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參數資料
| 型號： | HC-5502X |
| 廠商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數： | 4/17頁 |
| 文件大小： | 255K |
| 代理商： | HC-5502X |
4-4
The Transversal Amplifier (TA)
Whereas the feed amplifiers perform the 4W to 2W
transmission function, the transversal amplifier acts as the 2W
to 4W hybrid. The TA is a summing amplifier configured to
reject common mode signals. It will thus reject 2W common
mode signals. Figure 4 shows the single ended signal
transmission path. Given below is the design equation of the
2W to 4W signal transmission path. Given below is the design
equation of the 2W to 4W signal transmission. It can be seen
that RB2 and RB4 act as loop current sense resistors, and that
the voice signal output of the amplifier is a function of the
differential voltages appearing across RB2 and RB4.
Thus, the transversal amplifier also has a DC output
proportional to the metallic current in the loop. The output
voltage is given by:
V
TX
= 2(I
TIP
+ I
RING
) (R
B2
+ R
B4
)
where I
TIP
and I
RING
are assumed positive as indicated in
Figure 1. This DC level is used as an input to a comparator
whose output feeds into the logic circuitry as SH. This signal
is used to gate SHD.
Voice signals on the loop are transformed by the TA into
ground referenced signals as shown by the above equations.
Since the TA output has a DC offset it is necessary to AC
couple the output to any external circuitry. Note, that during
4W to 2W transmission, the transversal amplifier will have an
audio signal at its output proportional to the 4W audio
receive signal and the loop’s equivalent AC impedance. This
is called the transhybrid return, and must be cancelled (or
balanced) out to prevent an echo effect. This is discussed
more fully in Transhybrid Balancing.
Loop Current Limiting
The nominal loop length is equivalent to an 1800
load
across the feed amplifiers. However, on a short loop the line
resistance often approaches zero. Thus, a need exists to
control the maximum DC loop current that can flow around
the loop to prevent an excessive current drain from the
system battery. This limit is typically specified between
30mA and 40mA for general PBX applications. Figure 5
depicts the feedback network that modifies the R
F
voltage as
a function of metallic current. Figure 6 illustrates the loop
current characteristics as a function of line resistance.
As indicated above, the TA has a DC voltage output directly
proportional to the loop current. This voltage level is scaled by
R19 and R18. The scaled level forms the ‘Metallic’ input to one
side of a Transconductance Amplifier. The reference input to
this amplifier is generated in the bias network, and is equivalent
to 30mA or 40mA loop current, typically, for the HC-5502X and
HC-5504X, respectively. When the metallic input exceeds the
FIGURE 3. LINE FEED AMPLIFIERS
FIGURE 4. SINGLE ENDED AC SIGNAL EQUIVALENT CIRCUIT
HC-5502X/4X
FIGURE 5. LOOP CURRENT LIMIT CONTROL
Application Note 549
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