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CHAPTER 5: Technical Reference
Figure 5-6. CCITT 1.430.
5.3 AC Impedance
As shown in Figure 5-6, CCITT recommends that the AC impedance at 20 to
30 KHz should be greater than 2.5K ohms. To detect this, the Test Set puts
an R-sense resistor in series with the device under test (Figure 5-7). A 21- KHz
sinusoidal voltage, approximately 1V peak-to-peak, is generated. If the device
is properly terminated, the AC voltage will be equally divided between the R-
sense resistor and the terminating resistor. The AC impedance at 21 KHz is
negligible compared to 100 ohms in parallel. The voltage drop across the R-
sense resistor is fed through a pre-amp and then through a window
comparator circuit. The window is set to detect impedance of between 40 and
140 ohms. The low detect is 40 ohms. The high detect is 140 (min.) and 300
(max) ohms.
If there is no termination provided, the majority of the voltage will drop
across the device under test, and the resultant impedance will fall outside the
comparator window.
5.4 DC Continuity
The DC Continuity test assumes that the loop resistance will be within 0 to 5 K
ohms (see Figure 5-8). A maximum of 5 mA of loop current is generated.
