[SI-LIST] Re: loop antenna (mis)behavior
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: Gene theBean <mosfet_the_gr8@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>, <eric@xxxxxxxxxxxxxxx>, <a.ingraham@xxxxxxxx>
- 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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