Wind/Solar Electrics ???

[Joel Kolstad]

What do you think it means?

It means that perfect reconstruction of a signal requires sampling at at least
twice the _bandwidth_ of the signal present to insure that no aliasing occurs.

Two important points here:

1) It's the bandwidth of the signal that matters, not the highest frequency
present (this is kind of the analog version of the digitial guys' "it's the
edge rate that matters, not the clock speed"). This fact is frequently used
to great advantage in radio receivers (and plenty of other designs, I'm sure).
2) The assumption that aliasing is inherently detrimental is not always true.
I've seen designs where well-defined bandpass filters were stuck in front of
an ADC and the aliasing was used _to advantage_ to let the ADC sample at much
closer to 2x then one could have obtained with more traditional filter design.
(Although I'd admit that this seems to have been more common when ADCs were
slower and you had to use all the tricks you could to get performance out of
them.)

---Joel

 

[Me]

"MSW is a shysters sales pitch which misrepresents the product. "

Are there deterministic tests that tell when a device has a "good
enough" sine wave?
Or is there some sort of accepted "spec"?

I saw in another post where one of the EU2000 hondas had a beautiful
"looking" wave form, but failed to run a furnace.

What can we use to "know for sure" that the wave form of a device is
adequate BEFORE buying it?

Thanks
Phil

You can use a college education in Electrical Engineering, and $40K
worth of test equipment, to "KNOW FOR SURE"...... or you can fool
around and see what works........ or you can ask one who already
did the previous, and figured it out, and then actually believe what
they tell you........ other than that your on your own......


Me

 

[daestrom]

What do you think it means?

Nyquist figured out that higher frequency components of the input signal
will 'alias' and you will lose the ability to tell them from lower frequency
components. In order to avoid 'losing information' and not being able to
tell whether a particular sample stream was from a low or high frequency
component, Nyquist's theorem states you must sample at least twice as fast
as the highest component present.
http://www.cs.cf.ac.uk/Dave/Multimedia/node149.html
http://www.efunda.com/designstandards/sensors/methods/dsp_nyquist.cfm

A lot of folks mistake it to think you need to sample at least twice as fast
as the 'signal of interest' also, and try to ignore high frequency
components of the input because they're 'not interested in that noise'. But
what Nyquist proved was that any frequency in the sampled signal that is
more than 1/2 the sample frequency will 'alias' and 'wrap around' and be
*indistinguisable* from a frequency component that is less than 1/2 the
sample frequency.

For example, if sampling at 1000 hz, and the sampled signal is a 900 hz
'pure sine wave', the sampled data would look *exactly* the same as if you
had sampled a 100 hz 'pure sine wave'. They would be 'indistinguisable'.
So if/when you try to convert the sampled data back to analog, how do you
know whether it should reconstruct a 100 hz wave, or 900 hz? You don't, so
you have a conundrum.

So, to avoid losing this 'information' of being able to tell if you had a
100 hz or 900 hz input, the standard thing to do is filter the input so that
there is *no* 900 hz input. Then, the resulting sample data must have come
from the 100 hz component. And if/when you want to reconstruct it, you
*know* it should form a 100 hz signal because no 900 hz signal could
possibly been present (you eliminated it before sampling).

And as Joel mentioned earlier, since most low-pass filters do not have
perfect 'cutoff' (IIRC, simple first-orders 'roll off' at something like 3
db/decade), it is more common to eliminate any frequency component that is
more than about 40% of the sampling frequency.

daestrom

 

[Spehro Pefhany]

Nyquist figured out that higher frequency components of the input signal
will 'alias' and you will lose the ability to tell them from lower frequency
components. In order to avoid 'losing information' and not being able to
tell whether a particular sample stream was from a low or high frequency
component, Nyquist's theorem states you must sample at least twice as fast
as the highest component present.

<snip>

More than twice the bandwidth.


Best regards,
Spehro Pefhany

 

[SolarFlare]

With risers it is the same confusion.

message

 

[[email protected]]

1) It's the bandwidth of the signal that matters, not the highest frequency
present...

One might say "the highest frequency present" is the highest frequency
non-zero component of the power spectrum.

Nick

 

[Joel Kolstad]

One might say "the highest frequency present" is the highest frequency
non-zero component of the power spectrum.

Sure, but the point is that you can sample a signal that's has all (of a good
approximation thereof, e.g., 99%) of its energy between 144-148MHz (this is
the 2m amateur radio band) at 10MSps and recover everything. I.e., the
bandwidth of the signal is only 4MHz, so you only have to sample at something

 

[Ray Andraka]

Joel Kolstad wrote:

Hey Joel, what are you doing over here. Are you a pilot too?

I've used this for subsampling, although you have to be very careful of
clock jitter when you sub-sample. a couple picoseconds of jitter on the
sampling of a 100 MHz signal is going to add substantial noise to the
signal.


(subsampling, for those here who haven't a clue what we are talking
about...this is an airplane owner's forum after all...is taking
advantage of the nyquist theorum to sample at less than the frequencyt
of the signal when the bandwidth of the signal is narrow. For example,
if you have a signal centered at 100 Mhz that only has a 10 MHz
bandwidth, you can sample it at something less than 100 MHz and still
recover all of the information. The more generally held belief is that
you would need to sample it at greater than 200 MHz in order to not lose
information).

