what are the filters in sds100 scanners used for
SDS100/SDS200: sds 100 filters
- Thread starter joerobb23
- Start date
thank you much appreciated
tvengr
Well Known Member
If you are in Rockland County NY, I often have good results using the Wide Invert filter on sites with 700 MHz frequencies. Rockland County Public Safety Communications System (RCPSCS) Trunking System, Rockland County, New York
thank you
I_Am_Infinite
Member
Wide Invert works best on my SDS in my area also.
Ubbe
Member
It's a filter in the scanner, or actually two, that are 10MHz wide that let the RF signal pass to the rest of the receiver. When the filter are set to Off you are then receiving your monitored frequency in the middle of that 10MHz window.
As the SDS100/200 are super sensitive to be interfered by other frequencies within that 10MHz window there are different filter settings to move that window to the sides, in different directions and in different amounts of MHz.
With a Normal or Invert setting you are instead receiving your signal at the very edge of that window, the purpose being to try and block interfering frequencies that are outside of that window. So your selections will be to choose either the upper or lower frequency edge or go halfway out to the edge. So it all depends of where you have your local interference in your frequency band. It might be seen in the Waterfall spectrum display but usually its just a trial and error exercise to find what will give you the best continuos and steady reception.
The mistake from Uniden where to use a too poor receiver chip in the design and they tried to save the situation by letting the user solve it for them by these different filter options. Of course they could have chosen a much more narrow filter, maybe 0,1MHz instead of 10MHz but they cheap out and wanted to save some money as a 0,1MHz wide filter are way more expensive.
/Ubbe
Noob,
At this point you’re not explaining the SDS filter behavior, you’re forcing everything into a narrative that simply doesn’t survive contact with real-world results.
Your entire argument hinges on this idea that what we’re seeing is just classic front-end overload combined with a wide filter being “shifted” around to dodge interference. That might sound neat on paper, but it collapses the second you compare it to how these radios actually behave in the field.
If this were truly the overload scenario you keep insisting on, then the behavior would be consistent and predictable:
Instead, what users consistently see is the exact opposite:
Your “10 MHz filter shift” explanation doesn’t account for:
What’s actually happening is pretty obvious to anyone paying attention:
You’re not wrong that overload exists. You’re wrong that it explains this!
At this point you’re not explaining the SDS filter behavior, you’re forcing everything into a narrative that simply doesn’t survive contact with real-world results.
Your entire argument hinges on this idea that what we’re seeing is just classic front-end overload combined with a wide filter being “shifted” around to dodge interference. That might sound neat on paper, but it collapses the second you compare it to how these radios actually behave in the field.
If this were truly the overload scenario you keep insisting on, then the behavior would be consistent and predictable:
- attenuation would reliably improve decoding.
- “bad” signals would affect wide portions of the band, not isolated systems.
- filter changes would produce logical, repeatable outcomes.
Instead, what users consistently see is the exact opposite:
- one filter fixes a system completely while breaking another on the same band.
- sometimes OFF outperforms every filter.
- sometimes Invert variants outperform their non-invert counterparts for no “adjacent interference” reason.
- identical RF environments produce different results depending on modulation/system type.
Your “10 MHz filter shift” explanation doesn’t account for:
- why decode improves on clean, isolated systems with no adjacent interference
- why results change based on modulation (P25 vs DMR vs NXDN)
- why two sites on the same band behave completely differently
- or why users can degrade performance by adding attenuation in situations where overload should improve
What’s actually happening is pretty obvious to anyone paying attention:
- the filters are interacting with the SDR/DSP chain in a system-dependent way
- the impact is tied to signal structure, not just signal strength or proximity
- and there is no single “RF overload” explanation that fits all observed outcomes
You’re not wrong that overload exists. You’re wrong that it explains this!
Ubbe
Member
The 10MHz wide filter sit at the IF point in the SDR receiver, where the signal have been mixed to an IF frequency of 265MHz or 380MHz. One problem are that the SDR uses an "intelligent" attenuation called "smart power detector" to reduce its front-end gain whenever it sees a strong signal inside this 10MHz window, and that function cannot be disabled or changed, it uses a fixed setting within the chip.
This attenuation works fine when the SDR are used as a TV receiver or in a sat dish box where it always are a static signal level but fails when used with dynamic levels that constantly shifts, like in a scanner.
The chip manufacturer even states "To achieve world-class far-away-interference rejection performance" that indicates that it will have problems with nearby interference. It states that at the next channel, 6MHz away, it can handle an interference if it is more than 40dB lower in level than the monitored frequency. There's no specification if that interfering source are closer in frequency than 6MHz.
For its IP3 spec, where it starts to overload and produce a mix of different frequencies are -7dBm if the other frequency are more than 6MHz away. A good antenna amplifier have values like +40dBm, that's 50.000 times better. If the interference are in another frequency band, 36MHz away, its IP3 are 0dBm, almost equally bad at just a 7dB improvement. Keep in mind that Uniden uses a preamplifier in the scanner so actual values at the antenna input will be some 10dB worse.
"one filter fixes a system completely while breaking another on the same band."
Yes, as when the filter window shifts to one end it opens up the other end much wider to let other interfering signals pass within the filters 10MHz window. If you monitor with filter set to Off you pass frequencies +/-5MHz from the monitored one and a frequency 7MHz away are blocked. If using Normal or Invert you are blocking frequencies from passing the filter 1MHz away from your monitored frequency at one end but at the other end frequencies 9MHz away will instead go thru the filter and if that 7MHz interference are at the 9MHz end it will pass thru and interfere with reception.
"sometimes OFF outperforms every filter."
Filter settings are named inappropriately. It is always an IF filter in line and Off should be named Mid or IF shift Off. The Normal and Invert should be called IF shift + and IF shift -
/Ubbe
thank you very good information
Ubbe
Member
I believe you think that the SDR technology in Unidens SDS scanner could be compared to a real SDR receiver like Icoms R8600, but that are not the case as Uniden use a hybrid chip that are mostly analog and only digital at its output. If it where a true SDR receiver it wouldn't have the Function+7 for IF switch or use IF filters at 265MHz and 380MHz.
The block diagram of the receiver chip show how much analog it is, with a PLL to an oscillator signal being feed to the mixer section and to generate an I and a Q signal and the front end are a broadband amplifier with a weak tracking filter. A real SDR are converting the analog RF signal directly to digital data but those kind of hardware are too expensive to be used in a Uniden scanner.
/Ubbe
