SDS200: RSSI and noise levels

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Ubbe

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It works fine down to -100dBm, if that's the true value. The noise should preferably be below 500.

/Ubbe
 

kruser

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What's a good RSSI and noise level for VHF P25? I current have -75dBm RSSI.
As Ubbe said, that value should be fine.

On your detailed trunking display options, set one of the lower display fields to display D-ERROR. That's the one to watch on digital systems.
A value of zero (0) is best but even good quality signals will see that value bounce up into the 10's or 20's.
I forget how high it can go before audio really starts getting choppy but either way, adjust filters, antennas, IFX setting etc. for the lowest D-ERROR reading while they are talking on a talkgroup. It will also display a value when no chatter is on the system and only the control channel is being seen. You should also see the SYSID and RFSS ID as well as the Site ID when on a trunked system with no chatter while it is just watching the control channel. If any of them go away while it's sitting on an idle control channel, you may have reception problems.
 

radiopro52

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It works fine down to -100dBm, if that's the true value. The noise should preferably be below 500.

/Ubbe
I've been reading that anything below -80dBm is considered a very weak signal. -100dBm would be practically no signal. However my SDS200 shows any signal that reads -95dBm or higher as five bars of signal strength. I'm thinking that the signal strength readings on these scanners are very inaccurate and should probably be ignored, and one would be much better off by monitoring the D-ERROR as kruser has suggested.
 

kruser

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I've been reading that anything below -80dBm is considered a very weak signal. -100dBm would be practically no signal. However my SDS200 shows any signal that reads -95dBm or higher as five bars of signal strength. I'm thinking that the signal strength readings on these scanners are very inaccurate and should probably be ignored, and one would be much better off by monitoring the D-ERROR as kruser has suggested.
You are correct! The RSSI readings seem to be wrong more than they are right. Especially if you are an RF rich urban environment with tons of local transmitters nearby in the bands you would monitor. I've read from others that the readings behave fairly well in rural ares.
I find my SDS models will often decode many trunked 800 MHz systems even when the RSSI is showing -103 or so! Go figure.
In my area, even the weak signals I'm after will often show as -60 or -50dBm on the SDS displays. That's due to the 8 to 10 MHz wide TV tuner chip these models use. The available filters are intended to try and remedy this but in my case, they are still too wide to really work correctly.
With enough fiddling, I can usually make things work in the 7/800 MHz bands but the signals in the crowded 450 MHz band here can sometimes be impossible. We have a ton of DMR and NXDN signals in the 450 MHz UHF band now and they are all spaced so close together that the SDS models have a hard time with them. I'm surprised many commercial grade radios work as well as they do.

The funny thing is the VHF band is now pretty quiet here except for service specific users like railroad, marine and aircraft. Everyone jumped ship to 450 UHF and 7/800 UHF when the big digital push hit.
When our county installed a new 800 MHz system, one of the rules was that each municipalities had to cancel their old license and make the frequencies available to the pool again. Most of the users were VHF users and now those frequencies are quiet around here. I think we have some 90 Muni's with each having at least two frequencies they gave up. Several had many more than two. I also don't see many new licenses being granted on any of the old VHF freqs. Something must be planned for them but I've not heard what that may be.
The benefit to the scanner monitor was the fact that now you can pull in VHF stations from way over in Illinois or the very distant rural users in this state that are still on VHF.
 

Ubbe

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The SDS100 seems to have a sensitivity of 0,35uV, that's -116dBm.

If you read the link I posted you'll see how unreliable the RSSI works and how the RF AGC de-sense the receiver when a nearby frequency appears and at certain frequency distancies a carrier there could totally block reception.

/Ubbe
 
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