Re: Newbie material on HF+(more)

Leif Asbrink
 

Hi Joanne,

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.
I just think "There are rather few radios in the hands
of hams that are accurate with their S-Meter readings."
is an inappropriate statement.

In order of magnitude:
There are very few radios in the hands... (maybe 0.1% have one)
There are rather few radios in the hands... (maybe 1% have one)
There are few radios in the hands... (maybe 10% have one)

As I understand it many more than 1% of the ham community
owns an SDR.

Please correct me if my understanding of English is wrong.

The ham market needs a complementary HF transmitter to
match the HF+. That should lead into a complementary
transmitter for VHF on up.
Why is that? The old analog transmitter will continue
to serve well. The SDR can be added to the IF of an
analog radio or be used directly on the antenna in rx
mode.

(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.
I do not think the FCC would create problems for someone
selling test instruments (signal generators) capable
of delivering 20 dBm of power. There is the Softrock
that can transmit 1 W on any frequency within an octave
frequency range. It is a kit, but I do not build with
surface mount components so I bought an assembled unit.

The softrock is not a transmitter. It is a building block
by which a radio amateur can build a transmitter. Amateurs
building their own equipment have full responsibility for
what their equipment puts on the air.

I do not think the modest number of SDR transmitters is
because of FCC and PTT regulations. It is because amateurs
already have transmitters and replacing them with an SDR
would add no benefit. If you run Linrad you could use
the Linrad speech processor to increase the average
transmitted power from a conventional SSB transmitter
while reducing the splatter:
https://www.youtube.com/watch?v=VmxaZe3MM2A
There is no need for a Tx SDR.

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.
It is actually easier than it sounds. RMS power is computed
in the IF passband. The user is free to set the averaging
time as he wants.

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!!

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?
Dear Joanne, this question is irrelevant. Nobody would
try to do something like this.

You say you have I and Q samples separated by 10 ns. That
means clock is 100 MHz and the bandwidth is 200 MHz.
Anyone with the slightest insight in SDR technology knows
that a 2.4 kHz wide filter at a 100 MHz sampling rate
is too demanding even for the best processors we have.

What we do is to decimate, apply a filter with a bandwidth
of perhaps 40 MHz and then use every 4th data point.
(means 40 ns or 25 MHz bandwidth.) Then, filter and
decimate once more to get perhaps 640 ns or 1.5625 MHz
sampling. Then filter and decimate again to get perhaps
40.96 ms or 24.414 sampling. It is reasonable to apply a 2.4
kHz wide filter at this sampling rate but it is inefficient.
In Linrad a clever user would set the final sampling rate to
6 kHz or so.

If you would be really stupid and apply a 2.4 kHz filter
at a sampling rate of 100 MHz you could resample by a factor
of 100000/2.4 = 42000 without loosing any information at
all. Just square I and Q, add them, for every 42000th sample.

It is (of course) assumed that the frequency is shifted
so the 2.4 kHz baseband is in the range ±1.2 kHz.

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?
"the averaging above" ???

Anyway, after resampling to represent the desired frequency
range efficiently one cound do an fft with 3 kHz bandwidth,
then average 128 transforms and use the result for one line
in the waterfall.

Alternatively one could use a 128 times larger fft with
23 Hz bandwidth and take the average over 128 fft bins for
each pixel in the waterfall. The result should be identical.

We can however do much better. Use a 128 times larger
FFT. Average 10 transforms in the full fft size, then
pick the largest value with in each group of 120 average
powers and use for the waterfall. That strategy gives a
major improvement in sensitivity for narrowband signals.

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.
I think you need to study SDR technology. We use linear
transformations like frequency shift and filtering.
The RF signal is transferred to a baseband signal
with a sampling rate not much bigger than the signal
bandwidth. The transformation is IDEAL (done by digital
means) and it is done without any loss of information
whatsoever.

73

Leif

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