Re: Newbie material on HF+(more)

jdow
 

On 20180409 09:21, Leif Asbrink wrote:
Hi Joanne,

3) Of course SDRs have the potential for extremely
accurate S-Meters. It rather falls out of the concept.
You write "It rather falls out of the concept." and
I can not understand what you mean.
Colloquialism for "implicit in the SDR concept." D/A
converters are fairly nicely linear with high accuracy,
much higher than the analog AGC controls in empty state
electronics components. (Vacuum tubes)

You wrote "2) There are rather few radios in the hands
of hams that are accurate with their S-Meter readings."
and I disagree. There are many SDRs in the hands of
hams today - and most of them have extremely accurate
S-meters.
Many, yes. I suspect "most" is still, by far, older
analog empty and solid state equipment with no A/D
converters involved. This would be particularly true
on HF. At VHF etc I will grant a fairly large number
of RTL dongles exist out there and a few of the other
(far) more expensive models. Youssef is mining the
middle ground with some excellent equipment for the
price. I hope that will change at least the SWM market.
The ham market needs a complementary HF transmitter to
match the HF+. That should lead into a complementary
transmitter for VHF on up. (And both will lead to
annoying interaction with the FCC in the US. So I can
understand his holding off on it. "In the USA we only
send to the address found on the FCC web site for the
call letters supplied." The PITA level is sufficient
to keep a lot of entrepreneurs out of the market this
favoring the big boys already in the market. Personally,
I think there is something wrong with that effective
bias that has been setup. It's bassakwards.)

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.)
Well, I wrote peak power, but of course the meter
gives the peak RMS power since the detector is a
true rms detector. there should be no subtraction
of 3 dB.
True - depending on whether it is done sample by sample
or effectively millisecond by millisecond. RMS power in
the AF passband requires a millisecond by millisecond
average to get close. 10 ms is close. 100 ms starts
averaging out syllable level peaks. It's not as easy
as it sounds.

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.
Sorry I did not express myself clearer. When I mention
peak power of an RF signal I ALWAYS mean peak envelope
power. "peak envelope power (of a radio transmitter):
The average power supplied to the antenna transmission
line by a transmitter during one radio frequency cycle
at the crest of the modulation envelope taken under normal operating conditions." I have always thought that this
is a common practise when talking about radio;-)
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. I
can square I and Q, add them, and square root the result.
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?

That demands another question. Is there a material
difference between taking a 3 kHz wide set of 10 Hz wide
FFT samples and averaging the bin power levels compared
to filtering the signal to 3 kHz wide and measuring the
10 ns wide I/Q samples for the averaging above?

I may be missing something here. But visualizing a pure
CW signal that is on off keyed and sampled and processed
leads me to see what appear to be difficulties that are
best sorted out with some averaging of some type.

This is probably why there was some discussion from time
to time about PEP calculations for SSB back in the day
and why the FCC took some modest effort to describe how
to measure the output power of a transmitter.

I believe you with the discussion of your RMS
calculations. The calculations are, no doubt, correct.
The presumptions of accuracy depend on what it is you
think you are measuring. Measuring at RF frequencies
at some largish number of samples per cycle of RF
makes it easier to look for the instantaneous peak
which is 3 dB higher than the one cycle long (or
half cycle long) RMS. At AF, it's more awkward. But
some form of averaging over time is needed to get a
useful RMS value whereas a peak value less 3dB is
still probably as good a reading as you can get
with a very rapidly changing not particularly
repetitive waveform.

{^_^}

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