Using M24LR-DISCOVERY with 5V on VPSTX

[laepstein]

Posted on August 07, 2017 at 22:32

VPSTX is wired to 3.3Volts.  I want to give it +5V to increase the power.  This changes Rout from 27 ohms to 16 ohms.  I tried using the CR95HF EMI FILTER CALULATION tool to work out component values, without success.

As a baseline, I looked at the 3 volt case, since I already had the answer. I opened the tool and found most of the values default to the values on the board.  VPSTX is set to 3 volts and Rin is set to 27 ohms. I had to change Rrx from 0 to 330 and Crx to 1E+12F (infinite capacitor in series).  The result (section 2) was C11 = C12 = 41pF and C2 = 301pF.  That C11 and C12 value is half of the 82pF used on the board.

When I changed VPSTX to +5 and Rin to 16, the tool gave me C11 = C12 = 29pF and C2 = 301pF.  I tried 66pF (and lifted L8 so I could connect +5V) and got much worse performance than before.

In Summary, what am I misunderstanding about the tool, and what are the correct values for attaching +5V to VPSTX?

Thanks for reading this.

#m24lr-discovery #cr95hf-emi-filter-calulation

[Peter Boringer · Accepted solution]

Posted on August 09, 2017 at 10:05

Hello,

Keep in mind that the CR95HF is tuned to have performances at 3.3 volts. If you want to increase the emitted magnetic field you can increase the power supply VPS TX to 5 volts. The tools provided to calculate the component tuning are their to help you. Anyway by increasing the transmit power you will have also impact on the receive sensitivity. So you have also to consider ZRX1 and ZRX2, used to attenuate the signal coming from the antenna to the RX1 and RX2 inputs (when measured between GND and RX1/RX2, the minimum value of the signal must not be greater that 8 volts.)

This come in parallel with the antenna and has to be considered also in the matching calculation

So suggestion is to increase RX1 and RX2. (at 3.3v it is 330 ohms on the kit.) take care this might have also an impact on the capacitors values for matching the impedance between the antenna and the output of the EMI filter.

Regards

[laepstein]

Posted on August 09, 2017 at 19:12

Thank you for the reply.  I had some minor success with it.  For my '+5V at VPSTX' I increased RX1 and RX2 from 330 to 604, and (at the tool's recommendation) used 27pF for C11 and C12.  I verified that the signal at pins RX1 and RX2 did not exceed 8V.  The end result was performance roughly equal to the board's original performance.  So, there was no improvement, but no degradation either.  I will play with it some more.

Thanks again for your time.