Nice work on the power measurement Helmut :-)
I wanted to say a little about 0dB SNR and below operation. In the
attached plot, the lower three curves show the performance of modems in
AWGN channel - a channel that just has additive noise (like a very slow
fading HF channel or VHF).
These curves show how much energy/bit (Eb/No) we need for a certain Bit
Error Rate (BER).
The Blue curve just above the Red (ideal QPSK) perf is our modem in a
AWGN channel. Time for some math:
The energy/bit Eb = power/bit rate = S/Rb
The total noise the demod sees is No (noise power in 1Hz) multiplied by
the bandwidth B, N=NoB
Re-arranging a bit we get:
SNR = S/N = EbRb/NoB
or in dB:
SNR(db) = Eb/No(dB) + 10log10(Rb/B)
Now Rb = 700, B = 3000 Hz (for SNR in a 3000Hz bandwidth) so we get:
SNR = Eb/No - 6.3
Now, say we need a BER of 2% or 0.02 for speech, the lower blue curve
says we need an Eb/No = 4dB, so we get:
SNR = 4 - 6.3 = -2.3dB
So if the modem is working down to "just" 0dB we are about 2dB worse
than theoretical. This is due to the extra bandwidth taken by the pilot
symbols (which translates to 1.5dB) and some implementation loss in the
sync algorithms, and non linearities in the system.
I thought it worth explaining this a little more. These skills will be
just as important to Hams of the 21st century as Ohms law was in the 20th.
Cheers,
David
On 23/09/15 18:04, Helmut wrote:
Hi Glen,
Very difficult question, hi! I left the commercial arena more than 12 years
ago, so that have to use my old mixed surplus test equipment, certainly not
competitive with today's requirements, but good enough for ham radio.
The old R&S RMS-Powermeter with a tru-sensor provides a nominal accuracy of
+/- 5 %. A PEP- module for this RMS meter offers similar accuracy for
sinusoidal waveforms. From time to time I check the R&S meter with my (old)
HP 435A and a reference circuit.
The most accurate test equipment is finally my HPSDR setup. The feedback
loop for the pre-distortion delivers an exact and well defined portion of
the TX power to the RX input in real-time. As the ADC is driven to full
scale or in minimum 15 dB below the failure is within +/- 0.3 dB for average
or PEP indication (total dynamic range > 120 dB).
For the exciting tests with Alfred, HB9EPU, to get a feeling how capable
freeDV at small SNRs is, I always watch the displayed spectrum of the
incoming signal. The FFT bin width is 2,93 Hz, that means that a signal is
still visible, but no longer audible after processing in the phones.
Therefore it was amazing, that freeDV seems to produce reasonable
readability still with SNR =0 dB.
73, Helmut
p.s. No experience with AD8362. Boonton (Wireless Telecom Group) offers a
nice USB Peak Power Sensor.
-----Ursprüngliche Nachricht-----
Von: glen english [mailto:[email protected]]
Gesendet: Mittwoch, 23. September 2015 01:26
An: [email protected]
Betreff: Re: [Freetel-codec2] Tuning Aid 700B mode
Helmut
How well does your power meter respond to high peak to average signals,
IE faithfully measuring peak and PEP ? I've been involved in this with
DRM and DVB.
something to watch, anyway.
Maybe use a TRUPWR (R) device like the AD8362 to measure it.
73
On 23/09/2015 5:58 AM, wully wrote:
Hi all
Helmut, dc6ny, and I have done some more tests yesterday. The band
starts opening again (wonderful: we always want to test 700B).
As stated earlier, we never had any problem with the 700B-tuning: cohpsk
locks quicker than the fdmdv, at least, this is the impression we get
from our experiences. Yesterday, I started to reduce my power (measured
by an LP-100A just before the antenna coupler) when txing in 700B.
Helmut detected the signal and I reported about my power seting: Full
drive started with 58Watts peak. Then I reduced the power to 100mW peak
(reading the quickly changing "average" of LP-100A). This was the last
setting, that I could read on the LP-100A, my relative powerseting was
at 7 out of 256. Then I reduced to 6, then to 5, to 4, to 3, to 2. After
this experiment, Helmut reported, that he could fully decode the signal
at 6, but at 5 only about 50% and lower no decoding anymore. We
estimated, that setting 6 was about 50 mWatt.
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