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TRF7960 harmonics

Zvonko Antonic
Prodigy 20 points
Other Parts Discussed in Thread: TRF7960

I am looking for solution to get rid of high harmonics in RF signal. I am using " Application Schematic for the TRF796x EVM (SPI Mode)" from TI. I am getting RF emission from 8,9,10,11,12,19,20,21,22,23 harmonics. All of them exceed FCC and CE limits (most of them for 15dB or more). In the application there is a capacitor marked as "harmonic suppression" but there is no any explanation how it works. Is it effective just for one frequency (parallel resonance with 330uH) or I can tune this and achieve broader range suppression? Is there any other suggestion to attack this problem?

Regards,

Zvonko

  • 0
    Prodigy 115 points

    Hi Zvonko,

                In my RFID application we faced the same problem & struggled for 1.5 moths to get rid of it. We were getting harmonics emissions between 300 to 500 MHZ from our prototype.In FCC 15 Subpart C (intended Radiator) in threshold is 46dBuV whereas initially we failed even by 8dB.

                 Actually the harmonics are being generated from TRF796X chip itself.But,if you perform RE (we did as FCC 15 Subpart C) of TRF796x EVM as a stand alone unit powered from USB, the board will pass with a good margin.TI used suppression filter & low pass filter to avoid EMI/EMC issues.

                 What i can suggest for your design:

                                        1. First, check presence of any metallic object / another PCA (Printed Circuit Assembly) bethath the 13.56MHz RFID antenna.Proximity of any metallic object /PCA with contribute to increased harmonics.

                                        2. Check that your RFID antenna is tuned properly from 13.56MHz using a calibrated VNA.Perform the S11( Return Loss) testing after putting the antenna in the product enclosure.Detuning of antenna will also contribute for higher harmonics emission.

                 The 330nH & 10pF is being used as notch filter where it has been tuned for 87.611913MHz.

               I hope this will surely helpful for you.

                                                                                                                                                                                                                            Thanks & Regards,

                                                                                                                                                                                                                                                             Tuhin

  • 0 in reply to Tuhin
    TI__Guru* 91206 points

    Tuhin -

    Those are great suggestions! it turned out that we gave him some direct support in the end through Apps team - so (as not to lose this thread of info, as others may find it useful if they find themselves in same predicament)

    Zvonko -

    If you could repost the details (whatever you feel you can share that is not proprietary) behind why you saw all these harmonics in the first place (things like: you were using 5 TRF7960s at once with a distributed clock system on a board that is 45cm x 45cm with 5 mistuned antennas, distributed SPI, etc.) and what you did do fix those issues, that would be nice to have here on the forum.

    Thanks!

    Josh

  • 0 in reply to Josh Wyatt
    Prodigy 20 points

    Here is update on my emission problem I had with TRF7960:

    The emission came from two sources: CW signal (13.56Mhz clock distribution) and from antennas.Clock distribution traces was very long (supplying signal to 5 TRF7960 chips) with very fast edges and without signal conditioning components. I fixed this by changing oscillator driver and by introducing serial dumping resistor and C to GND. Antennas were also radiating harmonics of 13.56Mhz whenever intentional (fundamental 13.56Mhz) was present. We found out that antennas were not tuned properly (impedances were greatly mismatched). So we had to retuned them to 50ohms. Also, I figured out that this emission was generated by common mode mechanism. So, I have placed 5 CMM chokes (one for each antenna). All these measures helped us in reducing radiation at harmonic frequencies to acceptable levels.

    Regards,

    Zvonko