Caller ID Signal Levels (Part 2: Terminations)

Caller ID Signal Levels (Part 2: Terminations)


When setting up Caller ID signal levels for various international standards, it is very important to understand what, if any, terminating impedance is specified. This second article in a series of four explains how terminations effect signals on a telephone line, along with an example of how the Telecordia (Bellcore) series of standards uses terminations to set Caller ID signal levels.

AC Signal Model:

While Caller ID data can be sent using various modulation methods (FSK or DTMF), the information flow is always from the central office (CO) site to the customer premise equipment (CPE). Though for type II (off-hook) Caller ID, the CPE may be required to send various signals back to the CO, this article only deals with the signals generated by the CO.

The following diagram shows a basic model for the AC signal path from the CO to the CPE.

When sending the Caller ID information, the CO uses a voltage source to produce the FSK or DTMF signals. This voltage is represented by Vs in the figure above. However this is not the same voltage that is present at the CPE terminals (VTR). The voltage at the CPE terminals is determined by the ratio of the CO’s source impedance (Zs) and the CPE’s load impedance (ZL). The voltage at the CPE can be calculated as follows:


Normally the CO source impedance (Zs) is specified as either 900 ohms or 600 ohms. A typical telephone device (CPE) has a different load impedance between the on-hook and off-hook states. In the on-hook state, its load impedance is very high, typically greater than 1 Mohm, though dependent on the telephone design and national standards. From the above formula, this means that VTR is virtually the same as VS. However, in the off-hook state, a telephone’s AC load impedance is much lower. Typically it approximates 600 ohms at a frequency of 1 kHz, though this can vary from country to country depending on national standards.

As a quick example, assume the CO generates a 0 dBV signal with a source impedance of 900 ohms. If the CPE has a load impedance of 600 ohms, the voltage at the CPE terminals is:

0 dBV ==> 1 Vrms

0.4 Vrms ==> -7.96 dBV

In this example the voltage level present at the telephone is approximately 8 dB lower than the level generated by the CO.

Working with Caller ID Standards:

When defining Caller ID signal levels, the common standards used today will specify the levels either with a termination or without a termination.

If a standard does not use a termination, the load impedance in the above figure is assumed to be infinity. This means the voltage at an on-hook CPE is identical to the voltage generated by the CO. Thus when the standard requires a certain signal level to be applied to the CPE under test, the user can simply adjust the CO voltage source to the stated level.

In the second case, a standard may define signals levels as terminated into a certain impedance. If testing to the common Telecordia (Bellcore) SR-3004 Caller ID standard, the document specifies a 600 ohm terminating impedance with a 900 CO source impedance. It is important to note that this is independent of whether or not the CPE is in the on-hook or off-hook state. Using the above example, this means the CO voltage source must be set 7.96 dB higher than the level stated in the Telecordia standard in order to generate the proper levels at the CPE.

As an example, a typical FSK sending level is -13.5 dBm. So what would be the required signal level for the CO’s voltage source (VS)? Also, what is the voltage level at the CPE terminals (VTR) in both the on-hook and off-hook case?

A. Calculating the voltage level of the CO signal source:

Following the SR-3004 document, a FSK level of -13.5 dBm must be present at the tip and ring leads (VTR) with a 600 ohm termination (ZL) and a 900 source impedance (ZS). First convert the signal level into Vrms:

-13.5 dBm ==> 0.164 Vrm

Next, calculate the voltage level at the CO source using a re-arranged version of formula (a).

Thus, the CO voltage source has to produce 0.409 Vrms in order to match the -13.5 dBm level stated in Telecordia SR-3004.

B. Calculating the voltage level at the CPE in the on-hook state:

From part A above, the voltage source at the CO is set to produce 0.409 Vrms. Since a CPE in the on-hook state has a very high load impedance, it can be approximated that ZL is infinite. Using formula (a):

Thus the voltage at the CPE is the same as the CO source voltage. Note that converting 0.409 Vrms into dBm (600 ohms reference) yields -5.5 dBm, which is significantly higher than the -13.5 dBm specified level. This difference between the signal levels stated in some standards and the levels actually measured at the tip and ring leads can cause some confusion.

C. Calculating the voltage level at the CPE in the off-hook state:

In the off-hook state, for type II Caller ID transmissions, the CPE’s load impedance will be approximately 600 ohms, depending on national standards. Without knowing the exact load impedance the voltage present at the CPE terminals can not be calculated. Further to this, since a CPE’s load impedance changes with frequency, the voltage levels at the CPE will also change with frequency. The implication is that different CPE’s will have different voltages across their tip and ring terminals, even though the standard specifies a common level.


When testing Caller ID capable CPE’s to various standards it is important to know how the levels are defined within the standard. Some standards specify unterminated levels, while others specify terminated levels. In the unterminated case, the voltage level of the signal source is set to match the level set out in the standard. However, in the terminated case, the voltage level of the signal source must be calculated using the specified CO source impedance and termination impedance. In general, the signal levels stated in various testing standards are not meant to reflect the actual level at the CPE terminals (VTR). Rather, the stated levels reflect the level VTR only when the specified termination impedance is used. A common procedure used to correctly adjust the signal level is:

1) Connect the terminating impedance to the tip and ring leads

2) Adjust the voltage source (VS) such that VTR matches the specified level

3) Remove the terminating impedance

4) Connect the CPE and perform the tests

In conclusion, it should not be expected that the signal level at a CPE’s terminals match the stated level for any given test. The source levels used for testing should either be set using a procedure as above or calculated as shown, but not by measuring the voltage at the tip and ring leads with a CPE connected.

The next article in this series will provide a summary of various international standards in how they specify signal levels.