It's an challenging area to play in, I worked out the theoretical limits for DV here:
http://www.rowetel.com/blog/?p=4053 On 24/09/15 11:02, Bruce Perens wrote: > I was thinking, for my Pacificon talk on 700B next month, of doing a > take-off on the QST column "The World Above 50 MHz" called "The World > Below 0 dB S/N". > > On Wed, Sep 23, 2015 at 5:49 PM, Matthew Cook <[email protected] > <mailto:[email protected]>> 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. > > > > > ---------------------------------------------------------------------------- > -- > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > <mailto:[email protected]> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > ------------------------------------------------------------------------------ > Monitor Your Dynamic Infrastructure at Any Scale With Datadog! > Get real-time metrics from all of your servers, apps and tools > in one place. > SourceForge users - Click here to start your Free Trial of > Datadog now! > http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > <mailto:[email protected]> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > ------------------------------------------------------------------------------ > Monitor Your Dynamic Infrastructure at Any Scale With Datadog! > Get real-time metrics from all of your servers, apps and tools > in one place. > SourceForge users - Click here to start your Free Trial of > Datadog now! > http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > <mailto:[email protected]> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > > ------------------------------------------------------------------------------ > Monitor Your Dynamic Infrastructure at Any Scale With Datadog! > Get real-time metrics from all of your servers, apps and tools > in one place. > SourceForge users - Click here to start your Free Trial of Datadog now! > http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > <mailto:[email protected]> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > > ------------------------------------------------------------------------------ > Monitor Your Dynamic Infrastructure at Any Scale With Datadog! > Get real-time metrics from all of your servers, apps and tools > in one place. > SourceForge users - Click here to start your Free Trial of Datadog now! > http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 > > > > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
