System for locally testing a modem employing frequency shift keying
First Claim
1. A system for locally testing a modem employing frequency shift keying and being coupled to a two-wire transmission line, the two frequencies transmitted by the modem being lower than the two received frequencies and being produced by a modulator, in the form of multivibrator controlled by the data and connected to a transmission filter, the test system comprising means for looping the modem at the transmission line side in response to a test control signal, characterized in that said modulator comprises means for causing said modulator to produce, in response to said test control signal, two test frequencies, said two test frequencies being the respective products of said two frequencies transmitted by said modem multiplied by a multiplying factor, said multiplying factor being substantially equal to the ratio between the average values of said two received frequencies and said two frequencies transmitted by said modem, and said transmission filter comprises means for changing, in response to said test control signal, the transfer characteristics of said transmission filter in order that said two test frequencies may be transmitted therethrough.
1 Assignment
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Accused Products
Abstract
A test system is used to test a modem whose modulator is constituted by a multivibrator which during normal operation produces two frequencies f1, f2 in response to the data signal and which is connected to a transmission filter. In response to a test control signal, the multivibrator is modified to supply two test frequencies f'"'"'1 =nf1, f'"'"'2 =nf2, n being substantially equal to the ratio Fo /fo, Fo being the average value of the frequencies received by the modem during normal operation, fo being the average value of the frequencies f1, f2 ; at the same time the transmission filter is modified to transmit the two frequencies f'"'"'1, f'"'"'2 in response to the test control signal.
93 Citations
5 Claims
- 1. A system for locally testing a modem employing frequency shift keying and being coupled to a two-wire transmission line, the two frequencies transmitted by the modem being lower than the two received frequencies and being produced by a modulator, in the form of multivibrator controlled by the data and connected to a transmission filter, the test system comprising means for looping the modem at the transmission line side in response to a test control signal, characterized in that said modulator comprises means for causing said modulator to produce, in response to said test control signal, two test frequencies, said two test frequencies being the respective products of said two frequencies transmitted by said modem multiplied by a multiplying factor, said multiplying factor being substantially equal to the ratio between the average values of said two received frequencies and said two frequencies transmitted by said modem, and said transmission filter comprises means for changing, in response to said test control signal, the transfer characteristics of said transmission filter in order that said two test frequencies may be transmitted therethrough.
Specification