[time-nuts] TADD-1 Design (was Re: Stepping up the output of an OCXO (Dr Bruce Griffiths))

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 4 17:08:47 EST 2007


Christopher Hoover wrote:
>> The power supply noise may also limit the performance.
>>     
>
> This is my major concern with the design.  
>
> Any noise on the supply rail goes into the first stage via the bias network,
> and is transferred at whatever gain to the output.
>
> I noticed that when I used a particular bad wall wart to run one of my
> TADD-1's, I saw power supply noise on the outputs.  
>
> I hacked one of my TADD-1's to decouple the bias from the power supply, and
> I added a wide-band bead in front of the regulator.  Those changes helped
> considerably.  (I also tossed out that bad wall wart.)
>
> Using a high PSRR reference for the bias would be worth investigating.
> There are a number of these on the market, intended for RF apps.  E.g.,
> TPS794xx family.  http://focus.ti.com/docs/prod/folders/print/tps79401.html
>
> There's a good treatment of biasing single-supply op amps here: 
> http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/index.h
> tml
>
> -ch
>   
Chris

Yes the 1/2 supply biasing does raise some issues, however the inband 
component of the power supply noise will be attenuated by the low source 
impedance.
This will off course couple power supply noise into the source.
Motorboating used to be common in valve radios when the high voltage 
power supply electros dried out.

Using one of Wenzel's active power supply noise filters for each opamp 
would help considerably, in reducing the power supply noise and it only 
uses one npn transistor plus a handful of passive components:
http://www.wenzel.com/documents/finesse.html

The TI regulators are, from my perspective extremely noisy, one can do 
much better at least for higher output voltages.
They also don't have a high enough output for the TADD-1.
Lower power supply noise is possible using a buried zener reference with 
appropriate filtering as the regulator reference element.

Input reverse polarity protection with low dropout is possible using a 
power mosfet and a few passive components.
This avoids blowing fuses if the supply polarity is reversed. No 
heatsinking is required as the mosfet power dissipation is low.

The problems of instability and coupling of power supply noise into the 
signal path were also present when single supply discrete transistor 
amplifier were the norm.
Quite elaborate bias divider decoupling circuitry was used to minimise 
the effect.

Bruce



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