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]> 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]> 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]]
>>> 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.
>>>>
>>>
>>>
>>>
>>> ----------------------------------------------------------------------------
>>> --
>>> _______________________________________________
>>> Freetel-codec2 mailing list
>>> [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
>>>
>>>
>>
>> ------------------------------------------------------------------------------
>> 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
>>
>>
>
>
> ------------------------------------------------------------------------------
> 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
>
>
------------------------------------------------------------------------------
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
