CHAPTER 5: Technical Reference
CCITT 1.430
2.5K
OHMS
250
FREQUENCY
NT & TE IMPEDANCE TEMPLATE
FROM 0 - 30 KHz
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.
