[SI-LIST] Re: loop antenna (mis)behavior
- From: olaney@xxxxxxxx
- To: mosfet_the_gr8@xxxxxxxxx
- Date: Tue, 3 Jun 2008 08:06:06 -0700
Before we lose sight of the fundamental issue, it should be reiterated
that there are just two things to do for antennas. The first is to
shield the loop and its connection to the circuit, with a break in the
loop portion of the shield so that it is not a shorted turn. Information
on shielded loops is easy to find, so I won't rehash it here. The other
is to ground the shield so that it is effective and not just a floating
capacitor plate. Manipulations at the circuit end in an attempt to
address the shortcomings of an *unshielded* loop are, so to speak, "the
long way around". If shielding is done properly, electrostatic coupling
to the loop is eliminated. Gene, you mentioned that the loop has metal
around it, but never responded to the question about grounding status.
Are you convinced that these are implemented correctly? If they are,
whatever circuit and system problems remain are unlikely to be loop
related per se, and are issues of a different kind not necessarily within
the focus of this forum.
Orin Laney
On Tue, 3 Jun 2008 06:18:16 -0700 (PDT) Gene theBean
<mosfet_the_gr8@xxxxxxxxx> writes:
> Hi Richard,
> Thanks for the input. I think you misunderstand how this thing
> works, so let me explain it.
>
> The antenna develops a voltage across it, Vrx, due to some type of
> coupling - hopefully magnetic, but possibly e-field too. Without R3
> and R4, the there's no reference to much of anything. One one end
> of the antenna is +Vrx/2 and the other end has -Vrx/2. You can
> simulate this in spice and see it for youself. Anyway, that's a
> true differential mode signal, it just has no reference to my system
> ground - that is, it is floating relative to my system. That's
> where R3 and R4 come in. They are 100 ohms for 2 reasons:
> 1) they are more than 10X larger than the antenna resistance at 2MHz
> (around 2.5 ohm)
> 2) They are more than 10X smaller than the input resistance to
> opamp.
>
> OK, so far? Picture just the antenna with R3 and R4 only. There's
> the antenna that generates a differential signal with a common mode
> voltage of zero (my system ground). Now apply that signal to the
> opamp which is configured in differential mode. I could have used 3
> opamps, making an instrumentation amp, which would also work, but
> uses more parts and I don't have the room for it. The gain of this
> particular design is set at 10 (R5/R2 and R7/R6). If you analyze
> the circuit, you will find that the total gain is [Vrx/2 - (-Rrx/2)]
> * A, where A is R5/R2 and also R7/R6. So the output = A * Vrx. The
> nice thing is that any common mode signals on -Vrx and +Vrx get
> cancelled by the CMRR of the amp (yes the precision of
> R2,R5,R6,R7 change that somewhat). This is precisely what I'm
> looking for! Regardless of the common mode source, it gets rejected
> (either through capacitive coupling, or common mode on my
> grounds). Coolness :)
>
> The only reason for 2 amps is that the gain-bandwidth of them are
> fairly low. So at 2 MHz the most it'll do is around 10. In order
> to get the signal up to something that I can see on the scope, a 2nd
> gain stage is needed.
>
> I suppose center tapping the antenna is possible, which would give
> me a common mode voltage of zero, and a nice differential output,
> but the circuit topology would then change. Sadly, the system
> design dictated a non-center tapped antenna so I can't do it anyway.
>
> gene
>
>
>
>
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