[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[ghf at hoffmann-hochfrequenz.de]

Am 2022-07-13 15:50, schrieb Lux, Jim via time-nuts:
> On 7/13/22 5:24 AM, Dan Kemppainen via time-nuts wrote:

>> Just a thought. The 7805 in your schematic appears to be a big part of 
>> the signal chain. I've run into issues with 78xx series regulators 
>> being noisy. Spent a few weeks chasing down noise issues in some 
>> equipment and the 78xx regulator was a part of the problem. Replaced 
>> it with LT3042 as part of the solution.
>> 
>> Also note, not all 7805's are created equal. Different vendors may be 
>> better/worse than others.


> +1 for the 200 mA LT3042 (and the higher current 0.5A LT3045). There
> are now negative voltage versions, too, LT3094, also 0.5A.
> 
> Noise from 10Hz to 100kHz is around 0.8 microvolts.  At 10 kHz 2
> nV/sqrt(Hz).  But the real thing these things are good at is PSRR at
> higher frequencies.  >70dB at 1 MHz
> 
> They have enable inputs, can be paralleled willy-nilly, etc

I have measured the noise density of an assortment of regulators
under the same conditions and in the proposed circuits  of their
data sheets. I don't believe in different brands of 7805; they are
all ugly.

<   
https://www.flickr.com/photos/137684711@N07/24070698809/in/album-72157662535945536/ 
    >

You can increase Cset on the 3042 to 100u tantalum. That helps at the 
low
frequency corner. More does not help. For the output capacitor, 4u7 is
optimum. More produces a noise peak at 100 KHz or so.
The trace of the 3042 is sandbagged at the low end because of the input
capacitor issue with my then-preamp I explained a few days ago.

Paralleling lt304* is not cheap, althoug it drives the noise further 
down.
The circuit from the data sheet with the external NPN is about as good 
as
a single 3042. I tested it with 1.5A. It takes a little bit more input
voltage, abt 6V4 for 5V out, but is cheaper than many LT304* in 
parallel.

Gerhard

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