FreeLists / si-list / [SI-LIST] Re: How to couple noise to the PDN of oscillator

[Istvan Novak <istvan.novak@xxxxxxxxxxx>]

Zhangkun,

The 0.1-ohm impedance on your oscillator PDN requires 1A to create 0.1V 
injected noise.  You
can generate 1A AC current with a suitable opamp, or by a transformer or 
by a separate power
amplifier.  Each will have its own challenge.  Opamps down like to work 
across low load
impedance, and if you you a transformer to raise the load impedance 
presented to the opamp
output, you likely to run into slew-rate limits at higher frequencies.  
Regardless of how you
inject the noise, a major limiting factor is the inductance connecting 
to your final 0.1-ohm rail.
If my math is correct, a 25MHz corner frequency with 0.1-ohm requires a 
nanoHenry inductance
or less.  As long as you stay with sinewaves and dont need a flat 
transfer response from your
noise source to the 0.1-ohm PDN point, you can use brute force (a power 
amplifier), but it
takes a lot of power: to generate 1A with a 50-ohm source (without 
transformer), you need
50V.  Assuming rms voltages and currents, this corresponds to 50W.  
Another option to
consider is a two-stage step-down: first use a transformer to step from 
50 ohms say to one ohm,
followed by a resistive step-down from 1 ohm to 0.1 ohm.  The 1W 
resistive divider can be
small size and wide band.  Similarly the 50:1 impedance matching 
transformer can afford
up to 10nH inductance on the output connection, probably allowing you to 
create a flat
transfer response over the entire frequency range you are interested 
in.  This would allow you
to create a constant output level without manual or automated level 
corrections, plus you
could use waveforms other than sinewave without major distortion.

Regards,

Istvan Novak
SUN Microsystems


Zhangkun wrote:
> Dear all
>  
> Now we are studying the effect of power noise on jitter. We meet a problem,
> how to couple noise to the PDN. For example, we want to couple noise of
> 25MHz into the PDN of oscillator. However there is impedance dismatch. The
> impedance of signal source is about 50ohm and that of PDN is about 0.1ohm.
> The energy would be reflected.
>  
> Any advice would be helpful
>  
> Best Regards
>  
> Zhangkun
> 2008.5.14
>
>   


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