the leakage across the main directional coupler. To minimize the self-jamming signal a carrier cancellation circuit
is connected to the coupled port of the main directional coupler.
The carrier cancellation circuit is able to change its impedance and thereby is able to reflect a certain amount of
the coupled power back into the directional coupler again. This reflected signal will combine at the isolated port of
the main directional coupler with the self-jamming signal. The isolated port of the main directional coupler is
connected to the receive pins of ST25RU3993. In theory, if the signal reflected by the carrier cancellation circuit
has the same amplitude and the opposite phase then the self-jamming signal would cancel-out and vanishes.
To find the impedance setting of the carrier cancellation circuit that would result to a self-jamming signal
cancellation the power of the self-jamming signal needs to be measured. Such a measurement capability is
provided by ST25RU3993 and is referred to as reflected power measurement. An optimal impedance setting of
the carrier cancellation circuit would result to low reflected power measurement result. The main components of
the carrier cancellation circuit are three digital tunable capacitors, which are controlled by the STM32L476RG
microcontroller via the SPI interface. The fixed lumped components of the carrier cancellation circuit help to
center its impedance to 50 Ω and define the impedance step created by one LSB change of a digital tunable
capacitor.
The isolated port of the main directional coupler is connected to a 2:1 Balun to transform the incoming tag
response signal to a 100 Ω differential signal that is fed into ST25RU3993 at the pins MIX_INP (4) and MIX_INN
(6).
5.2
Microcontroller and connections
Table 2.
MCU interfaces and buttons
Button
Description
A
MCU reset button
B
USB receptacle: micro, B-type
C
JTAG interface: for BT module programming
D
BT module reset button (not installed)
E
BT module boot mode jumper (not installed)
F
UART interface connections
G
MCU Boot mode button
H
STM32L476RGT6 (Arm
®
32-bit Cortex
®
-M4)
J
SWD debug interface connections
K, L and M
SPI debug interface connections
N
UART_LOG connection for debugging purposes
P
Enable power amplifier LDO
R
ST25RU3993 RAIN
®
(UHF) RFID reader device
UM2353
Microcontroller and connections
UM2353
-
Rev 2
page 9/48
Summary of Contents for ST25RU3993-EVAL
Page 25: ...Figure 16 Schematic of UHF RFID reader device UM2353 Rev 2 page 25 48 UM2353...
Page 26: ...Figure 17 Schematic of external power amplifier UM2353 Rev 2 page 26 48 UM2353...
Page 28: ...Figure 20 Schematic of USB supply UART bridge UM2353 Rev 2 page 28 48 UM2353...
Page 29: ...Figure 21 Schematic of microcontroller UM2353 Rev 2 page 29 48 UM2353...
Page 30: ...Figure 22 Schematic of supply LDOs UM2353 Rev 2 page 30 48 UM2353...
Page 31: ...Figure 23 Schematic of external interfaces and connections UM2353 Rev 2 page 31 48 UM2353...
Page 32: ...Figure 24 Schematic of BT classic module UM2353 Rev 2 page 32 48 UM2353...
Page 33: ...Figure 25 Schematic of RF shield UM2353 Rev 2 page 33 48 UM2353...
Page 39: ...8 4 GND layer Figure 30 Layout GND layer UM2353 GND layer UM2353 Rev 2 page 39 48...