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參數(shù)資料
| 型號： | T5761-TG |
| 廠商： | TEMIC SEMICONDUCTORS |
| 元件分類： | 通信及網(wǎng)絡(luò) |
| 英文描述： | UHF ASK/FSK Receiver |
| 中文描述： | SPECIALTY TELECOM CIRCUIT, PDSO20 |
| 封裝： | SO-20 |
| 文件頁數(shù)： | 4/32頁 |
| 文件大小： | 765K |
| 代理商： | T5761-TG |
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T5760 T5761
/
Rev. A2, 19-Oct-00
Preliminary Information
4 (32)
The value of C
L
should be optimized for the individual
board layout to achieve the exact value of f
XTO
(the best
way is to use a crystal with known load resonance fre-
quency to find the right value for this capacitor) and
hereby of f
LO
. When designing the system in terms of re-
ceiving bandwidth and local oscillator accuracy, the
accuracy of the crystal and the XTO must be considered.
If a crystal with
,
50ppm over Temperature
–
40 C to 105 C,
of total aging and a CM ( motional capacitance ) of 7 fF
is used, an additional XTO pulling of
added.
30 ppm adjustment tolerance at 25 C
10 ppm
30 ppm has to be
The resulting total LO tolerance of
the
receiving
T5760/T5761 if the T5750 has also a total LO tolerance
of
120 ppm.
120ppm agrees with
specification
bandwidth
of
the
DVCC
XTAL
TEST 3
TEST 2
n.c.
V
C
S
L
Figure 4. XTO peripherals
The nominal frequency f
LO
is determined by the RF input
frequency f
RF
and the IF frequency f
IF
using the following
formula (low side injection):
f
LO
= f
RF
–
f
IF
To determine f
LO
, the construction of the IF filter must
be considered at this point. The nominal IF frequency is
f
IF
= 950 kHz. To achieve a good accuracy of the filter
corner frequencies, the filter is tuned by the crystal fre-
quency f
XTO
. This means that there is a fixed relation
between f
IF
and f
LO
.
f
IF
= f
LO
/ 915
The relation is designed to achieve the nominal IF fre-
quency of f
IF
= 950 kHz for the 868.3 MHz version. For
the 915 MHz version an IF frequency of f
IF
= 1.0 MHz
results.
The RF input either from an antenna or from a RF genera-
tor must be transformed to the RF input Pin LNA_IN. The
input impedance of that pin is provided in the electrical
parameters. The parasitic board inductances and capaci-
tances influence the input matching. The RF receiver
T5760/T5761 exhibits its highest sensitivity if the LNA
is power matched. This makes the matching to an SAW
filter as well as to 50 or an antenna more easy.
Figure 33 shows a typical input matching network for f
RF
= 868.3 MHz to 50 . Figure 34 illustrates an according
input matching for 868.3 MHz to an SAW. The input
matching network shown in Figure 33 is the reference net-
work for the parameters given in the electrical
characteristics.
Analog Signal Processing
IF Filter
The signals coming from the RF front end are filtered by
the fully integrated 4th-order IF filter. The IF center fre-
quency is f
IF
= 950 kHz for applications where f
RF
=
868.3 MHz and f
IF
=1.0 MHz for f
RF
= 915 MHz. The
nominal bandwidth is 600 kHz.
Limiting RSSI Amplifier
The subsequent RSSI amplifier enhances the output
signal of the IF amplifier before it is fed into the demod-
ulator. The dynamic range of this amplifier is
DR
RSSI
= 60 dB. If the RSSI amplifier is operated within
its linear range, the best S/N ratio is maintained in ASK
mode. If the dynamic range is exceeded by the transmitter
signal, the S/N ratio is defined by the ratio of the maxi-
mum RSSI output voltage and the RSSI output voltage
due to a disturber. The dynamic range of the RSSI ampli-
fier is exceeded if the RF input signal is about 60 dB
higher compared to the RF input signal at full sensitivity.
In FSK mode the S/N ratio is not affected by the dynamic
range of the RSSI amplifier, because only the hard limited
signal from a high gain limiting amplifier is used by the
demodulator.
The output voltage of the RSSI amplifier is internally
compared to a threshold voltage V
Th_red
. V
Th_red
is deter-
mined by the value of the external resistor R
Sens
. R
Sens
is
connected between Pin SENS and GND or V
S
. The output
of the comparator is fed into the digital control logic. By
this means it is possible to operate the receiver at a lower
sensitivity.
If R
Sens
is connected to GND, the receiver switches to full
sensitivity. It is also possible to connect the Pin SENS di-
rectly to GND to get the maximum sensitivity.
If R
Sens
is connected to V
S
, the receiver operates at a
lower sensitivity. The reduced sensitivity is defined by the
value of R
Sens
, the maximum sensitivity by the signal-to-
noise ratio of the LNA input. The reduced sensitivity
depends on the signal strength at the output of the RSSI
amplifier.
Since different RF input networks may exhibit slightly
different values for the LNA gain, the sensitivity values
given in the electrical characteristics refer to a specific
input matching. This matching is illustrated in figure 33
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