Re: SMPS with two control loops

In article <4387e0d5$0$18203$afc38c87@xxxxxxxxxxxxxxxxxxxx>,
[...]
>But the switching noise makes it impossible to compare two tiny sine
>waves on my CRO.

If you have a digital storage scope, triggering the scope from the signal
and using the averaging mode in the scope works. You have to fiddle the
frequency a bit to get the minimum noise in the sum.


You can make a simple commutating filter using a CD4052, if you can create
two squarewaves at the signal frequency, with a 90 degree phase
relationship:

Scope
! -------- C2A
! ! X0 !-----!!--GND
C1 R1 ! ! ! C2B
Vi-!!-/\/\/\-+--!In X1 !-----!!--GND
! !
SQR0---!A X2 !-----!!--GND
SQR90--!B !
!en X3 !-----!!--GND
--------

You get a steppy waveform on the scope. The nice thing is that the phase
between the sine wave and the squarewaves doesn't matter much. You
calibrate by hooking Vi to the function generator. You pick off the
voltages for (T0 and T180) and (T90 and T270).

The R1,C1 time constant should be low enough to let the desired signals
through. R1 needs to be large enough that the currents in the 4052 are
kept below about 1mA.

The R1,C2 time constant should be long.

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