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In Reply to: RE: How useful is a spectrum analyzer in tackling noise in a sound system? posted by Karma16 on March 13, 2009 at 19:21:17
Thanks for your very helpful reply. I am primarily interested in thorough elimination of EMI/RFI and other types of noise without impairing the stiffness (i.e., increasing the impedance) of the power supply any more than absolutely necessary. I have started reading up on this subject and I get the impression that depending on the devices being used, for best results it may be desirable to take measurements and tweak until the problem diminishes/disappears (at which point one should obviously stop). I'm aware that for certain other types of devices, calculations following standard formulas or even ballpark guesstimation may be adequate to do the job.
-- Chris
Follow Ups:
"I am primarily interested in thorough elimination of EMI/RFI".
Then a spectrum analyzer is the best thing since sliced bread. I don't think they are hard to use, probably easier than a scope but may be hard to interpret. I've used both for so long that they are second nature so I can't reliably predict how much difficulty you will have. Hp has some excellent Ap notes on-line that you can download and I bet you would have it well in hand after a weekend of playing around.
You do need both instruments, but not especially at the same time. Unfortunately I now have to beg a spectrum which I'm reluctant to do and sorely miss having one at hand. While a scope is indespensible on a daily basis, you will only use a spectrum occasionally unless you are working on RF design. But it's just shocking what you can miss on a scope sometimes that's grossly obvious with a spectrum. I could tell you tales...
For RFI stuff probably a 1GHz machine is an adequate choice and they are pretty obsolete now since so much stuff has moved to S band. Like John says they don't have the narrow IF and frequency stability to look close in at audio, but that's not the issue if you are working on EMI concerns.
What it boils down to is if you buy a spectrum you won't use it a lot. But when you do you will find out information that most everyone else has to just guess at and should make much faster (and quantifiable) strides in improving the situation.
And if you get one cheap, but don't want it any longer, think of me...
Regards, Rick
If you are talking about a switchmode supply below then I'd get at least a GHz of bandwidth but if you are referring to a 50/60 rectified supply then what the others have said below is true.
I've done a fiar bit of SMPS power supply compliance/noise tweaking using nothing more than a 2GHz tek spectrum analyzer and a set of rabbit ears with cheap RF amp. Not ideal but it allowed me to come up with a method of designing them which rarely ever requires me to pull out and use the spectrum analyzer anymore. lol.
I have a very good used HP spectrum analyzer that was $35K originally which I picked up for $900 on ebay. I specificlly bought it to measure digital audio clock spectrums (ie phase noise). It does a very good job of this.
Its actually also quite usefull for exactly what you are describing. I hook a probe up and put a 1" piece of wire on the end of the probe and move it around power supplies, power cords, transformers etc and see what it picks up. Its much better at this task then a scope because it discriminates between the higher frequency stuff and the 60/120 Hz stuff. With a scope the larger component is pretty much all you see.
I did a lot of searching to find just the right one to get. Most of the ones you list are lousy for audio use, they go up into the GHz range but have resolutions in the 10KHz range or so, this is pretty much useless for audio. The one I got "only" goes up to 40MHz but has a 1Hz resolution, way better for audio use.
As Karma16 mentions these are rather complex devices to master, it takes quite a while to figure out how to get it to do what you want. And then there is trying to figure out WHAT you want to do!
Most of the ones mentioned are hybrid digital/analog devices, they are digitally controlled and displayed analog spectrum analyzers. They still use analog mixers, oscillators and filters which are digitally controlled and the final output is digitized and displayed on the screen. One nice thing about some of the new ones is that they are designed to be used with computers for the control and display jobs.
There are also fully digital ones, but they are limited by the dynamic range and speed of their AD converters. These are typically much less expensive and smaller, but more restricted as to what they can measure, after carefully looking at what was available I decided on the hybrid model. I'm glad I did, its a womderful instrument to have. BUT huge and HEAVY, it weighs 90 pounds, not exactly a portable device.
John S.
HI Chris,
Without exception, the outout impedance of a power supply should be zero. In practice, none actually are. However, one of the characteristics of the finest amplifiers is the massive size, current capability and stability of their power supplies. They really do attempt zero ohms. It is important that the amplification load not modulate the power supply voltages.An example are the power supplies used by Audio Research power amps. They are extraordinary. My D250 Mk II Servo amp uses the most impressive power supply I have ever seen in an audio amplifier. I think this accounts for the amazing linearity and control exhibited by the amp.
Why is noise such a concern for you? Is this just an academic exercise or do you have a real problem? How does it exhibit itself? Over my many years with a large number of amps, I have never had a problem that I could assign to power supply noise. I have had amps with less than perfect power supplies. It's almost always related to the cost of the amp. Great power supplies are expensive.
BTW, your lack of an equipment profile prevents us from putting your questions into a context.
Sparky
Edits: 03/14/09
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