that4315 THAT Corporation, that4315 Datasheet - Page 11

• Manufacturer Part Number: that4315
• Description: Pre-trimmed Low-voltage Low-power Analog Engine
• Manufacturer: THAT Corporation

Document 600065 Rev 04
companding system. As before, the columns labeled
Encoder VCA Gain, Encoder Out, Decoder VCA
Gain, and Decoder Out use the equations derived
previously in the section titled “The Mathematics of
Log Based Companding Systems”. The values in the
column labeled RMS In are derived using the equa-
tion:
R12, since this sort of break point behavior is diffi-
cult to model in a spreadsheet.
readily be modeled in SPICE.
Encoder
(dBV)
-100
-110
-10
-20
-30
-40
-50
-60
-70
-80
-90
12
13
Table 2 shows the transfer characteristics of this
I
This table does not show the effect of R6 and
In
-0
RMS In
Op-Amp
Table 3. 3:1 compander transfer characteristics
U6D
Encoder
14
(In dB)
=
VCA
Gain
-35
-28
-21
-15
12
19
25
32
39
-8
-1
10
5
R
C11
Encoder Out
1u
RMS In
20
Decoder
Encoder
(dBV)
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
C20
8n2
Out/
-12
-15
-19
-22
-25
-29
-32
-35
-39
-42
-45
-9
In
6k65
R8
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
3k01
R14
V+
C13
10u
0.1472
0.1003
0.0683
0.0466
0.0317
0.0216
0.0147
0.0100
0.0068
0.0047
0.0032
0.0022
I
(mA)
RMS In
U4B
THAT4315
2
RMS In
RMS Out
4
Decoder
CT
However, is can
(In dB)
R10
80k6
VCA
Gain
-12
-19
-25
-32
-39
35
28
21
15
-5
C12
10u
8
1
Copyright © 2009, THAT Corporation
C10
1u
5
Vref
Decoder
Figure 9. 3:1 Decoder circuit
(dBV)
-100
-110
200k
Out
-10
-20
-30
-40
-50
-60
-70
-80
-90
-0
220p
R9
C19
R11
2k0
Vref
R13
5k1
3
2
R12
C18
2k0
1n
U6A
Op-Amp
7
Op-Amp
±30 dB are well within specification, as is the RMS
detector input current. A maximum decoder output
level of -9 dBV (0.5 V
the user decides to use a diode clipper at the output
of the VCA for over-modulation protection.
3:1 Compander
Engine
of other than 2:1, and this feature can be particu-
larly advantageous in situations where RF band-
width is at a premium. The circuits in Figures 8
and 9 demonstrate a basic 3:1 companding system.
The topology of this system is reminiscent of the
previous example.
of 26 dB;
used to “floor out ” the detectors at low levels for the
reasons mentioned previously.
the detector to stop responding to input signals that
are 16 dB below level match. The value is deter-
mined by
companding system. As before, the columns labeled
1
U6B
You’ll note that the VCA gain ranges that vary
The configuration of THAT Corporation’s “Analog
The encoder shown in Figure 8 has a static gain
R4 in the encoder and R10 in the decoder are
The values of R4 and R10 were chose to cause
R
Table 3 shows the transfer characteristics of this
A
V
floor
5
6
15
®
=
C15
22u
VCA In
200
ICs” allows compression/expansion ratios
10
=
11
k
( .
EC+
EC-
k
7 5
(
Vref
Filt
W
16
W
12
6
7
U4A
THAT4315
m
VCA Out
dB
@
A
Gnd
Vcc
9
8
20
×
×
10
6
dB
mV
-
U4C
THAT
4315
20
16
)
13
)
PEAK
@
80 6
10
) was chosen in the event
9
.
R15
10k0
1k1
C16
100n
R7
k
U6C
Op-Amp
W
8
C14
C21
10n
22p
C17
100n
V+
Decode Out
Page 11