Tom,

 It was done with a combination of Switching and linear regulation.
 Also, the switching frequency of the switching regulator is >1Mhz.
 This allows for small components and easier filtering.  There is a
 common mode choke used.  Plus the transformers are part of the filter
 circuit.   Beyond that, I can't say much more.

 As far as measuring the noise floor, we used a SRS Spectrum analyzer.



                                           Regards,

                                             Jim




> Jim,
>
> If you don't mind giving away some secrets, what was the general method
> you used to achieve these numbers?  Not looking for schematics or
> anything, just curious how you could get a switcher so quiet.
>
> And also, how did you verify the -140db?  It seems to me not a trivial
> achievment.
>
> Tom
>
> At 02:41 PM 6/25/04 +0500, you wrote:
>> Lawrence,
>>
>> I agree 100% from a regulation standpoint.  But from a noise
>> perspective, you'd be hard put to gain the same performance from such
>> inexpensive hardware.   We have spent many manhours designing a power
>> supply for a system we designed and build.   The noise performance for
>> this system is around the -140+ (or should that be minus for lower
>> noise)db range. This is not easily achieved by weither a linear type
>> regulator or a switching type regulator.  We have achieved a small
>> switching supply that has an input range of about 3 VDC to about 18
>> VDC with an output of plus and minus 5 AVDC @ ~200ma (each leg).  And
>> this with a noise figure of better than -140db.   Not easily achieved,
>> and not inexpensive, but doable.
>>
>> But as a practical matter, your design is sufficient for most lab uses
>> you will probably run into in general work.   But if you have to have
>> a very quiet supply, figure on spending a few more bucks than this.
>>
>>                                               Regards,
>>
>>                                                 Jim
>
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