Wind/Solar Electrics ???

In article .com,
"philkryder" wrote:

"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

wrote in message
...
Joel Kolstad wrote:

(I can't tell you how many times I've seen people stating something like,
'The Nyquist theorem requires sampling at at least twice the highest
frequency present in the signal," when of course it says no such thing.)

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/designstandard...sp_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

On Fri, 23 Dec 2005 18:46:49 GMT, the renowned "daestrom"
wrote:


wrote in message
...
Joel Kolstad wrote:

(I can't tell you how many times I've seen people stating something like,
'The Nyquist theorem requires sampling at at least twice the highest
frequency present in the signal," when of course it says no such thing.)

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.
snip

More than twice the bandwidth.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

With risers it is the same confusion.

"Rich Grise" wrote in
message
news
On Thu, 22 Dec 2005 22:01:02 -0500, SolarFlare
top-posted:

Three risers though.

Yabbut, that misses the point of the gag. It's easy
to point at the three
risers:

. ------------
. | -- 1
, -----
. | --2
. -----
. | -- 3
. -------------

And, obviously, the middle one is #2.

But, while stepping up or down the stairs, the way
most people count steps,
if you're going down, (to the right) you'd go:


. ------------
. | 1
, -----
. | 2
. -----
. | 3
. -------------

And count 3 steps. But if you're going up, which is
right-to-left in
this exsample, you'd go:


3
. ------------
. | 2
, -----
. | 1
. -----
. |
. -------------

because where you started from is zero in either
case, but step 2 is
different if you're going up or down.

Hope This Hemps!^H^H^H^Hlps! %-}
Rich



"Roger" wrote
in
message
...
Or this one: Imagine a short staircase, say to a
"sunken living
room" or some such, of 3 steps:


------------
|
-----
|
-----
|

---------------------------

There are only two steps. on the stairway. The
others are landings

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Joel Kolstad wrote:

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

wrote in message
...
Joel Kolstad wrote:
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.

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
8MSps.

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).

"Ray Andraka" wrote in message
news:deZqf.31975$Mi5.3388@dukeread07...
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

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.

philkryder wrote:
"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

A fine example of the correct approach.

wmbjk wrote:
On 22 Dec 2005 19:49:37 -0800, "philkryder"
wrote:


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


I've purchased a couple of ~$1500 machines from a local welding
supplier on condition that if there were any problems running them off
my SW inverters then the machines could be returned in as-new
condition the following day and I'd buy a different model instead.
That flexibility, and being able to see the machines in person, made
the extra cost of buying locally worthwhile.

Wayne