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Old 06-14-2018, 12:41 PM
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Default RSSI on SDS 100 calibrated?

Interested in the new RSSI (Received Signal Strength Indicator) screen on the Uniden SDS 100. I've seen the RSSI display pictured in some promotional ads or reviews.

Can anyone advise if the RSSI (the signal strength level displayed) is simply RELATIVE like a un-calibrated S Meter, or actually CALIBRATED to the input voltage level seen at the antenna jack?

One way to check this would be to hook an RF Generator to the antenna input at a sufficiently low level and see if the displayed RSSI level matches the output level of the generator.

Of specific interest is the calibration of the RSSI level at 100.0 MHz and at 850.0 MHz.

Can anyone check this and see, or perhaps Uniden can advise?

Much appreciated!!
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Old 06-14-2018, 1:28 PM
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0dBm = 1mV signal level.

I doubt it's precisely calibrated, but it's a bit more meaningful than x signal bars.
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Old 06-14-2018, 2:40 PM
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RSSI is relative and useless for calibration. I'd prefer a value expressed in dBm or uV. That's a useful value you can collect scientific data with.
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Old 06-14-2018, 3:38 PM
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Quote:
Originally Posted by KC8ESL View Post
I'd prefer a value expressed in dBm or uV. That's a useful value you can collect scientific data with.
The RSSI on the SDS100 is denominated in dBm.
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Old 06-14-2018, 6:29 PM
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This is great information so far.

Some of the newest entry level products (SDRPlay for example) feature a true reading calibrated RF meter, wherein the voltage at the antennae terminals is accurately reflected on the meter. So I'm hoping SDS can do this either now or with a future firmware upgrade.

Can anyone perform those basic measurements at 100 and 850 MHz?

***Also hoping that the UPMan will comment on the accuracy of the RSSI information as well.
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Old 06-15-2018, 5:37 PM
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Default can this?

Quote:
Originally Posted by jonwienke View Post
The RSSI on the SDS100 is denominated in dBm.
Jon based on what you have said I should be able to see higher RSSI readings in same channels same radio location and of course hear improvements with a better Antenna?

Can you message me in FB PM I want to talk to you on a few matters as I am as of Monday going towards GPS recieving and talk antennas!
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Old 06-15-2018, 6:38 PM
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Quote:
Originally Posted by jonwienke View Post
0dBm = 1mV signal level.

I doubt it's precisely calibrated, but it's a bit more meaningful than x signal bars.
So you injected a 1mV signal and the 100 showed 0 dBm? I'm guessing since what you injected is the same as what the 100 showed you must have accounted for cable loss, lots of people miss that step.

Did you try any other levels to see if it's consistent and did you try multiple bands?

Or were you just telling us 0 dBm is the same as 1mV?
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Old 06-15-2018, 6:44 PM
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Here is a great write up of RSSI values coming out of an SDR. Different chip, but should be very close to the same situation with the SDS100.


https://dsp.stackexchange.com/questi...ower-from-rssi


Marcus Miller is an undeniable genius for all things SDR. Here is text from the above, and nearly all his words and absolutely none of them mine:
Quote:
[The] Received Signal Strength [Indicator is] always relative to some signal model, incorporating considered bandwidth, assumptions on the modulation scheme, duration of transmission, generally: It's a estimation of received signal strength based on some property of the received signal.

Yes, I wrote that, and I still think it applies, so:

What is the difference between RSSI measurements and the dBm values that I record from the spectrum analyzer?

Well, the RSSI is calculated based on some signal-model based estimator, whereas the dBm value on your spectrum analyzer is calculated with another metric – namely, the power measured going through a filter with the filter bandwidth you configured at your spectrum analyzer.

These two might, or might not be related, taking into account that even a signal with very low power spectral density values might be sufficiently good for something that has e.g. matched filters to extract the signal from noise, or any other form of processing gain.

