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TMS3705: Read Range

Part Number: TMS3705
Other Parts Discussed in Thread: TXU0102

Hi Team,

This issue is an extension of a previous matter regarding read range for a RO Transponder and a TMS3705E. A new design was created that uses the TMS3705E along with the UCC27424DGN driver chip to increase the drive voltage and energy supplied to the antenna. The UCC27424DGN drives a differential passive circuit that allows the antenna resonance to be adjusted to 134kHz with selected capacitors. This circuit provides greater signal swing at the antenna with reasonable supply voltage (<9vdc) on the UCC27424DGN. The additional benefit of this design is that the antenna common mode is at DC ground to provide safer operation.

The read path is also differential from the antenna resonance circuit and is filtered (bandpass) and amplified by a differential to single-ended amplifier with gain selection greater than 100 (currently). The single-ended receive signal is AC coupled to the TMS3705E. An additional 1k ohm shunt resistor was added just before the 47k series resistor connected to pin 1 of the TMS3705E. This was done to provide a DC path to ground for the first stage amplifier that has been described in other documentation. 

The image below shows the FFT (20msec transponder transmit) at the input to the TMS3705E before the AC coupling cap. The signal level at pin 2 (output of first stage amplifier) of the TM#3705E is about 10dB higher. The TMS3705 mode control register is programmed with the Division factor adapting automatically (DATA_BIT [4:1] = 1111). The transponder is positioned about 8 inches above the antenna loop in the optimal orientation to receive and transmit EM energy. With the transponder at this height (8 ") above the antenna the data is being read about 50% of the time, so basically at the limit of the read range. 

We would like more read range so I have attempted to increase the gain of the differential to single-ended amplifier but the resulting improvement in read range is not happening. One question that I need your help with is what signal level should we be providing to the TMS3705 (or can we provide) to increase the read range? Is the current signal level too high, not high enough, too noisy? Are there other factors to consider to achieve more read range? 

Best regards,

Jim  

  

  • Hello Jim,

    the signal level at the SFB pin should be 5 mVpp. For noise reduction you should add a bandpass filter with a range of ca. 115 kHz to 140 KHz.

    Best regards,

    Andreas.

  • Hello Andreas,

    In previous correspondence through the customer support channel it was mentioned that increasing the signal level to 20mVpp would be better. What signal level would be too high and would saturate the first stage and following BP filter (internal to the TMS3705)?

    As mentioned, I have a 1k shunt resistor preceding the 47k series resistor to provide a DC path to ground for the TMS3705. Is this an appropriate value or should this be changed to a better value and what would that be? It was also mentioned that the gain of the RF amplifier should not be changed as this would alter the RF amplifier bias and limit the signal to the following stage. The same 47k / 150k values have been retained from the design shown in the TMS3705 datasheet. I presume this is near the optimal gain for the first stage RF amplifier, please confirm.

    Best regards,

     Jim

  • Hello Jim,

    the saturation level is not specified. There is only a related note available in chapter 5.3 of the Application Note "Integrated TIRIS RF Module TMS3705A Introduction to Low Frequency Reader". Since the signal level should be at least 5mV, it sounds reasonable to set it to 20mV.

    The need for the 1k shunt resistor would be eliminated with the following circuitry:

    The dc blocking capacitor should be al least a few nF but depends also on the output resistance of the external amplifier.

    The 47k / 150k values should be reasonable.

    Best regards,

    Andreas.

  • Hello Andreas,

    Thanks for the information and suggested change to the pre-amplifier connection. I will implement this change and test the reading range. 

    Best regards,

     Jim

  • Hi Andreas,

    I made the change as you suggested and the signal level at the SFB pin is much larger. I will reduce the gain of the pre-amplifier if required.

    During the testing it was discovered that the signals, TXCT and OSC1, originate from a 3v3 domain. I checked the TMS3705 datasheet and confirmed that the signal levels should be > 0.7*Vdd for a high and < 0.3*Vdd for a low. Up to this time the TMS3705 has been driven with 3v3 level signals and still seemed to operate properly except for read range.

    I added level shifting to the TXCT and OSC1 signals (see image) and the operation of the TMS3705 changed radically. In particular, the OSC1 signal measured at the TMS3705 is a few millivolts DC. I'm not yet sure if the oscillator is shutdown and in sleep mode or if another problem exists. We are working with code developers to troubleshoot. Prior to the addition of the level shifting, a clock signal directly connected from the STM32 was observed at the OSC1 pin no matter what the state of the TMS3705.

    Please confirm this configuration is correct to drive the TXCT and OSC1 signals. The datasheet mentions internal pull-up resistors (100k to 500k) on the TXCT and OSC1 pins. The external 10k pull-up resistors could be removed. 

    Best regards,

     Jim 

     

  • Hello Jim,

    in general your level-shifter circuitry seems to be OK. The component values of cause depend on the selected transistor. Please be aware that this kind of level shifter is inverting the signal. You could try your circuitry without the TMS3705 connected to confirm functionality. Alternatively you could also use integrated level-shifters such as TXU0102.

    Best regards,

    Andreas.