CW RX Filter not centered

Randy Hollingsworth

In the past, my cw filters were always set to the center of the passband. My cw filters now are not centered on the filter bandwidth. If I try to adjust the filter to center it, the signal is only heard at the upper end of the filter passband on the wider filters, and not at all on the two narrowest passbands. The USB and LSB filters seem to be OK. Has any one else seen this and is there a setting that I have somehow **** up or overlooked that is causing this?

I am running a FLEX 6500 with the latest radio software and the latest version of SmartSDR (3.8.21).

KD0RC

Hi Randy, can you post a screen shot of SmartSDR? It would help if the panadapter is zoomed very far in so that the filter is visible on the screen. Also open the mode part of the flag so that we can see the selected filter (see example, below). Please post the whole SmartSDR screen.

Randy Hollingsworth

Sure …here is the screenshot as requested:

KD0RC

Hi Randy, can you post the whole SmartSDR screen?

I suspect that your offset is something other than zero:

Randy Hollingsworth

Sure …here is the screenshot as requested:

KD0RC

Hi Randy, I need the entire SmartSDR screen, not the small snippet that I did as an example to show the zoom.

Randy Hollingsworth

Here is a second shot of the 400 Hz filter and the filter adjustment module:

I have tried adjusting the filter characteristics, but cannot get it back to center, and even when close, the signal still only responds near the tight edge of the passband.

Randy Hollingsworth

should say "……right edge" of the passband"

KD0RC

OK, the problem is that your filter is offset by 100 HZ. Re-select the active filter (or click on any filter choice) and it will re-center the filter.

Randy Hollingsworth

Except it doesn't…when I click on the active filter (or any other filter) it does not reset to zero

KD0RC

That's odd… It works for me. The other way to re-center the filter is to hover the mouse over the filter diagram in the control panel (yellow circle around the "100" above) and drag the filter until it changes from 100 to zero. It is kind of touchy doing it that way, but that should work. I'm not sure if you can click it to re-center. I will try that tomorrow when I have the radio on.

Randy Hollingsworth

I have tried the alternate process of dragging the filter till it reads zero, but I can not get it on zero, it jumps from a number above zero to one below. I tried clicking it as well, no change. It's puzzling, because it was doing this on my older version of SmartSDR, and continues doing it after I updated the software to the latest version. If my SmartSDR software was corrupted, I would have expected loading the new version would have fixed it. Hoping it is not a hardware issue.

Craig_KØCF

The problem is that "Pitch" is set to 100, which places the carrier frequency 100 Hz from the lower edge of the passband. Change the pitch to a reasonable audible frequency. I use 550 Hz, but that is a personal preference.

73, Craig KØCF

KD0RC

Good job Craig, I never knew that there was a relationship of filter offset and sidetone pitch!

I did some fooling around and think that I understand the relationship now. Sidetone pitch has to be at least half the value of the filter width for the filter to automatically center (zero offset). The difference between the pitch and (filter width / 2) is the offset that will be applied. You can still manually drag the filter edges or the offset to get to zero offset, but it is a bit fiddly. The offset direction is based on whether you have selected upper or lower CW sideband (CWU or CWL in the Phone/CW tab of Radio Setup).

If you set a 400 Hz sidetone pitch, the 1.0K and 3.0K filter settings will have a 100 HZ and 1100 HZ shift, respectively. If you have a 500 Hz sidetone, the 3.0K filter will have a 1000 Hz shift. For Randy's situation, a 100 Hz pitch will only auto center the filter when using the 50 or 100 Hz filter widths.

So Craig's solution is best for most situations, and is the reason that I never tripped over it. I like an 800 Hz sidetone, so only the 3.0K filter setting is offset for me. I have noticed that offset, but I thought it was a quirk of the 3.0K filter. Because I tend to use pretty narrow CW filtering, I never gave it much thought.

I don't know why this is implemented this way. Maybe someone has some insight as to why you might want this.

Randy Hollingsworth

Thanks to both Len & Craig for their assistance. I will give Craig's suggestion a try and see how it works. A couple of questions. First, is their a way to set the filter center other than using the slider? A numerical entry would be a lot easier. Second, is the sidetone/filter center relationship documented anywhere in FLEX manuals, videos or white papers? Sure seems like it should be.

I will report on the results after I give it a try.

KD0RC

Hi Randy, that is the only way that I know, unless you have a physical control mapped to the offset function. I think the Maestro and possibly the Flex Control can adjust it. If you use a DJ box, you would be able to use NodeRed. My TeensyMaestro has a control for it.

I have not seen this documented anywhere, and was unaware of it until Craig's post. I think that most keep their sidetone pitch at 600 or above, so it hasn't had any conversation around it until now.

Randy Hollingsworth

OK, more confused than ever. First, I loaded a new copy of SmartSDR 3.8.21 onto a different PC using my same FLEX-6500, and all of the the CW filters are set at zero, except for the 3.0 K filter, just as Len had observed. On this copy of the software, with the filter set at zero, when I center the cw signal at the vertical line in the passband, I get a tone of about 600 Hz.

Going back to my original PC and software copy, when I try to set the "pitch" to about 600 the signal winds up outside the filter bandwidth for the all of the filters at 500 Hz and below. With the filter set to as close to zero as I can get, the signal is still offset from the vertical line and I have to shift the frequency away from the line to receive the signal, and the tone is different depending on the filter used.

At this point, I think I need to send in a trouble ticket to FLEX, not sure what else to try.

73……Randy, K0FA

KD0RC

Hi Randy, on the original PC, what do you have the CW Pitch set to (not the pitch that you hear, but the pitch setting in the control panel)? It sounds like it is still set at 100 Hz.

Randy Hollingsworth

It helps to realize that the Pitch Setting is different than the midpoint setting on the filter - DUH! Once I realized/remembered that there was a Pitch setting on the console, I checked and as Craig & Len suspected it was set to 100. I reset it to 600 HZ and everything is back to normal :) Thanks again to both Len and Craig for correctly diagnosing the problem and helping me finally understand what needed to be done.

KD0RC

Glad you got it straightened out Randy. I learned something new as well!