Limit Testing in Modem Manufacturing

The following figure shows how limit testing is used to check if a signal meets specifications:

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.