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Wind/Solar Electrics ???



 
 
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  #161  
Old December 30th 05, 02:06 AM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

Your sampling of only the bandwidth information is not
complete for decoding.

"Ray Andraka" wrote in message
news:zIdsf.34359$Mi5.7331@dukeread07...
SolarFlare wrote:

The point is the sampling rate has to be done at

just
over double the frequency of the signal and not the
bandwidth.



No, that is not correct. It only needs to be sampled

at 2x the
bandwidth, assuming the spectrum has been properly

filtered to energy
outside the signal of interest. See my earlier post.



  #162  
Old December 30th 05, 02:46 AM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

Now you have to provide samples ***AND*** changing
information with an algorthm to decode successfully.
Your samples are then not complete and useless without
other information supplied.

Superhetrodyning in a radio assumes a variable
superhetrodyning frequency when it gets decoded

Let's see you regenerate the original carrier and
information from that without the carrier frequency
known.

When you listen to the audio on your radio can you tell
the carrier frequency without the dial?

"Joel Kolstad" wrote in
message ...
Assuming all the "information" (the carrier and

whatever sideband(s) you care
about) is still within your bandpass frequencies,

you've lost nothing and
there is no aliasing with any non-zero signals.


Superheterodyning is still common to get the RF down

to an IF that can be
digitized directly. As Ray mentioned earlier, the

problem with trying to
digitize, say, a narrowband 900MHz signal using a

5Msps ADC is that the effect
of any clock jitter going into the ADC gets

multiplied by the 900/5, so at
some point obtaining a decent oscillator becomes

impractically expensive.

---Joel




  #163  
Old December 30th 05, 04:41 AM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

"SolarFlare" wrote in message
...
Now you have to provide samples ***AND*** changing
information with an algorthm to decode successfully.
Your samples are then not complete and useless without
other information supplied.


This is true, but it's pretty much true for all communication systems, not
just sub-sampled digital ones. What changes is that sometimes the extra
'information' supplied can be done by something as sophisticated as a human's
brain as he tunes across the dial to find the 'best' sound -- this
corresponding to finding the carrier. (At some point this becomes a very
philosophical discussion... 'information' only has _meaning_ to an observer
who presumably knows something about or has a hunch as to what they're
observing is. Although one can compute the 'information' within signal in an
attempt to ascertain whether it resembles a random process or whether it's
conveying what we may 'intelligence.' Hence you can probably recognize the
difference between someone speaking random jibberish and an actual language
even without knowing that language, but on the other hand a good way to
conceal information is to make it appear almost completely random -- when in
fact it isn't --, which is exactly what cryptography does.)

Let's see you regenerate the original carrier and
information from that without the carrier frequency
known.


Example: Take an antenna that's about a meter long... feed its output to an
LNA and then a reasonably steep bandpass filter passing 144-148MHz... sample
with a 16 bit, 12MSps ADC (I chose 12 just because it shifts 144MHz to
baseband, although there's no reason you can't use any frequency 8Msps).
Feed this digital word to a 16 bit DAC clocked at 10MSps. Lowpass filter the
DAC's output with a reasonably steep 4MHz low-pass filter. Feed this signal
to one port of a mixer and 144MHz to the other port. Poof! There's your
original signal back again! Feed this through another 144-148MHz bandpass
filter if you don't like the image response at 140-144MHz.

There are a few caveats he

1) Clock jitter will tend to broaden out the specctra of the original signals
a bit (how good is your clock?)
2) The track & hold (analog) circuitry in the ADC has to be good to a couple
hundred MHz to avoid distortion.
3) Your noise floor is limited to no better than ~-100dB (and potentially
_much_ worse if you haven't been careful in your layout, power supply
decoupling, etc.). Note that everything described above also applies to
switched capacitor circuits (a technology whose time has just about passed,
but a neat idea); in that case analog noise rather than quantization noise
will dictate the noise floor (and realistically it'll probably be much worse
than -100dB...)
4) A typical DAC holds its output (e.g., a first-order hold) rather than
generating impulses, so the spectrum reproduced has a sin(x)/x profile to it
(frequencies closer to 148MHz will have less gain than those at 144MHz); this
can be fixed in the digital domain with the use of a FIR or IIR filter. In
some systems the droop is small enough that people just ignore it.

