SDR# Filters ???

[jeatock]

In SDR# | Radio there are six types of filters in the pull down box, and a setting for filter order.

The resources that I have found seems to be from two diverse points of view: One says that SDR# has six types of filters (I can see that). The other simply states that x[n] over [fx] equals y0[n] over 2 times 4 over 6 times [fx] multiplied by the difference of the Superbowl scores is the answer to all my questions.

The EE416 and CALC210 classes were well over thirty years ago. There are two problems with growing older, but I can't recall what they are...

So here is my request: Is there anyone who would be so kind as to explain (simply. please!) what the differences in the filters are, and how the order value affects them? And (again, simply), how?

My end result is easy: I use SDR# and a RTL dongle to watch 2MHz of spectrum centered on 155 MHz, NFM. A two-brazillion watt paging transmitter eight blocks away on 158.1MHz is not a good neighbor to have. Turning the RF gain down flushes the baby out with the bathwater, so I am wondering if a change in filter setting might help. Physical RF filtering of that big sloppy transmitter is like trying to filter sunlight.

Taking that a step further, different modes (AM/USB/etc) and the performance of DSD is no doubt affected by filtering, but how?

Trial and error is frustrating.

 

[marksmith]

If these filters do anything other than narrow the bandwidth that you receive they probably won't help with adjacent interference.

BCD536HP/HP2/996XT/PSR800/396XT/996T

 

[jeatock]

Strong possibility, but will they make it worse, or the same, or degrade/improve the NFM signals I want? Bandwidth is a separate setting.

 

[jeatock]

Anyone?

 

[marksmith]

Personal opinion but I'd work with the narrower bandwidth and forget the filters. Filtering won't silence a big loud strong adjacent signal.

BCD536HP/HP2/996XT/PSR800/396XT/996T

 

[jeatock]

I do, and it helps. Lowering gain to 'just enough' for a known signal helps as well, but when hunting or researching it is too deaf.

This is now more of a curiosity- there are six filters put there for a reason, but what are they for, what is the difference between them and how does filter order change their operation?

 

[w4rez]

I seem to recall there being a notch filter plugin available for SDR#. I've never used it so I can't speak for it's effectiveness. Notching out that nasty pager might do the trick.

I guess I'm lucky. All the pagers around here seem to have moved to 931 MHz. They can stay there for all I care.

 

[jeatock]

Paging companies are traditionally bad neighbors. In the VHF-Marine band (even closer to 158.1 MHz) radios are useless in some metro areas. At my local 911 PSAP (a mile from the page transmitter) I have to install physical notch filters on single resource fixed receivers to keep my front ends from smoking.

Once upon a time a paging company shared a site with a public safety critical VHF repeater (with four big carefully tuned SInclair cans as a duplexer). The paging company slipped in one night and upped their horsepower to >300w (with a 6db antenna that gave them an ERP of 1,200w), but couldn't be bothered to put a notch filter on their transmitter. Every time they went in TX the public safety repeater would key up also. The PS manager (who owned the site) called them, and when he was told they would have a tech stop by in a month or so the site owner reached out and pulled their plug, evicting them on the spot and telling them to come get their equipment. I could only grin as I packed up my equipment.

They moved their transmitter a few miles away to the cable TV head end tower, and now when they key up they scramble QVC. Durn the luck.


My OP still stands: what is the difference between the six offered SDR# filter modes, and how does filter order change their functioning?


Anyone?

 

[DaveNF2G]

I cannot answer your question directly. I believe the "filters" are involved in processing the signal during the demodulation process, not directly at the point of reception. I suggest searching on the names of the various filter types in the SDR# menu for possible further clarification.

I also second the notch filter plugin suggestion. The one I have is very flexible. Totally eliminating the interference might not be possible, however. At least you can chop it down to something more tolerable.

I think it's high time the FCC started applying the same engineering standards to paging transmitters as they do to everything else. Pagers have always been exempted from mandatory improvements to spectrum occupancy inflicted on all other users. On the VHF and UHF bands at least, they are still allowed to use parameters that are two or three generations out of date.