moog hp

[Osamu Hoshuyama]

Dear List Members,

I'm not good at English, but I can contribute about this topic.

> Date:    Mon, 29 Mar 1999 13:03:13 +0200
> From:    jh <jhaible at primus-online.de>
> > Not too bad, pretty good actually. 
> 
> It must be, as it's close to Moog's design (;->).
> 
> Seriously, I was wondering why Moog used transistors
> instead of diodes here. Matching diodes should have
> been easier than matching npn / pnp pairs.
> 
> JH.

I guess that is for decoupling of signal through CV lines.
If transisters are used, the signal coupling is reduced to 1/beta.

I had some experiments of 4-stage and 12-stage phase shifters based on
moog-style HPF structure using matched diodes.  This circuit is close
to the Tomg's moog-ap.  However, It oscilated at MHz due to signal
coupling through CV lines.  When equivalent resistance of diode ladders
is small, signal coupling through CV lines is not negligeble.  This
effect is significant at high frequencies because impedance of CV
lines is not sufficiently small.  When equivalent resistance is small,
a capacitior in signal path can be heavy capacitive load for opamp,
which may be another cause of instability.

To stabilize it, the following techniques were effective:

* Decoupling using capacitors for CV lines  (I used three 2.2uF films for
  each CV line)
* Decoupling using resisters (22 ohm) between CV line and GND
  (This technique is used in MS-50 diode-bridge VCF)
* Inseartion of small resister (22--100 ohm) between the capacitor and
  the center of diode ladder (Limitation of range of equivalent
  resistance)
* Limitation of frequency response of opamps (TL072) using capaciters
* Limitation of the CV range 

Off course PCB layout should be important.

Hope this helps,

- -- ---- ----------- --- --- - ----- -- --- --- - - ------ - - - --
 Osamu HOSHUYAMA          Digital Signal Processing Technology Group
   Dr. (Eng.)       C&C Media Research Laboratories, NEC Corporation
               Email: houshu at ccm.CL.nec.co.jp,  Fax: +81-44-856-2232
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