On 20180409 09:21, Leif Asbrink wrote:
Hi Joanne,Colloquialism for "implicit in the SDR concept." D/AYou write "It rather falls out of the concept." and3) Of course SDRs have the potential for extremely
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 handsMany, 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.)
True - depending on whether it is done sample by sampleThat makes sense. I think I'd go with signal peakWell, I wrote peak power, but of course the meter
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.
If I make a single 10 ns wide I and Q sample of RF asSorry I did not express myself clearer. When I mentionRegardless of what units one presents the data inAh, but is it "instantaneous" power (peak voltage
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