[SI-LIST] Re: How to couple noise to the PDN of oscillator
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: Zhangkun <zhang_kun@xxxxxxxxxx>
- Date: Thu, 15 May 2008 07:58:51 -0400
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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