 

[Joel Kolstad]

Hey Joel, what are you doing over here. Are you a pilot too?

Hi Ray!

Hmmm... no, I'm not a pilot, I just got sucked in by the cross-posting
(starting from sci.electronics.design) and the topic has drifted considerably
since it started.

I've used this for subsampling, although you have to be very careful of
clock jitter when you sub-sample. a couple picoseconds of jitter on the
sampling of a 100 MHz signal is going to add substantial noise to the
signal.

Yes it is... I suspect that's why that projects such as GNURadio (which
sub-samples using something like 80 or 100MSps ADCs) tend not to be as
sensitive as more traditional analog receivers. (Someone made the comment
that the FM decoder in GNURadio doesn't really even work as well as a $5
transistor radio, which is true enough albeit perhaps missing the point of how
cool/fun it is to be able to write any modulator/demodulator you like if
you're not looking for the ultimate sensitivty.)

(For example, if you have a signal centered at 100 Mhz that only has a 10
MHz bandwidth, you can sample it at something less than 100 MHz and still
recover all of the information. The more generally held belief is that you
would need to sample it at greater than 200 MHz in order to not lose
information).

I believe that folks who think you need to sample at 200MHz (the intuitively
reaosnable answer) are those who were never made to /had the opportunity to
open up an undergraduate signals & systems book. Thinking about things
like modulation are so much cleaner in the frequency domain once one gets the
whole "multiplication in one domain is convolution in the other" bit down.

---Joel

 

[George Ghio]

Yes this is the problem. While there are people who will tell you
anything to make a sale, how do you know what you are really getting.

One test is the "Modified Square Wave" test.

When you hear these words you know you are dealing either with a shyster
or an ignorant person who should not be selling things he does not
understand.

It is hard, what with a flood of imports at bargain basement prices.

Still, as long as people are willing to believe that a $59 3000W
"modified sine wave" inverter from Walmart, Cost Co, etc, etc has the
same specs as a $900 3000W sine inverter is, at best, fooling themselves.

I buy inverters from known manufacturers who are willing to provide spec
sheets that out line the full parameters of the inverter. You know
things like:

Efficiency curves
Max continuous output
1/2 hour rating
Surge
Standby
Max DC in

Well everything really.

I did build a kit inverter, once, years ago. It had a max rating of
150W, Which it met.

It had a half hour rating of 0W

And a surge of about 300W

Still it did the job it was built to do for many years.

Put your supplier on the spot. Tell them your load and buy on the
condition that what they are selling you will do what they claim or you
get a full refund, no questions asked.

 

[George Ghio]

A fine example of the correct approach.

 

[wmbjk]

One test is the "Modified Square Wave" test.

When you hear these words you know you are dealing either with a shyster
or an ignorant person who should not be selling things he does not
understand.

The only thing your test proves is that you're irrationally
judgmental.

It is hard, what with a flood of imports at bargain basement prices.

Still, as long as people are willing to believe that a $59 3000W
"modified sine wave" inverter from Walmart, Cost Co, etc, etc has the
same specs as a $900 3000W sine inverter is, at best, fooling themselves.

Why do you give buyers so little credit? And where does one buy a
3000W sine-wave inverter for $900?

Wayne

 

[wmbjk]

wmbjk wrote:

A fine example of the correct approach.

Oh crap. Since your agreement has to be counted as a negative, now
anyone reading will have to wait for someone credible to concur.

Wayne

 

[[email protected]]

I did build a kit inverter, once, years ago. It had a max rating of
150W, Which it met.

It had a half hour rating of 0W

And a surge of about 300W

I'm still scratching my head over that one.


NT

 

[[email protected]]

Sure, but the point is that you can sample a signal that's has all (of a good
approximation thereof, e.g., 99%) of its energy between 144-148MHz (this is
the 2m amateur radio band) at 10MSps and recover everything. I.e., the
bandwidth of the signal is only 4MHz, so you only have to sample at something

IIUC that woudl demodulate the signal too?

NT

 

[Steve Spence]

George Ghio wrote:




I'm still scratching my head over that one.


NT

That's standard with george's posts. Don't get a splinter.

 

[daestrom]

Sure, but the point is that you can sample a signal that's has all (of a
good approximation thereof, e.g., 99%) of its energy between 144-148MHz
(this is the 2m amateur radio band) at 10MSps and recover everything.
I.e., the bandwidth of the signal is only 4MHz, so you only have to sample
at something

Only if you demodulate the signal first.

daestrom

 

[daestrom]

<snip>

More than twice the bandwidth.

So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz,
you think I can sample it at just 150 hz and get accurate reproduction?
That's just wrong.

It is the maximum frequency component in the signal that is important. The
bandwidth is not related unless the lower edge of the band is at 0 hz
(whereupon the upper side of the band is equal to the max frequency).

daestrom

 

[daestrom]

Yes this is the problem. While there are people who will tell you anything
to make a sale, how do you know what you are really getting.

One test is the "Modified Square Wave" test.

When you hear these words you know you are dealing either with a shyster
or an ignorant person who should not be selling things he does not
understand.

Judging from your previous posts, I think you mean when you hear "Modified
*sine* wave", then you know you are dealing with shyster or an ignorant
person...."

A salesperson that says their unit puts out a modified *square* wave would
be a sign[sic] of a knowledgable salesperson.

daestrom

 

[SolarFlare]

Off your medication again Steve?