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T5753 Datasheet(PDF) 5 Page - ATMEL Corporation

Part No. T5753
Description  UHF ASK/FSK TRANSMITTER
Download  13 Pages
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Maker  ATMEL [ATMEL Corporation]
Homepage  http://www.atmel.com
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T5753 Datasheet(HTML) 5 Page - ATMEL Corporation

 
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5
T5753
4510F–RKE–07/04
Output Matching and Power
Setting
The output power is set by the load impedance of the antenna. The maximum output
power is achieved with a load impedance of Z
Load,opt = (255 + j192) Ω. There must be a
low resistive path to V
S to deliver the DC current.
The delivered current pulse of the power amplifier is 9 mA and the maximum output
power is delivered to a resistive load of 400
Ω if the 1.0 pF output capacitance of the
power amplifier is compensated by the load impedance.
An optimum load impedance of:
Z
Load = 400 Ω || j/(2 ×π 1.0 pF) = (255 + j192) Ω thus results for the maximum output
power of 8 dBm.
The load impedance is defined as the impedance seen from the T5753’s ANT1, ANT2
into the matching network. Do not confuse this large signal load impedance with a small
signal input impedance delivered as input characteristic of RF amplifiers and measured
from the application into the IC instead of from the IC into the application for a power
amplifier.
Less output power is achieved by lowering the real parallel part of 400
Ω where the
parallel imaginary part should be kept constant.
Output power measurement can be done with the circuit of Figure 5. Note that the com-
ponent values must be changed to compensate the individual board parasitics until the
T5753 has the right load impedance Z
Load,opt = (255 + j192) Ω. Also the damping of the
cable used to measure the output power must be calibrated out.
Figure 5. Output Power Measurement
Application Circuit
For the blocking of the supply voltage a capacitor value of C
3 = 68 nF/X7R is recom-
mended (see Figure 6 on page 6 and Figure 7 on page 7). C
1 and C2 are used to match
the loop antenna to the power amplifier where C
1 typically is 22 pF/NP0 and C2 is
10.8 pF/NP0 (18 pF + 27 pF in series); for C
2 two capacitors in series should be used to
achieve a better tolerance value and to have the possibility to realize the Z
Load,opt by
using standard valued capacitors.
C
1 forms together with the pins of T5753 and the PCB board wires a series resonance
loop that suppresses the 1st harmonic, hence the position of C
1 on the PCB is important.
Normally the best suppression is achieved when C
1 is placed as close as possible to the
pins ANT1 and ANT2.
The loop antenna should not exceed a width of 1.5 mm, otherwise the Q-factor of the
loop antenna is too high.
L
1 ([50 nH to 100 nH) can be printed on PCB. C4 should be selected that the XTO runs
on the load resonance frequency of the crystal. Normally, a value of 12 pF results for a
15 pF load-capacitance crystal.
~
ANT2
ANT1
R
in
Power
meter
C
1 = 1n
L
1 = 56n
C
2 = 3.3p
Z
Lopt
V
S
Z = 50
50


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