[SI-LIST] Re: Imbalance in outout from single ended drivers driving RF (13.56 MHz) antenna

If your antenna is electrically small and designed for local (near field)
coupling, you can get by with more harmonic energy relative to the
fundamental than if the antenna is designed as a formal radiating
structure for far field work.  The point is that 13.56 MHz is an
internationally accepted ISM frequency but the harmonics are not.  If you
radiate those over the limit, you will not receive regulatory approval. 
Of course, you can run a completely dirty source in a shielded room and
still comply, but how often does that happen?  I presume that is not the
case here.  If parasitic elements are having a big effect on your
bandpass network, either the chosen components are not correct for the
purpose, or the layout is bad.  Sometimes the abstract design is
inadequate.  Lots of app notes and demo boards show primitive RF
interfaces that are sufficient for demo purposes but not for production
designs.  That takes either a proven reference design or the intellectual
muscle to create your own.  Which did you use?
That said, good filter design uses some fairly specific construction
methods to maximize out of band rejection.  A nominally competent filter
implementation should reach a floor at least 60 dB down without too much
trouble.  An expertly implemented LC filter can reach -80 dB and hold
that to the 10th harmonic and beyond.  But, you can't just throw the
components on a board and get that performance because dealing with
parasitic effects through proper component selection, layout, and
shielding is part of the design.  If you are struggling with a 20 or 30
dB rejection floor, the filter design or the implementation definitely
needs rethinking.  The problem might be as simple as lousy Q, too few
sections, or common/diff mode or grounding problems that create sneak
paths around the filter.  It takes a practiced eye to take a physical
layout and visualize the hidden schematic of parasitics hiding behind the
formal one, but that it is necessary for good results.  You are probably
facing a fairly straightforward problem - if you know what to look for.

My consulting rates are reasonable...

Orin

On Fri, 20 Feb 2009 22:48:44 +0530 (IST) Bharathkumar Raju
<bharathrajupcb@xxxxxxxxxxx> writes:
Dear Orin Laney,
        Parasitics of the bandpass fiter inductor & capacitor are not
allowing 'Maximising' the mismatch for harmonics. Impednace offered by
the bandpass filter is very low for the higher order harmonics and all
higher oreder harmonics reaches the antenna. Spectral measurement at the
antenna terminals (end of matching circuit) shows strong presence of all
harmonics. Here too I find imbalance between the 2 wings. I guess there
will be considerable current flow through the midpoint (ground) which is
supposed to be zero.
         Radiated emission is going to be a trouble I guess. Will there
be any other trouble(s)?
 
Bharath




From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
To: bharathrajupcb@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Sent: Friday, 20 February, 2009 9:10:32 PM
Subject: [SI-LIST] Re: Imbalance in outout from single ended drivers
driving RF (13.56 MHz) antenna

This is not a matching problem like making sure you have good edges. 
Here, you want only the fundamental to reach the antenna.  To do that the
bandpass filter must MAXIMIZE the mismatch at all harmonic frequencies. 
Since you are (hopefully) not using the harmonic energy, it doesn't
matter whether harmonics levels are the same at both pins or not, just
the fundamental.

Orin Laney


On Fri, 20 Feb 2009 20:14:35 +0530 (IST) Bharathkumar Raju
<bharathrajupcb@xxxxxxxxxxx> writes:
> Dear List memebrs,
> Â Â Â  We use NxP (Philips) chip MFRC531 in our RFID card reader. I 
> am measuring the output at the Tx1 & Tx2 pins when both pins are 
> left open. I use TDS 7154 Tek scope and in spectral mode I measure 
> the output. The out put from these pins is digital 5 volts @ 13.56 
> MHz. 
>      At 13.56 MHz the outputs are equal and reasonably balanced 
> between them. But as I try to measure the output at higher 
> frequecnies I find mismatch between the outputs. At some haromincs 
> of 13.56 MHz Tx1 output is higher and at some other harmonics Tx2 
> output is higher.
>      Will this imablance bring any issues? How to reduce the 
> mismatch? I request your expert guidance here.
> Â Â Â  We have a LC filter between the pins and ground where the 
> single ended output of the Tx pins get converetd to differntial 
> signal. Output of this LC filter goes to the antenna matching 
> circuitry and the 13.56 loop antenna in the PCB as trace. 
> 
> Bharath 
> 
> 
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