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Ralph,
These are very common issues, and we write and lecture on the topic
frequently. We also offer workshops on the subject. A good place to start
is with my new book, Power Integrity, that discusses the many noise paths.
Unfortunately the simulation is very difficult, for a number of reasons.
1) While it might seem like a simple S21 measurement, the output is the
mixing product of many signals that vary in frequency and amplitudes
2) Many of the signals are dependent on the control loop stability of
the LDO, which is in itself a function of the operating current,
differential voltage and input output capacitance
3) The noise potential is much larger than what we see, much like in a
rogue wave, require a lot of simulation to identify the "potential" noise
4) Efficiency improvement techniques, such as pulse skipping, burst
mode and PFM from the VRM's aggravates this situation and these are
difficult to predict.
5) Simulations need to include the VRM, LDO, LNA and any other noise
generating circuits in addition to the PCB.
6) While the issues appear in high frequency output, they are often
high order harmonics of low frequency signals, below the range we consider
in our testing and simulations. We have actually seen such issues above the
100th harmonic of a low frequency signal
We have shown how to determine the maximum noise level of a rogue wave using
Keysight ADS. You can search for it as How to Design for Power Integrity:
Finding Power Delivery Noise Problems or locate it here
https://youtu.be/oL6qjhJH_m4?list=PLtq84kH8xZ9HIYgBYDsP7TbqBpftidzI8. We
are just starting to look at solutions for evaluating LNA and clock noise
related to the power supply in ADS. The Keysight ADS simulator can include
the PCB, analog circuits (VRM and LDOs) as well as clocks and LNA's. It
does require high fidelity models of all of these components.
Also not all LDO's are created equal . Some have much higher PSRR than
others. We have regulators in our products with more than 100dB PSRR, even
at higher frequencies. We also showed a GaN linear regulator at space power
that is ideal for ultra-high PSRR, ultra-low impedance and for the most part
noise free. Some LDO's even have spurious signals that can interfere with
RF, clock jitter and especially sensitive sensors.
The system needs to be considered holistically, and the power integrity is a
central distribution highway for noise, so must be very carefully
considered. Also, we frequently find that the RF isolation beads are one of
the more significant issues, as it increases the impedance on the load side,
resulting in much higher noise, generally at the low frequency range. The
resulting ringing generates a harmonic comb with a very large number of
harmonics. We also have many tools for helping to identify the potential
for such noise interference.
If you would like to write to me directly, I can share some images and
articles with you. You can contact me at steve@xxxxxxxxxxxx
Regards,
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