Re: Newbie material on HF+(more)


jdow
 

On 20180408 08:07, Leif Asbrink wrote:
Hi Joanne,

3) Of course SDRs have the potential for extremely
accurate S-Meters. It rather falls out of the concept.
?? Why is that?
What mechanism other than an intentionally non-linear
table lookup or a grossly non-linear amplifier would
provide the non-linearity? The "precise" data exists.
I suppose somebody could intentionally create 3 dB or
15.213 dB S units going out of their way to create the
errors. Is there another reason other than willful
malice or woeful ignorance on the part of the coder?

BUT, how many of them give good readings with SSB as
well as CW signals? How does the CW WPM affect
the S-Meters?
I can not tell what other programmers have done,
but I have implemented two alternative S-meters
in Linrad. One showing peak power, the other true
RMS.
That makes sense. I think I'd go with signal peak
power less 3 dB to approximate the 1 RF cycle average
power. (Correct for peak to RMS on a sine wave
carrier.) And I'd report it as peak dBm to people
who ask for signal reports. Comparing this to typical
ham equipment with meter dynamics involved would give
considerably different readings, though. This is
revisiting the VU meter design. {^_-}

(How SHOULD the meters react to transient signals
(fast CW and SSB?))
For CW they should report the key-down power level,
Yes. In the past physical meter dynamics may have
spoiled the accuracy a little if the decay rate is
set too high. (I used the digital VU meter trick of
showing short term peaks and the current peak reading.
The short term peak decays abruptly after about one
second.)

for SSB it is more complicated. Showing peak power
within a selected time interval as well as true RMS
over the same interval would give the operator all
the information. What to give as signal report is
then up to the judgement of the operator. With heavy
speech processing the peak to average power may be as
low as 3 dB, but nice sound quality for local QSOing
would require something like 10 dB.
I believe that is what I just typed up above. We are
in agreement. Now the VU meter people might treat this
is up for grabs. {^_-}

4) Indeed, the S-Meter range is woefully inadequate.
It's amusing to find a solid copy signal that is down
somewhere into the -150 dB range, S -6? Straight
dBm readings at the radio's input with an indication
of the antenna design and height can allow extraction
of some sort of meaningful data. Should I modify the
S-Meter reading downwards one for a 6 dB gain antenna?
Gimme dBm or death?
The "S-meter" is intended to assist the amateur in
giving signal reports in the RS(T) system. I do not
think that is any good idea at all - better use the
original definition:
http://www.acsu.buffalo.edu/~maxwell/RSTInfo.html
That is why I get obnoxious and report in dBm or S units
that are as accurate as I can make them. They want more
than an "Armchair copy OM!" report they'll get something
more scientific than they intended. (Of course, I have a
few times reported "strong but severely distorted OM!"
for people who overdrive their ALC. And before I make a
report like that I make sure the RF noise blanker is OFF.)

Obviously the report is how the signal is received
by the operator and antenna gain should not be subtracted.
The RS(T) report gets its full meaning when station
characteristics is transferred during the qso or
perhaps on a qsl card. (antenna, receiver, LNA,...)
Of course, I was imagining the humor of doing so instead
of a dBm report. {^_-}

Amplitude is one of those contextually defined words.
In describing a signal's amplitude I tended to use
units such as dBm. That implies a power type quantity.
But, given it's a sloppy word I try to extract what
the writer means from context and leave off worrying
about it.
Regardless of what units one presents the data in
amplitude and power have the same meaning for an
RF signal. What I wanted to say is that amplitude
might implicate a peak detector. Particularly
if we talk about a CW signal. Amplitude is likely
to be inthuitively interpreted as the amplitude
during keydown while power is more likely to be
understood as the average power. None of the
interpretations is formally more correct than its
opposite, when specifying "dB" or "dBm" for an RF
signal one has to specify the detector used.
Ah, but is it "instantaneous" power (peak voltage
of the sine wave squared divided by the impedance)
or an average power over either precisely one RF
cycle or a large enough number of cycles that the
error becomes small? Instantaneous less 3 dB is a
simple way with really fast modulation compared
to carrier frequency.

Good old analog spectrum analyzers do not give the
correct noise floor. They do not have true RMS
detectors. If I remember correctly the error
is 2.51 dB
(Probably carried out to an unwarranted level of
precision, too.) This is one of the nice advantages
of SDRs.

The origin of this thread was about measuring changes
in antenna noise. For that purpose I think a true
RMS detector is the appropriate tool - and the HF+
has such a tool in "Signal diagnostics" One just
has to be careful to not be fooled by the digital
AGC function that can not be disabled.
Yes, for best accuracy. However, "the same detector
every time" can give quite useful results if you do
not need to get more than some loose qualitative
reporting. The noise here dropped about 10 dB. RMS
readings might get you into the 7.834 dB silliness
range. Of course a mark one eyeball view of a
spectrum can note that the discrete spikes I have
from my USB, Ethernet, and DSL wires are unchanged
but between then the noise went down about 8 dB
between lightning strikes. Without the eyeball I
am not sure how I'd accurately report improvements
or degradation.

"No, OM, I did not mean 80dB over S-9. I simply
said 80 dBuV. Subtract 73 dB if you want dB over S9."

{^_-}

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