and an 1:1 Balun transform the output of the internal power amplifier to a 50 Ω single-ended signal. The internal
power amplifier is supplied by the on-chip generated and regulated voltage VDD_PA.
L1, L2 act as RF chokes, C12 and C14 are bypass capacitors. After the Balun the output power of the internal
power amplifier can be measured as being approximately 21 dBm when switched to the short range configuration.
The output of the external power amplifier and the output of the internal power amplifier are connected to an RF-
switch, which can be controlled via the GUI of the ST25RU3993-EVAL reader. Only one power amplifier option
can be active at a time.
Note that in a typical reader design only one power amplifier option will be used depending on the application and
read range requirements. So for a commercial reader design the RF-switch will be removed and so it’s introduced
insertion loss of ~0.5 dB.
The RF-switch requires DC blocking capacitors on all its RF terminal. The output of the RF-switch is connected to
a low pass filter, which should attenuate the second and third harmonic of the carrier frequency.
The filtered transmit signal is then connected to a 20 dB directional coupler, which takes a portion of the transmit
power (<10 mW). This sample power is further attenuated by a pi-pad attenuator to limit the input power of the
logarithmic power detector. The power detector generates a DC voltage, which corresponds to the transmit power.
The characteristic of output DC voltage of the power detector versus the generated RF power is shown in the
chart below.
Figure 6.
Power detector voltage characteristic
The DC output of the power detector is connected to an ADC (PA1) of the microcontroller.
After the 20 dB directional coupler the main directional coupler is placed, which has a coupling factor of 10 dB.
The purpose of this device is to isolate the transmit signal from the receiving inputs of ST25RU3993 and has
thereby a major influence on the sensitivity of the reader.
The output port of the main directional coupler is connected to a second RF-switch that allows to direct the
transmit power to either antenna port 1 or antenna port 2. The RF-switch again requires DC blocking capacitors
on all its RF terminal. The antenna ports both are of SMB (female) type.
In order to maintain the sensitivity of the reader the self-jamming signal reaching the receive inputs of
ST25RU3993 need to be minimized. The self-jamming signal comprises of reflections from the antenna (S11) and
UM2353
RF circuit
UM2353
-
Rev 2
page 8/48
Содержание ST25RU3993-EVAL
Страница 25: ...Figure 16 Schematic of UHF RFID reader device UM2353 Rev 2 page 25 48 UM2353...
Страница 26: ...Figure 17 Schematic of external power amplifier UM2353 Rev 2 page 26 48 UM2353...
Страница 28: ...Figure 20 Schematic of USB supply UART bridge UM2353 Rev 2 page 28 48 UM2353...
Страница 29: ...Figure 21 Schematic of microcontroller UM2353 Rev 2 page 29 48 UM2353...
Страница 30: ...Figure 22 Schematic of supply LDOs UM2353 Rev 2 page 30 48 UM2353...
Страница 31: ...Figure 23 Schematic of external interfaces and connections UM2353 Rev 2 page 31 48 UM2353...
Страница 32: ...Figure 24 Schematic of BT classic module UM2353 Rev 2 page 32 48 UM2353...
Страница 33: ...Figure 25 Schematic of RF shield UM2353 Rev 2 page 33 48 UM2353...
Страница 39: ...8 4 GND layer Figure 30 Layout GND layer UM2353 GND layer UM2353 Rev 2 page 39 48...