10
January 24, 2005
SECTION 3:
AC Transmission Theory and
Design
Introduction
The information in the next section describes the signal flow
of the RSLIC and CODEC PCM to 2W transmission circuits.
Appendices A and B contain the block diagrams and AC
transmission equations needed to tailor the AC transmission
parameters to specific user requirements.
The scope is limited to the design of the telephony T
X
and
R
X
voice circuits within the 0.3kHz to 3.4kHz frequency
band. The user should refer to the specific RSLIC and
CODEC data sheet design equations for all other BORSCHT
functions and performance parameters.
General Description (refer to Appendices A and B)
The evaluation board implements a complete half-channel
line circuit with the user’s choice of fixed gain 5V and 3.3V
CODECs interfaced to the appropriate RSLIC. The following
AC transmission parameters are set by resistors that control
the RSLIC transmission gain paths:
• 2W impedance
• Receive (R
X
) D-A gain
• Transmit (T
X
) A-D gain
• Transhybrid balance
The RSLIC and associated components implement a
voltage-feed, current-sense architecture for setting the
above parameters. The receive path begins as PCM code
input to the CODEC, which performs the D-A conversion at
fixed-gain and supplies the analog receive signal to the
RSLIC high impedance 4W V
rx
input. The RSLIC G
42
R
x
amplifier has a fixed inverting voltage gain of -2 (voltage-
feed) with a floating differential output at the Tip and Ring
terminals.
The G
24
T
x
gain path begins at the RSLIC 2W tip and ring
input. The RSLIC provides a termination impedance to the
externally applied T
x
signal appearing across the Tip and
Ring terminals. A pair of series-connected current sense
resistors in the RSLIC are connected to a differential sense
amplifier that develops a voltage output proportional to the
sensed AC signal current. This voltage is applied to the non-
inverting input of a second amplifier whose output is
summed with the incoming Rx signal from the CODEC,
forming the impedance matching circuit. The gain of the
second stage is controlled with external feedback resistor
(Rs) allowing for adjustment of the level of feedback, and
therefore the RSLIC 2W synthesized impedance.
The HC55185 (5V) and ISL5585 (3.3V) RSLICs have the
same internal architecture, however, the G
42
R
x
gain paths
are connected differently. The HC55185 G
42
R
X
input is at
Vrs, and the gain is fixed at 0dB. The ISL5585, however, is
required to compensate for the 3.3V CODEC attenuation
(i.e. lower 0dBm0 reference level) with a gain increase. This
increase in R
X
gain is accomplished by injecting the CODEC
R
X
output at the RSLIC second stage T
X
amplifier summing
junction input through the external input resistor, Rin.
Connecting the RSLIC R
X
input in this fashion enables the
R
X
gain to be adjusted to any desired CODEC 0dBm0
reference level and any desired PCM to 2W R
X
gain. The
HC55185 R
X
path my also be configured this way, if RSLIC
R
X
gains greater than 0dB are needed.
The resistor component values used to set each AC
transmission parameter have a strong influence on the
remaining parameters. Therefore, an ordered approach is
required to complete the setting of all 4 AC transmission
parameters. These are described next in the correct order.
RSLIC 2W Impedance Matching
Correct matching of the RSLIC 2W terminal impedance with
the load impedance is a requirement before the R
X
and T
X
gains can be set. There are 2 components that make up the
total RSLIC impedance; the protection resistors (2 X R
P
) and
the synthesized terminal impedance of the RSLIC (Z
O
). The
impedance matching circuit and design equations are
included in Appendices A and B and are identical for both
the HC55185 (Table A1) and the ISL5585 (Table B1).
The RSLIC synthesized impedance has a strong
dependence on the protection resistance Rp. When
performing the impedance calculation, the Rp term should
include all resistance elements in series with the RSLIC tip
and ring terminals. This is especially important when
resistive current limiting devices, such as PTCs, are used.
The evaluation board components provide a 600
Ω
impedance using the on-board protection resistor values of
51
Ω
in the tip and ring leads.
Receive Gain Path, R
X
The D-A gain of the complete line circuit is the sum of the
CODEC D-A gain and the RSLIC G
42
R
X
gain.
R
X
Gain for the 3.3V ISL5585 and 3.3V CODEC
The receive gain block diagram and equations for the 3.3V
ISL5585 and 3.3V CODEC are contained in Appendix A.
The receive gain path is defined by the following;
(EQ. 3)
G
PCM - 2W
= G
RX
(CODEC) + G
42
(RSLIC)
The 3.3V CODEC receive gain is fixed at -5dB and the
overall PCM to 2W gain is set by the ISL5585 RSLIC G
42
R
X
gain according to;
(EQ. 4)
G
42
=G
PCM-2W
- G
RX
(CODEC)
For a G
PCM-2W
receive gain equal to 0dB, the RSLIC R
X
gain is given by:
(EQ. 5)
G
42
= 0dB -(-5dB) = 5dB
Application Note 1168
