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