[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: Tue, 3 Jun 2008 06:49:30 -0700
Gene.
I have fixed this ground plane noise common mode noise problem about 5
times in the past. On phase lock loops, radio receivers, 310 Mhz filter
noise problems etc. I am only trying to help. Noise can occur when circuits
are not terminated
right, impedance matching is not right, injection from another adjacent plane..
etc etc.
I am pretty sure one needs to do some clever thing with the subtractor circuit
U2 connected to ground plane on the receive side or center tap of L1.
Try simulation with a center tap on L1 just to prove a point. I would be
interested in knowing the results. I would'nt
worry about what exists in the current design, U may end up replacing
L1 anyway. You may have to go to management with a new concept and having
simulation results gives info you need to prove your point..
Try something a bit exotic, this is how great discoveries
are made. Be not afraid to try something just a bit out of the
ordinary, you may be pleasantly suprized of the results.
Richard Jungert
> Date: Tue, 3 Jun 2008 06:18:16 -0700
> From: mosfet_the_gr8@xxxxxxxxx
> Subject: [SI-LIST] Re: loop antenna (mis)behavior
> To: si-list@xxxxxxxxxxxxx
>
> 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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