So, your only option is to understand both estimators, with the Spectrum Analyzer estimator being relatively simple, and the flex estimator being free and open source (so you can just read the code ), and relate the two; so basically, you'll have to find, given a (class of) signal(s) s(t), the functions fSA, fflex and finally g from

Pspectrum analyzer(s(t))Pflex frame sync RSSI(s(t))⇝P′flex(Pspectrum analyzer)=fSA(s(t))=fflex(s(t))=g(fSA(s(t))) .
I've just skimmed the liquiddsp code for the RSSI estimation, and the interesting part seems to happen in qdetector_cccf_execute_align.

However, this can never work without making a lot of assumptions. For example, if your signal happens to have a very narrow, but very strong single tone interferer, it will probably not break flex reception, and if that RSSI estimator is any good, not change RSSI very much, but it will drastically change the dBm value displayed by your spectrum analyzer. So if the question was

Can I find an estimator that always gives me the same reading as the liquidDSP flex RSSI based on the dBm value displayed by a spectrum analyzer?

then the answer is a

No. See above proof by counterexample (single tone interferer).

However:

If I cannot relate these two measurements by a correction factor, is there a way to obtain the receive signal power measurements (in dBm) programmatically (GNUradio perhaps)?

Sure. Why not? You got the raw samples, don't you? So do a magnitude squared of them, and find a linear correction factor. That will work (have done so multiple times), because now you're actually measuring to things that relate to each other (signal amplitude as seen by your SDR frontend's ADC and signal power as seen by the spectrum analyzer simply have a quadratic relation).

Note that you will have to re-calibrate for different frequencies, sampling rates, if settable, analog frontend widths (only applies to B2xx, E3xx, and a few very old daughterboards for your N2xx currently; by the way, the TVRX* boards have always-on AGC, so they can't be used for this).

The relationship between input voltage and digital number amplitude is linear for quite a large range for the Ettus frontends – however, as soon as your input amplifier and mixers start reaching saturation, this will no longer apply, so you did good to use a few dB of attenuation.

If you happen to have a WBX, SBX, CBX or UBX daughterboard, there's IP2/IP3 measurements (along with effective vs set gain) available that will help you estimate the region of linearity on files.ettus.com:/performance_data/.

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edited May 21 '16 at 14:03
answered May 19 '16 at 10:08

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Old 06-15-2018, 7:05 PM
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Quote:
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Or were you just telling us 0 dBm is the same as 1mV?
That is the definition of 0dBm. I did not say the SDS100's RSSI is precisely calibrated to that standard, I expressed doubt that was so. But even if there is a ±10dB inaccuracy in the SDS100's RSSI display, it is still a more precise and useful measurement of signal strength than "2 signal bars".
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Old 06-15-2018, 7:22 PM
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Quote:
Originally Posted by jonwienke View Post
That is the definition of 0dBm.
I know that, I was just wondering if you had actually tested or not, that's all.
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Old 06-15-2018, 10:10 PM
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And of course we all know that 0 dBm = 1 mW, not 1 mV. I failed to catch my mistake.
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Old 06-17-2018, 11:56 AM
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** "Different chip, but should be very close to the same situation with the SDS100"

This does lead directly to the question, whose SDR chip is utilized inside the SDS100?

Of course it is not Ettus Research chip; It would be nice to find out without having to physically look under the hood.

Also of interest, since this radio is SDR based, what is the current ability to view the spectrum using external software (Sentinal?) and are there any enhanced software capabilities for visual spectrum analysis to take full advantage of the real power of SDR being planned or mulled over?

It might be that with the announcement of a new SDR radio on the horizon from Whistler, Uniden can tap in to unleash the full power of SDR to differentiate SDS100 from its competitor via advanced software capabilities.--

--The also leads to the thought that since SDS100 is an SDR based receiver, Uniden might find a new and distinct customer base for the radio within the SDR community. A second market!

SDS100 had a unique "duality" here and is the first entrant
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