This example is reasonably practical. Strictly speaking, to make it simlper
you can just bandpass filter the output of the DAC directly and be OK, but the
sin(x) profile along with the limited analog bandwidth of the DAC tend to make
this approach impractical (you end up with very little SNR); this approach if
often used for proof-of-concept demos, though.

If you happen to have a carrier at 146.23MHz in the input signal, it'll most
certainly still be there in the output signal, yet the system didn't have to
'know' where the carrier was.

When you listen to the audio on your radio can you tell
the carrier frequency without the dial?


Sure, I can measure it! In fact, a much more interesting problem is how one
generates a carrier when none exists in the first place. There are plenty of
modulation schemes out there specifically don't use a carrier to either save
power (TV transmissions -- which have a very small albeit not eliminated
carrier -- are a good example of this) or to conceal transmissions (in
military systems nothing attracts unwanted attention more than a carrier some
60dB above the noise floor).

---Joel


  #164  
Old December 30th 05, 05:10 PM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

daestrom wrote:


So what you're saying is, *if* you know the carrier frequency and band-width
of the signal imposed on that carrier, you can design a system that will be
able to reproduce the imposed signal using a relatively low sample rate (low
when compared to the carrier frequency).

But if the carrier frequency changes, then you need to modify the sample
rate to avoid a lot of aliasing issues. So in radio reception, the sample
rate is adjusted along with tuning the receiver? Or is this done at the
intermediate frequency which is fixed so that sample rate adjustment is
fixed with the intermediate frequency? (do they even still use
superheterodyning in tuners?? ;-)

It's been a long time since I did any RF stuff. But A/D and D/A stuff at AF
and lower has been quite a passion for me for some time. And the basic
Nyquist hasn't changed.

daestrom



The carrier frequency has nothing to do with it. What is important is
the bandwidth and the center frequency of the pass-band. Note that your
signal needn't take up the whole bandwidth, and in a typical radio
system the signal you are tuning to is a very small fraction of the
pass-band. In any case, the pass band is defined by the anti-alias
filter, so practically speaking, it is a fixed, known pass band.
Therefore your *if* is satisfied.

What subsampling buys you is a way to sample an IF that is at a higher
frequency than the sample rate of your system, which may be limited
either by the ADC or by your computational power.

BTW, I never said nyquist changed. I was simply stating that it is more
general than the commonly held belief that the sample rate has to be at
least 2x the highest frequency. The truth is, the sample rate has to be
at least 2x the bandwidth of the signal.
  #165  
Old December 30th 05, 05:30 PM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

SolarFlare wrote:

OK let's go with your analogy example of 1234 being
represnted by 234 only.

You have no way of decoding 234 into 1234 without
passing information of 1000 as your baseband info and
therefore the the number 1234 has not been successfuly
representedm as being reproduced without further
information.

Now we could further argue algorythms as part of the
information or part of the sample.



Likewise, you have no way of discerning 234 is actually 234 and not 1234
with a 3 digit decimal number system. The problem is not unique to
sub-sampling, it exists at baseband as well. The only difference is
that at baseband the representation looks the same as the signal. In
either case, you need to know the fixed constraints of the system to
fully comprehend the meaning of the representation. For example, in a 3
decimal digit system, you have no way of knowing that 234 really is 234
and not 1234 or 2234 unless you also know that the inputs are limited to
the range 0 to 999. The only way around that is to have an infinite
number of "symbols" to represent all the possible data when the set of
possible data is infinite. As soon as that set is not infinite, we can
take advantage of our knowledge of the system to reduce the set of
symbols to a manageable number of elements. I'd argue that any
engineering requires a set of implied constraints in order to make the
problem solvable.

In the case of the subsampling, we know by design what the pass-band of
the anti-alias filter is. That is a constant parameter designed into
the system, so presumably it is know to designers of all the components
of the system.

