Silicon Laboratories SI2494/39, SI2493/57/34/15/04 AC Termination, Ringer Impedance and Threshold

AN93

3.2. AC Termination

The ISOmodem has four ac termination impedances when used with the Si3018 line-side device, selected by the ACT bits in Register U63. The four available settings for the Si3018 are listed in Table 31. If an ACT[3:0] setting other than the four listed in Table 31 is selected, the ac termination is forced to 600 Ω (ACT[3:0] = 0000).

Table 31. AC Termination Settings for the Si3018 Line-Side Device

ACT[3:0]			AC Termination
0000	600	Ω
0011	220	Ω + (820	Ω 120 nF) and 220 Ω + (820 Ω 115 nF)
0100	370	Ω + (620	Ω 310 nF)
1111	Global complex impedance

3.3. Ringer Impedance and Threshold

The ring detector in many DAAs is ac coupled to the line with a large 1 µF, 250 V decoupling capacitor. The ring detector on the ISOmodem is resistively coupled to the line. This produces a high ringer impedance to the line of approximately 20 MΩ. This meets the majority of country PTT specifications, including FCC and ETSI ES 203 021.

Several countries, including Poland, South Africa, and Slovenia, require a maximum ringer impedance that can be met with an internally synthesized impedance by setting the RZ bit (Register 67, bit 1).

Some countries specify different ringer thresholds. The RT bit (Register U67, bit 0) selects between two different ringer thresholds: 15 V ±10% and 21.5 V ±10%. These two settings satisfy ringer threshold requirements worldwide. The thresholds are set so that a ring signal is guaranteed to be detected above the maximum and not detected below the minimum.

3.4. Pulse Dialing and Spark Quenching

Pulse dialing is accomplished by going off- and on-hook at a certain cadence to generate make and break pulses. The nominal rate is ten pulses per second. Some countries have strict specifications for pulse fidelity that include make and break times, make resistance, and rise and fall times. In a traditional, solid-state dc holding circuit, there are many problems in meeting these requirements. The ISOmodem dc holding circuit actively controls the on-hook and off-hook transients to maintain pulse dialing fidelity.

Spark-quenching requirements in countries such as Italy, the Netherlands, South Africa and Australia deal with the on-hook transition during pulse dialing. These tests provide an inductive dc feed resulting in a large voltage spike. This spike is caused by the line inductance and sudden decrease in current through the loop when going on-hook. The traditional solution to the problem is to put a parallel resistive capacitor (RC) shunt across the hookswitch relay. However, the capacitor required is bulky (~1 µF, 250 V) and relatively costly. In the ISOmodem, the loop current can be controlled to achieve three distinct on-hook speeds to pass spark-quenching tests without additional BOM components. Through settings of two bits in two registers, OHS (Register U67, bit 6) and OHS2 (Register U62, bit 8), a delay between the time the OH bit is cleared and the time the DAA actually goes on-hook, can be created, which induces a slow ramp-down of the loop current.

3.5. Line Voltage and Loop Current Sensing

There are two methods for line voltage and loop current sensing. The first method is the legacy mode using U79 (LVCS) [4:0]. The legacy mode is intended for backward compatibility in applications originally designed for the previous generation ISOmodem. This mode is used in the intrusion detection algorithm implemented on the device.

The second method of measuring line voltage and loop current takes advantage of the improved resolution available on the Si3018 and Si3010 DAA chips. U63 (LCS) [15:8] represents the value of off-hook loop current as a non-polar binary number with 1.1 mA/bit resolution. Accuracy is not guaranteed if the loop current is less than the minimum required for normal DAA operation. U6C (LVS) [15:8] represents the value of on-hook and off-hook loop voltage as a signed, two’s complement number with a resolution of 1 V/bit.

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