SDS100/SDS200: Suddenly becoming less discriminating...

[Fubar]

Lately I've noticed the SDS100 is starting to hear adjacent signals it never did before. I've never enabled the attenuator before, but I've started to have to turn it on lately, which is a bummer because I listen to a lot of analog traffic.

For example a repeater that is 19.5 miles away from me transmitting on 147.120 is bleeding through on another repeater channel in the SDS100 on 147.380. I feel like a repeater 20 miles away shouldn't be overdriving the scanner to where it needs attenuation turned on. There's a fed frequency 163.9875 that likes to let a NOAA broadcast 162.550 bleed through for another example.

I'm just curious if this is a known problem with the SDS100 or if perhaps during one of the firmware updates something was changed and I didn't notice until now? Or if this is some sort of issue that the scanner has developed and is there a way to fix it?

Thanks everyone!

 

[RFI-EMI-GUY]

Do you have the waterfall software? Tune around to see if there are any strong signals in the band causing your receiver to overload, experience IM distortion. Also if you are charging the SDS100 while listening, USB chargers create high frequency noise which can create IMD effects. See if it happens on battery alone. Turn off any powered device nearby or connected to the radio.

 

[Trucker700]

It could be the result of high activity from the sun. On days with an extremely high MUF( Maximum Usable Frequency) VHF and even UHF signals can be heard further away than normal. As we approach Fall a phenomenon known as ducting can cause signals that you would normally not hear to appear for brief periods of time.
James

 

[Ubbe]

I'm just curious if this is a known problem with the SDS100

Yes. Any signal at -80dBm or stronger can turn up in your SDS100 at another frequency due to mixing products in its SDR receiver.

SDS100.mp4

drive.google.com

/Ubbe

 

[n1chu]

It could be the result of high activity from the sun. On days with an extremely high MUF( Maximum Usable Frequency) VHF and even UHF signals can be heard further away than normal. As we approach Fall a phenomenon known as ducting can cause signals that you would normally not hear to appear for brief periods of time.
James

You are referring to “band openings” in Amateur Radio (Ham) jargon or “skip” in Citizens Band jargon… both refer to atmospheric conditions that change depending upon the time of day, which is to say where the sun is in the sky in relation to you. It’s how hams talk around the world, commonly known as shortwave radio. Sunspot activity also affects the HF band and I’ve experienced band openings as high as the VHF band. That’s not to say the UHF band isn’t affected, I just never witnessed it. The point is these atmospheric conditions are less prevalent the higher you go in frequency. “Ducting” is another phenomenon which allows the weather to create conditions favorable to band openings. I’ve worked ducting when a weather front set up on the east coast, talking to ham repeaters from the Delaware Memorial Bridge (antenna site) to Cape Cod, with me in the middle (Ct.). The repeaters were all on the same frequency a BBB d any differing PL’s were not in play.

Some receivers (scanners) allow for an intermediate frequency to be changed. If you have the ability to change this give it a try.

 

[Ubbe]

Some receivers (scanners) allow for an intermediate frequency to be changed. If you have the ability to change this give it a try.

Additional to the IFX setting there's also the different filter settings that can be tried, Normal Invert and so on.

/Ubbe

 

[crippledchicken]

I remember years back, in my CB days the skip would run on for weeks sometimes.

 

[RFI-EMI-GUY]

Yes. Any signal at -80dBm or stronger can turn up in your SDS100 at another frequency due to mixing products in its SDR receiver.

SDS100.mp4

drive.google.com

/Ubbe

Yeah that is pretty bad when you consider that a typical commercial radio might have a 70 dB IM spec which would equate to say ~ -118 dBM sensitivity + 70 dB = -48 dBm signal required to mess with linearity. A good public safety radio might be 10 dB better. An analog scanner might have a threshold of -58 dBm.

 

[Ubbe]

Yeah that is pretty bad

SDS100 goes into heavy IM at -60dBm and a BCD436 gets some IM at -40dBm. The SDS100 desense 20dB if a -60dBm signal are sensed 100KHz from the monitored one.
SDS100 measurements

/Ubbe

 

[RFI-EMI-GUY]

It could be that poor receiver specs are a bigger problem that simulcast discrimination in urban environments. I have a somewhat low Q tuneable Coleman 800 MHz filter on my 536 and it seems to have settled it down quite a bit.

 

[Ubbe]

I have a somewhat low Q tuneable Coleman 800 MHz filter on my 536 and it seems to have settled it down quite a bit.

It's the total RF level of all frequencies that reach the receiver that could create problems. That Coleman are a bandpass, standard 5MHz wide, so you have their custom made wider bandwidth? But then you can only scan the 800MHz band, or do you use a diplexer/triplexer to split up to different frequency bands?

A BCD536 are using a bandpass that covers the whole 700-1300MHz range, the same for most of Unidens scanners, so that a lot of cellular signals goes thru its filter.

/Ubbe

 

[Fubar]

Thanks everyone for your replies, I sure appreciate it! I unfortunately do not have the waterfall software, but a new transmitter nearby would be certainly something to investigate. I will also do some testing with having the scanner running on battery versus plugged in, I don't recall a difference, but I wasn't looking either.

Perhaps I don't understand the physics of "band skip" but those signals have always been fairly weak when it happens and certainly the SDS100 shouldn't be overpowered by them. It's an interesting theory though, as the scanner did just fine until recently when I noticed interference being more of an issue. It could be something environmental.

 

[RFI-EMI-GUY]

It's the total RF level of all frequencies that reach the receiver that could create problems. That Coleman are a bandpass, standard 5MHz wide, so you have their custom made wider bandwidth? But then you can only scan the 800MHz band, or do you use a diplexer/triplexer to split up to different frequency bands?

A BCD536 are using a bandpass that covers the whole 700-1300MHz range, the same for most of Unidens scanners, so that a lot of cellular signals goes thru its filter.

/Ubbe

I don't know the 3 dB bandwidth of this Coleman it does have 5 MHz increments. It was apparently used as an IF or LO filter for some satellite or telemetry receiver so it is arbitrarily marked 990 to 1590 MHz yet actually tunes about 690 to 990 MHz. My 536 is used primarily to monitor local 800 MHz P25 so not a problem. I have a slew of other receivers.

 

[Fubar]

Did some more testing. Ensured the scanner was running on battery power and the bleed through still occurred both in the middle of the night and during the day (in case atmospheric conditions were a factor).

I just discovered though that attenuation is per-channel, so I've only had to deafen a couple of channels. I'll keep experimenting, hopefully this isn't a case of the scanner having a fault.