[SI-LIST] Re: loop antenna (mis)behavior
- From: Gene theBean <mosfet_the_gr8@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 3 Jun 2008 06:18:16 -0700 (PDT)
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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