A test instrument connectable to a device under test, DUT, for causing the DUT to transmit a predetermined modulation test pattern signal, the test instrument comprising: a test instrument host interface to couple to a data interface of the DUT; a test pattern generator to generate transmit non-return-to-zero, NRZ, signals and to communicate the transmit NRZ signals to the DUT via the test instrument host interface, wherein the DUT is to receive the transmit NRZ signals, and convert the transmit NRZ signals to modulation signals for transmission via a line interface of the DUT, and the DUT is to receive, via the line interface, signals responsive to the transmitted modulation signals and convert the received signals to the return NRZ signals; and a test pattern analyzer to receive the return NRZ signals from the DUT via the test instrument host interface, and determine, based on the transmit NRZ signals and the return NRZ signals, signal parameters of conversion processes performed internal to the DUT to convert the transmit NRZ signals to the modulation signals and to convert the received signals to the return NRZ signals, characterised in that the test pattern generator is to generate, based on the determined signal parameters, a modified test pattern signal, to be communicated to the DUT to cause the DUT to transmit a predetermined modulation test pattern signal via the line interface of the DUT. a test instrument host interface to couple to a data interface of the DUT; a test pattern generator to generate transmit non-return-to-zero, NRZ, signals and to communicate the transmit NRZ signals to the DUT via the test instrument host interface, wherein the DUT is to receive the transmit NRZ signals, and convert the transmit NRZ signals to modulation signals for transmission via a line interface of the DUT, and the DUT is to receive, via the line interface, signals responsive to the transmitted modulation signals and convert the received signals to the return NRZ signals; and a test pattern analyzer to receive the return NRZ signals from the DUT via the test instrument host interface, and determine, based on the transmit NRZ signals and the return NRZ signals, signal parameters of conversion processes performed internal to the DUT to convert the transmit NRZ signals to the modulation signals and to convert the received signals to the return NRZ signals, characterised in that the test pattern generator is to generate, based on the determined signal parameters, a modified test pattern signal, to be communicated to the DUT to cause the DUT to transmit a predetermined modulation test pattern signal via the line interface of the DUT. receive the return NRZ signals from the DUT via the test instrument host interface, and determine, based on the transmit NRZ signals and the return NRZ signals, signal parameters of conversion processes performed internal to the DUT to convert the transmit NRZ signals to the modulation signals and to convert the received signals to the return NRZ signals, characterised in that the test pattern generator is to generate, based on the determined signal parameters, a modified test pattern signal, to be communicated to the DUT to cause the DUT to transmit a predetermined modulation test pattern signal via the line interface of the DUT.