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Wrap Test for Modems

IP.com Disclosure Number: IPCOM000088106D
Original Publication Date: 1977-Apr-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Coburn, RL: AUTHOR

Abstract

Existing wrap tests are performed by connecting the modem transmitter to its receiver. The data transmission equipment transfers data from the transmitter to the receiver and checks the received data for errors. In present wrap tests the received signals are nothing more than the attenuated transmitted signals. The signals are very stable and clean. The attenuated signals will check that the modem threshold circuit is working. However, the modem receiver amplifies these signals back to normal levels with the addition of only a few microvolts of circuit noise. Under these conditions, the signal-to-noise ratio remains high. The improved test circuit illustrated attenuates the signal as was the case previously. However, random or Johnson noise is added to it.

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Wrap Test for Modems

Existing wrap tests are performed by connecting the modem transmitter to its receiver. The data transmission equipment transfers data from the transmitter to the receiver and checks the received data for errors. In present wrap tests the received signals are nothing more than the attenuated transmitted signals. The signals are very stable and clean. The attenuated signals will check that the modem threshold circuit is working. However, the modem receiver amplifies these signals back to normal levels with the addition of only a few microvolts of circuit noise. Under these conditions, the signal-to-noise ratio remains high. The improved test circuit illustrated attenuates the signal as was the case previously. However, random or Johnson noise is added to it.

In view of the addition of random noise, no matter how much the signal is amplified, no improvement in signal-to-noise ratio can be obtained. The signal- to-noise ratio is set to cause a marginal modem to fail. The circuit illustrated in the drawing includes an operational amplifier A1 which provides the low impedance noise source which is summed in the second amplifier A2. The Johnson noise is generated by the carbon resistor R1, which is of a high value within the range of to 5 megohms and is buffered by amplifier A1 so that it appears as a low impedance noise source. Capacitor C is used to limit the bandwidth of the noise. The output of amplifier A2 is the test signal used and has a rela...