In the example case, then, we set as a system constraint the fact that
all inputs are in the range of 1000 to 1234. That constraint is a
constant, and is implied by the design. No information is lost by not
transmitting the constant that is already known throughout the system.
Doing so simply wastes bandwidth on your communications channel.
  #166  
Old December 30th 05, 11:28 PM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???


"Ray Andraka" wrote in message
news:wZctf.58905$4l5.50283@dukeread05...
daestrom wrote:


So what you're saying is, *if* you know the carrier frequency and

band-width
of the signal imposed on that carrier, you can design a system that will

be
able to reproduce the imposed signal using a relatively low sample rate

(low
when compared to the carrier frequency).

But if the carrier frequency changes, then you need to modify the sample
rate to avoid a lot of aliasing issues. So in radio reception, the

sample
rate is adjusted along with tuning the receiver? Or is this done at the
intermediate frequency which is fixed so that sample rate adjustment is
fixed with the intermediate frequency? (do they even still use
superheterodyning in tuners?? ;-)

It's been a long time since I did any RF stuff. But A/D and D/A stuff

at AF
and lower has been quite a passion for me for some time. And the basic
Nyquist hasn't changed.

daestrom



The carrier frequency has nothing to do with it. What is important is
the bandwidth and the center frequency of the pass-band. Note that your
signal needn't take up the whole bandwidth, and in a typical radio
system the signal you are tuning to is a very small fraction of the
pass-band. In any case, the pass band is defined by the anti-alias
filter, so practically speaking, it is a fixed, known pass band.
Therefore your *if* is satisfied.

What subsampling buys you is a way to sample an IF that is at a higher
frequency than the sample rate of your system, which may be limited
either by the ADC or by your computational power.

BTW, I never said nyquist changed. I was simply stating that it is more
general than the commonly held belief that the sample rate has to be at
least 2x the highest frequency. The truth is, the sample rate has to be
at least 2x the bandwidth of the signal.


After proper filtering of the input.


  #167  
Old December 31st 05, 03:38 AM posted to rec.aviation.owning,sci.electronics.design,alt.solar.photovoltaic
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Default Wind/Solar Electrics ???

You still had to supply the constraints so the sampling
is not complete for any waveform.

This is like supplying $234 to buy that big screen TV
when you have to supply $1000 under the table to
actualy get it delivered. All the money is not upfront
and the $234 is a lie.

"Ray Andraka" wrote in message
news:2gdtf.58906$4l5.30943@dukeread05...
SolarFlare wrote:

OK let's go with your analogy example of 1234 being
represnted by 234 only.

You have no way of decoding 234 into 1234 without
passing information of 1000 as your baseband info

and
therefore the the number 1234 has not been

successfuly
representedm as being reproduced without further
information.

Now we could further argue algorythms as part of

the
information or part of the sample.



Likewise, you have no way of discerning 234 is

actually 234 and not 1234
with a 3 digit decimal number system. The problem is

not unique to
sub-sampling, it exists at baseband as well. The

only difference is
that at baseband the representation looks the same as

the signal. In
either case, you need to know the fixed constraints

of the system to
fully comprehend the meaning of the representation.

For example, in a 3
decimal digit system, you have no way of knowing that

234 really is 234
and not 1234 or 2234 unless you also know that the

inputs are limited to
the range 0 to 999. The only way around that is to

have an infinite
number of "symbols" to represent all the possible

data when the set of
possible data is infinite. As soon as that set is

not infinite, we can
take advantage of our knowledge of the system to

reduce the set of
symbols to a manageable number of elements. I'd

argue that any
engineering requires a set of implied constraints in

order to make the
problem solvable.

In the case of the subsampling, we know by design

what the pass-band of
the anti-alias filter is. That is a constant

parameter designed into
the system, so presumably it is know to designers of

all the components
of the system.

In the example case, then, we set as a system

constraint the fact that
all inputs are in the range of 1000 to 1234. That

constraint is a
constant, and is implied by the design. No

information is lost by not
transmitting the constant that is already known

throughout the system.
Doing so simply wastes bandwidth on your

communications channel.


 




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