can use limit testing to make sure the frequency response of the filter always meets these specifications. The first step in this process is to specify the limits. You can specify a lower limit of –10 dB in the passband region and an upper limit of –60 dB in the stopband region, as shown in the following figure. After you specify these limits, you can run the actual test repeatedly to make sure that all the frequency responses of all the filters are designed to meet these specifications.

Pulse Mask Testing Example

The ITU-T G.703 recommendation specifies the pulse mask for signals with bit rates, n × 64, where n is between 2 and 31. The following figure shows the pulse mask for interface at 1,544 kbits/s. Signals with this bit rate also are referred to as T1 signals. T1 signals must lie in the mask specified by the upper and lower limit. These limits are set to properly enable the interconnection of digital network components to form a digital path or connection.