SDS100/SDS200: Signal strength with open squelch.

[WD6ABC]

I noticed that in some parts of town, if I set squelch to 0 on a quiet channel, I get 5 bars of signal (-60db), and other areas maybe 1 bar (-110db). This is on Uhf channels (450-600mhz).
Doesn't happen on the 356HP scanner..
Any ideas?
John

 

[jonwienke]

It's normal for signal strength to fluctuate as you drive around. The 5-segment signal strength display on the x36 models is far less precise than the RSSI -dB reading. RSSI has a formally defined meaning, "signal bars" does not. There is no standard for defining what signal strength range equates to "three bars" or whatever. The SDS models give a more accurate measurement of signal strength than the x36 models.

 

[WD6ABC]

I noticed that in some parts of town, if I set squelch to 0 on a quiet channel, I get 5 bars of signal (-60db), and other areas maybe 1 bar (-110db). This is on Uhf channels (450-600mhz).
Doesn't happen on the 356HP scanner..
Any ideas?
John

I've been experimenting, and it looks like Filters setting has something to do with it.
Me - John

 

[WD6ABC]

It's normal for signal strength to fluctuate as you drive around. The 5-segment signal strength display on the x36 models is far less precise than the RSSI -dB reading. RSSI has a formally defined meaning, "signal bars" does not. There is no standard for defining what signal strength range equates to "three bars" or whatever. The SDS models give a more accurate measurement of signal strength than the x36 models.

So does it mean there is a constant signal @ -60 db into the scanner only when the squelch is open? If I close the squelch, the bars go await.

John

 

[jonwienke]

You're only going to see a RSSI reading in a frequency when squelch is open. When you close the squelch, the scanner is scanning.

 

[WD6ABC]

You're only going to see a RSSI reading in a frequency when squelch is open. When you close the squelch, the scanner is scanning.

I have the scanner on Hold. When Squelch is closed RSSI is zero (no bars) If I open squelch it goes to 5 bars (-60db). This only happens in certain areas of the city.

 

[jonwienke]

Hold or not, I don't think the scanner is intended to show signal strength when squelch is closed. Even if you're holding, the scanner can still be scanning. When you hold on a trunked system talkgroup, the scanner can still scan multiple frequencies on multiple sites.

 

[WD6ABC]

Hold or not, I don't think the scanner is intended to show signal strength when squelch is closed. Even if you're holding, the scanner can still be scanning. When you hold on a trunked system talkgroup, the scanner can still scan multiple frequencies on multiple sites.

Hold or not, I don't think the scanner is intended to show signal strength when squelch is closed. Even if you're holding, the scanner can still be scanning. When you hold on a trunked system talkgroup, the scanner can still scan multiple frequencies on multiple sites.

This is hold on a conventional channel (not trucked)
If you stop your scanner on a channel (no PL) and open squelch, do you get any signal bars? (while not active)

 

[kruser]

This is hold on a conventional channel (not trucked)
If you stop your scanner on a channel (no PL) and open squelch, do you get any signal bars? (while not active)

It's the design of the SDS models. They use a TV Tuner front end chip made by Rafael. I don't recall the chip number but there are threads about it here in the various Uniden forums.
Being designed for TV, the chip has a very wide front end. I'm told it is near 8 MHz wide.
The Rafael chip is not much different than what is in the cheap SDR sticks you can buy for under $20 bucks on eBay and Amazon.
At least the basis behind its operation is similar. The Rafael chip may be better quality, I don't know much more about it other than what I've read here.

Using different filters and sometimes the IFX setting can tame down the signal indications you see in many cases.
It's not ideal from what you may be used to but that's the way Uniden designed the things so they can do a decent job at LSM reception.

That -60 dBm reading is probably real but just from another signal within the tuner chips passband and not the frequency you are tuning.
Mine also does the same and does not show an active RSSI (or any signal bars) when the squelch is closed. That's a Uniden thing again as some radios will still show signal level even though it may be below the squelch threshold.

 

[Ubbe]

Most receiver displays do not show signal strenght when the squelch are closed as it will only distract. Most receivers also do not use signal level squelches but noise level squelches. It detects the high frequency component of the FM modulation at something like 4KHz-5Khz, outside of the speech band and allowed datamodulations frequency range. When a carrier are received, that 5KHz signal of noise are reduced and can be detected to open the squelch. In scanners the FM detector are used for squelch detect even when receiving in AM mode.

So there could be an interference on the frequency that makes the signal strenght indicate a strong signal but there's too much noise in that 5Khz frequency to make the squelch open, which is good. Plugins in SDR# for SDR receivers that offer a scanner and squelch function only use a level controlled squelch which makes it very difficult to use as the signal level varies depending of frequency.

SDS100/200 probably uses the AGC function to indicate signal strenght. The more signal the SDR chip receives the more the AGC tries to reduce gain to not overload the receiver and can be used to give the signal strenght, but as mentioned it is a widebanded receiver as it is designed for extra wide modulated digital broadcast signals and will react to signal levels from neighbouring channels.

/Ubbe