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Topic: OVI 40 canal I/Q (RX) (Read 5437 times)
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DD4WH
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alter Hase
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Posts: 462
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Re:OVI 40 canal I/Q (RX)
« Reply #16 on: 14. April 2018, 18:22:41 »
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The discussion should be backed up by real measurements and not be based on the dBm values on the TFT screen. See my last post.
73 Frank
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DD4WH
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Re:OVI 40 canal I/Q (RX)
« Reply #18 on: 15. April 2018, 11:35:08 »
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Hi Andreas,
you probably misunderstood me.
I was assuming this thread was about (assumed) differences in IQ signal processing in OVI40 and mcHF (and not about comparing RX properties in mcHF RF board and your prototype RF board).
And those IQ "measurements" made here in this thread are not valid, as I stated in my last post, because they have been made with the internal dBm display.
So I would like to see real measurements on those IQ signals, before I can believe there is something wrong in software or hardware.
BTW, the internal dBm display does this:
for (int c = (int)Lbin; c <= (int)Ubin; c++) // sum up all the values of all the bins in the passband
{
sum_db = sum_db + sd.FFT_Samples[c];
}
This means that measurements with 256-point-FFT --> (dBm display in mcHF UI board) and 512-point FFT (dBm display in OVI40 UI) will always differ by 3dB, because the 512-point-FFT has twice the number of bins to sum up ;-).
So we can take home two messages:
* do not compare dBm measurements between mcHF UI and OVI40 UI
* do not believe in measured signal strengths lower than -102dBm in SSB bandwidth or lower than -112dBm in CW bandwidth
All the best 73s
Frank DD4WH
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SP3OSJ
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Re:OVI 40 canal I/Q (RX)
« Reply #20 on: 15. April 2018, 12:08:29 »
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I wanted to write that if there is any disturbance, the receiver does not receive the signal.
If the interference is not present, the receiver reads the signal.
The small display does low noise, the large display (more pixels) makes big noise.
Just enough.
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SP9BSL
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Re:OVI 40 canal I/Q (RX)
« Reply #21 on: 15. April 2018, 12:48:58 »
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Hi,
This means that measurements with 256-point-FFT --> (dBm display in mcHF UI board) and 512-point FFT (dBm display in OVI40 UI) will always differ by 3dB, because the 512-point-FFT has twice the number of bins to sum up ;-).
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You're right, and that's why Danilo with 6th reply wrote dBm/Hz which normalizes all measurements:)
* do not believe in Measured signal strengths lower than -102dBm in SSB bandwidth or lower than -112dBm in CW bandwidth
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fully agree
I have other proposal: we can use PC via CAT/DIQ OUT and measure received noise without LCD plugged in for OVI/mcHF. With PC we can have deeper FFT (more acurate) than the one in UHSDR.
Will do experiment with slowdown of PARALLEL bus for LCD (it has VeryHigh speed mode on GPIO).
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73 Slawek
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SP3OSJ
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Re:OVI 40 canal I/Q (RX)
« Reply #22 on: 15. April 2018, 13:35:48 »
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Is the receiver that has -102dBm/2.7kHz a good receiver on the 50/70/144/432MHz band?
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DD4WH
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Re:OVI 40 canal I/Q (RX)
« Reply #24 on: 15. April 2018, 17:48:25 »
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@Slawek:
| You're right, and that's why Danilo with 6th reply wrote dBm/Hz which normalizes all measurements:) |
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You are right, dBm/Hz-display does normalize and account for filter bandwidth. BUT the figure is still wrong: the damage has already been done, because the sum that is calculated for the dBm AND the dBm/Hz display is dependent on the number of FFT bins. It is a bug in the software, that is what I tried to explain in my last post (see line 2180 to 2192 in spectrum.c). We have to substract 3dB from the reading when we use an FFT of size 512, because the intercept constant in the dBm code has been empirically determined for an FFT size of 256 !
| I have other proposal: we can use PC via CAT/DIQ OUT and measure received noise without LCD plugged in for OVI/mcHF. With PC we can have deeper FFT (more acurate) than the one in UHSDR. |
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That is a very good idea, not to use the dBm display but an SDR program to deliver measurements of signal strength.
@Artur: please read carefully: I said:
Christian, DL9NL has shown nicely and very precisely that the mcHF is capable of measuring signals down to -102dBm (SSB-bandwidth 2.4k) accurately --> forget measuring lower signals in SSB bandwidth accurately! In CW bandwidth (300 Hz), signals can be measured accurately down to -119dBm, but not lower.
http://funkamateure-dresden-ov-s06.de/index.php?article_id=462&clang=0 |
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I never mentioned a receiver having an MDS of -102dBm
@Andreas: this thread is about a potential IQ problem with the OVI40 UI PCB and not about the prototype RX PCB :-). Nice to hear about the features of that OVI40 RX PCB prototype (looking forward to it ;-)), but we cannot reproduce your results (because we do not know your prototype) and they are unrelated to the potential IQ problem.
So, if somebody thinks we have an IQ problem with the OVI40 UI vs the mcHF UI, please provide reliable data that we can reproduce. At the moment I cannot see a problem that is described accurately enough to even identify and reproduce it.
Have fun with the UHSDR,
73 Frank
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DB4PLE
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Urgestein
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Re:OVI 40 canal I/Q (RX)
« Reply #25 on: 15. April 2018, 18:08:00 »
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Hi Frank,
it is absolutely no problem to account for the FFT size and reduce the 512 FFT values accordingly so that both values are comparable.
This is a no brainer. On the other hand, what I did was to calibrate both so that I measured the same value for the same more or less white noise signal from antenna and that should take into account the dbm difference, shouldn't it?
73
Danilo
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| « Last Edit: 15. April 2018, 18:08:21 by DB4PLE » |
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DD4WH
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Re:OVI 40 canal I/Q (RX)
« Reply #26 on: 15. April 2018, 18:38:17 »
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Hi Danilo,
yes, you are right, I missed that bit of your post.
If you calibrate the 256FFT and the 512FFT to the same signal, their subsequent measurements should be comparable.
I remember we already had a thread on S-Meter reading confusions ;-). Must be somewhere in this forum.
If time allows, I will have a look into other peoples code, how they solved the problem of S-meter readings and if there is potential for improvement (it seems many different measurement types are used even in semiprofessional RXs). First of all, we should add a 3dB subtraction in the FFT512 dBm code.
73 Frank
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Pages: 1 [2] 
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. My answer was directed to Artur who assumes that mcHF has better low signal performance of mcHF compared to OVI40.
