The function principle of markers according to ISO 11785 in HDX mode
The figure below (extract from ISO 11785) shows the encoding principle "0" and "1" and the time distribution of the pump and reset signals.
It is important that the transfer of mark data begins only after the pump (activation field) signal disappears. The signal from the label itself is frequency-manipulated with a 10 kHz band, and it can be received, amplified, and detected using a standard chip for a household FM receiver with some additions.
Description of the circuit fragment
The figure below shows a fragment of the RFID reader circuit, its receiving part for processing HDX signals.
(to view the full size, click on the image).
The FM signal reception and processing circuit is based on the well-known SA615 (DA2) chip . For intermediate frequency filtering, standard LTP455A piezo filters (BQ1 and BQ2) with a center frequency of 455 and a 10 kHz band are used. The central frequency of the input signal is 129.2 kHz in the signal manipulation band. Therefore, the upper setting is applied for the heterodyne, and a MEMS-type generator is used, operating at a frequency of 584.2 kHz (GEN2). Thus, the frequency of the mirror channel is 1039.2 kHz. That is if the Q-factor of the antenna loop is not less than 20 (really it is higher), the mirror channel suppression is not less than 18 dB that is enough for a common case.
The heterodyne signal is fed to the input of the internal circuit of the generator with a broken feedback. Thus, the internal circuit operates in a pure amplifier mode.
The SA615 chip has a differential input. This fact is used in the circuit. The antenna loop is connected to terminals J2 and J4. The VT1 and VT2 transistors are used as powerful keys to protect the receiver when pumping. The control signal is applied as a "SW_FM" net.
All other circuits are assembled according to a typical application circuit. The entire setup is reduced to tuning the L4 coil of the frequency discriminator.
The circuit is elementary and starts immediately. Only, the nominal of the capacitors C28 and C32 should be trimmed. Transistors VT1 and VT2 are high-frequency, in this case they are prone to self-excitation. Better sensitivity without capacitors, but without them, parasitic "ringing" may occur.
Further processing is standard: a quantization by levels, strobing, etc.etc. The est way is to do this using a signal processor, but one can also do it using discrete components.
The antenna implementation
An important aspect of the successful implementation of an RFID reader is the antenna. To minimize losses (i.e. maximize Q-factor), one need to use a wire with a sufficiently large cross section. To maximize the range of operation, one need an antenna with a large area.
Making such an antenna is a technological problem.
For this case, it was decided to use a cable from the Elevator type KPL 16x0. 75 .
The antenna layout is shown in the following figure.
The internal dimensions of the frame are 400 x 500 mm. The confident reception range is exactly according to the theory, 640 mm along the antenna axis for standard livestock markers. The unsure range, which requires additional processing, extends up to 1.5 m along the antenna axis.
The inductance of this frame is approximately 380 uH.