Re: Newbie material on HF+(more)

jdow
 

On 20180410 17:57, Leif Asbrink wrote:
Hi Joanne,

And without that reduction why is a fancy SDR needed for
reception?
Because it allows better interference suppression and
some other tools that allow copying weaker signals.
It also allows a highly sensitive waterfall and perhaps
CW skimmer.
Not necessarily true if there are weak signals within the
splatter zone of the really strong signals. There are times
of the day when that defines the entire HF bands when the
band is open. It doesn't help that hams (and more so CBers)
tend to completely overdrive their ALC on transmit. That's
bad on several levels.

The FFT calculated number is an average already. In fact
if you make a really fine grain FFT and don't use overlapped
computation techniques you're averaging over syllables which
won't catch the peak.
NO NO!!
The RMS power is computed in the time domain on the
filtered signal.
How long does it take to collect a set of FFT data that
allows 10 Hz resolution. You are in essence averaging
over that period in your FFT output. Right?

Many digital modulation techniques
featuring multiple tones have a very high peak to average
power. That's why they are best run at power levels well
below the peak the transmitter can develop. The human
voice also has that problem. By comparison CW is really
simple. And will your RMS give the same reading for an
extended key down, a series of Morse code dahs, and a
series of Morse code dits?
Yes.
Brain just kicked in. You are measuring the I/Q output of
the filtered demodulator input? If so I can see how that
would give you a real power reading. I^2 plus Q^2 is indeed
a constant regardless of when the sample is taken. My head
was not thinking properly on that detail. I was too much
envisioning the detector in an analog radio with the "Q"
signal. So information is lost and you get funky results.
Sorry about that.

Of course, the average power over a period of seconds
is probably more important for the transfer of
information for most modes. But you need to know the
peaks in order to avoid overloading the A/D or D/A
elements in the system.
Now you are on really thin ice...
The signal of interest is rarely (or more adequately NEVER)
the dominating signal in the wideband signal fed to the
ADC in a SDR.
True - and even then if you take I^2 plus Q^2 the results
will work out to a real instantaneous power reading. So you
can snag the peak reading and go.

If I make a single 10 ns wide I and Q sample of RF as
filtered to the bandwidth of interest what do I have.
The bandwidth of interest is perhaps 2.4 kHz for SSB.
A "a single 10 ns wide I and Q sample" has no meaning,
that implicates that you represent the 2.4 kHz bandwidth
with a sampling frequency of 100 MHz. Grotesque!!
That is on the order of the sample rate for the ANAN SDRs.
What is grotesque about that?
To believe that the power for the S-meter is computed
from a signal filtered to 2.4 kHz bandwidth and presented
at a sampling rate of 100 MHz.
It should be possible if "stupid". Otherwise there is
unreasonable magic taking place. And now that I have my
brain into I *AND* Q mode sense is coming out.

I can square I and Q, add them, and square root the result.
Shame on me! {o.o}

What do I have? If the RF carrier is say 1 MHz how long
do I have to average successive 10 ns wide samples to get
a decent true RMS reading by squaring I's and Q's, adding
I's and Q's, dividing by the number if I and Q pairs, and
square rooting the result? What happens if I average 125
samples instead of 100 samples? Does it matter what part
of the 1 MHz sine wave I start with?
Dear Joanne, this question is irrelevant. Nobody would
try to do something like this.
It is done every day.
I suggest you give me a single example. I am convinced
you will not be able to do that. (Please remember we are
discussing S-meters in receivers.)
In the immortal words of Emily Litella "https://www.youtube.com/watch?v=OjYoNL4g5Vg". (Thank you "Laugh In" for great cultural
references.)

But usually the filter is placed at
the lower frequency after filtering and decimation to
minimize computation.
I would say always.
In a sane world one does not throw away CPU cycles. I was
not postulating sanity. I was postulating a world of
wretched excess that should, aside from burning CPU cycles,
should behave the same.

(More I/Q mental lapse on my part removed. One single sample
would work just fine if you get I and Q data rather than just
I as my brain was thinking despite saying I/Q. "Duh!")

(And we get back to something else of interest which makes
typical S-Meter readings "misleading". Most of the time
average power is more important than peak power. Although
clipped peaks are lost information, too.)

And that number does matter with respect
to limiting in equipment. But for things like SSB or
high peak to average digital waveforms the average power
over seconds is what matters more in the information
theory world.
Sure, given a max pep power (1.5 kW) one would get the
best information transfer with very hard compression
and an average power of about 900W. That would be
when the signal has a constant level in a background
of white noise. Such a signal is heavily distorted and
not easy to copy even at higher signal levels. If there
is some qsb or if signal levels are a little higher
it is much better to use less compression and perhaps
500W average power.
In other words, this might be why heavy metal music
cuts through ambient noise better than most symphonic
music. {^_-}

On HF as I understand the report is always 59 or
599 (to save time.)
For SSB one starts to wonder if a dual reading might
be worthwhile, peak over a 1 second measurement (not
averaging) interval and average over that same one
second measurement period. The signal is as good as
the average and the peak suggests a limit on how
much better you could make it with fancy compression
techniques, some of which introduce annoying distortion,
some of which introduce subliminal distortion, and some
of which merely keep the peaks controlled. (I suspect
digital speech codecs can step in to the really annoying
distortion realm. {^_-})

I hope I'm making more sense at the moment than when I
was mentioning I and Q but only thinking in terms of I.
Picture me sitting here feeling silly.

{o.o}

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