[SI-LIST] FW: Re: loop antenna (mis)behavior
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: <mosfet_the_gr8@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>, <eric@xxxxxxxxxxxxxxx>, <a.ingraham@xxxxxxxx>
- Date: Mon, 2 Jun 2008 20:44:33 -0700
Gene.
Here is another idea. Take R3, R4 ground side in your schematic and lift them
from ground plane and tie them to the new center tap of L1. Now take this new
node ( center tap of L1 ) and run that new line into the -side of the op amp
thru a new resistor setting up a new subtractor around U2. This is better
Idea right?
Richard Jungert > From: r_jungert@xxxxxxxxxxx> To: mosfet_the_gr8@xxxxxxxxx;
si-list@xxxxxxxxxxxxx; eric@xxxxxxxxxxxxxxx; a.ingraham@xxxxxxxx> Subject:
[SI-LIST] Re: loop antenna (mis)behavior> Date: Mon, 2 Jun 2008 20:22:37 -0700>
> Gene.> > Try center tapping L1 and running the center tap wire thru a
resistor into the 2nd stage op amp and set up U2 like a subtractor circuit.
Subtract common noise this way. Or try center tap simply to ground. To get true
differential I think you will have to center tap L1. Center tap on L1 could
also allow you to omit R4 and R3 RIGHT? > > An RF balun works this way in that
the primary is like single ended and secondary is a balanced center tapped coil
on the secondary of the transformer. > > Also, your loading the tuned circuit
with 1K to -side of U1 which is virtual ground. The tuned circuit is heavily
loaded. > System gain will be low with all these resistors hanging on the tuned
circuit. > > I have designed in the past taking the ground line from a shielded
RF cable input strait into the -side of the op amp subtracting common mode
noise in video systems. It works nice. > > Does this make sense? > > Richard
Jungert> > > > > > Date: Thu, 29 May 2008 12:47:32 -0700> > From:
mosfet_the_gr8@xxxxxxxxx> > Subject: [SI-LIST] Re: loop antenna (mis)behavior>
> To: a.ingraham@xxxxxxxx; si-list@xxxxxxxxxxxxx; eric@xxxxxxxxxxxxxxx> > > >
Hi all - again, thanks for the help.> > I've come up with, what I think is the
solution. Check out this circuit:> > > >
http://www.geocities.com/fazool1_2000/si_list/si_list_loop_problems.pdf> > > >
L1+R1 represent the antenna loop at 2MHz, as measured by an LCR meter. C1 is a
tuning cap that negates the inductive reactance of the loop. R3 and R4 give
both ends of the antenna a ground reference. In principle, the induced signal
on the antenna produces a +V/2 on one side, and -V/2 on the other. SPICE
simulation concurs. The opamp is setup in differential mode with some gain that
is sufficient to make the signal visible on the scope. Although the design
shows 10X gain on first stage, and 10X on second stage, the gain bandwidth of
this opamp degrades that somewhat.> > > > In the lab, the +V/2 and -V/2 signals
are clearly visible, 180 degrees out of phase, and both have zero volts common
mode. The range of reception works for around 6 feet or so, in any direction
away from the TX. In the immediate vicinity of the TX antenna, the system
behaves in a fashion that looks like mutual coupling between the loops. As I
wrote earlier, the phase within an area surrounding the TX changes drastically
depending upon position. But once outside this zone, then the phase between 2
RX antenna remain constant. Swapping lead polarity on an antenna changes phase
by 180 degrees. All cool!> > > > The major difference between this setup and
the one that fails, is the way in which the antenna is connected. It's more of
a single ended design. I think the reason it misbehaves is that there's some
common mode signal on the antenna (e-field) that the opamp picks up on simply
due to single-ended construction of circuit. This alternate design, using a
balanced design, exploits the cmrr of the opamp nulling out any such affects.
Sound logical?> > > > BTW, the tin foil test (on this balanced design) revealed
that the signal is indeed magnetic, not e-field. Thanks, Andy!> > > > regards,>
> > > gene> > > > > > > > --- On Thu, 5/29/08, Eric Bogatin
<eric@xxxxxxxxxxxxxxx> wrote:> > From: Eric Bogatin
<eric@xxxxxxxxxxxxxxx>> > Subject: [SI-LIST] Re: loop antenna
(mis)behavior> > To: "'Andrew Ingraham'" <a.ingraham@xxxxxxxx>,
si-list@xxxxxxxxxxxxx> > Date: Thursday, May 29, 2008, 5:18 PM> > > > Gene-
Andy has a good idea, especially if you are measuring the voltage across the
coils directly with a scope probe and the input to the scope is set to 1 Meg
Ohms. In this case you probably are sensitive to capacitive coupling. Maybe put
a low resistor (like 100 Ohms) across the ends of the coils and measure the
voltage across the resistor. Keep in mind that at 2 MHz, the wavelength is 500
feet, so in the lab you are always in the near field coupling. If you are using
an rf pre-amp, then try to find one with a low input impedance. --eric
************************************** Dr. Eric Bogatin, Signal Integrity
Evangelist Bogatin Enterprises, LLC Setting the Standard for Signal Integrity
Training 26235 w 110th terr Olathe, KS 66061 v: 913-393-1305 f: 913-393-0929
c:913-424-4333 e:eric@xxxxxxxxxxxxxxx www.BeTheSignal.com Upcoming Signal
Integrity Classes San Diego: EPSI, BBDP, July 28-31, 2008 San Jose, SICT, Aug
12-13 San Jose, EPSI, BBDP,> > Sept 29-Oct 2
**************************************** -----Original Message----- From:
si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of
Andrew Ingraham Sent: Thursday, May 29, 2008 8:44 AM To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: loop antenna (mis)behavior Gene, I may not be correct in
my use of "electrostatic," and yes, the prefix does imply static conditions;
but I believe that's not always the case.. What I was referring to was plain
old capacitive effects, not an RF field. (Which admittedly is somewhat
questionable this close, relative to a wavelength, between antennas.) What I
was trying to imply, is that you have two pieces of metal which are near one
another. The fact that they are formed into loops doesn't matter. You've
excited one of them (the TX one) with an overall voltage relative to ground, by
driving it unbalanced. You take the other one and measure its voltage relative
to ground, because> > it also isn't balanced. And you detect a signal. If you
flip the polarity of the RX "loop" by reversing the grounded and sense ends,
you're still going to detect the same exact signal, because it is being picked
up by the capacitive coupling between the two pieces of metal that you call
antennas. Or, perhaps, by the (capacitive) coupling into the scope probe/leads,
or the leads to your RF preamp. Andy
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