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.
>>>
>>
>>
>>
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