Bookmarked tnx David !
73
/M.
On 24 September 2015 at 10:26, David Rowe <[email protected]> wrote:
> Yep, I thought it worth blogging on as I keep forgetting it myself:
>
> http://www.rowetel.com/blog/?p=4621
>
> Have some other ideas for more later....
>
> - David
>
> On 24/09/15 10:19, Matthew Cook wrote:
> > David,
> >
> > Why can I see a discussion paper or magazine article entitled "do you
> > know your Eb/No?" *grin*.
> >
> > 73
> >
> > Matthew
> > VK5ZM
> >
> > On 24 September 2015 at 05:46, David Rowe <[email protected]
> > <mailto:[email protected]>> wrote:
> >
> > 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]
> > <mailto:[email protected]>]
> > Gesendet: Mittwoch, 23. September 2015 01:26
> > An: [email protected]
> > <mailto:[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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> >
> >
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