Suggestions on attenuating a specific frequency...

[zz0468]

This part of the description bothers me a bit "pass 158.700 rej 152.390"

It does not really tell me what the attenuation will be at let's say 154.000.

The cavity is a DB4140. I believe that the '-1A' part indicates that it was once part of a larger filter system, and the label reflects that. The two 'TEE' connectors on the top are also a clue. You would want to remove one, for your purposes.

It's a single cavity notch, and doesn't have band pass/band reject adjustments. I'm quite familiar with those particular cavities. It's a single notch, without a separately tunable pass frequency.

I wasn't able to find a data sheet on it showing the attenuation curves, but I was able to locate the tuning instructions:

http://www.repeater-builder.com/db/pdfs/db-4104-40-75-notch-filter-tuning-instructions.pdf

 

[gmclam]

The cavity is a DB4140. I believe that the '-1A' part indicates that it was once part of a larger filter system, and the label reflects that. The two 'TEE' connectors on the top are also a clue. You would want to remove one, for your purposes.

I saw the two Ts and noted they are SO-239 connections. No problem there.

It's a single cavity notch, and doesn't have band pass/band reject adjustments. I'm quite familiar with those particular cavities. It's a single notch, without a separately tunable pass frequency.

This looks to me like it is designed more for a 'transmission' site, rather than a receive site. The issue I am concerned with is how narrow the notch is.

I wasn't able to find a data sheet on it showing the attenuation curves, but I was able to locate the tuning instructions:

http://www.repeater-builder.com/db/pdfs/db-4104-40-75-notch-filter-tuning-instructions.pdf

I found that web site too, but didn't find the instructions, thanks. This is what I expect, but again I am still concerned about just how much bandwidth it attenuates. If it weren't for the text "pass 158.700 rej 152.390", I'd be a little less concerned.

 

[zz0468]

... If it weren't for the text "pass 158.700 rej 152.390", I'd be a little less concerned.

Yeah, without the original paperwork, it's difficult to tell from here. But in general, a single notch cavity is going to create a dent maybe about 250 KHz wide, and maybe 25 db deep. It'll taper off to lower loss as you get further from the center frequency, and the response is not going to be symmetrical. It'll be a bit lossier on one side of the notch than it is on the other, by maybe a couple of db - which you'll probably never notice.

It is what it is. For the type of requirements that you've spelled out, that's pretty much the best tool that physics have given us to deal with it. Anything less than a large cavity will have a wider notch. Larger diameter cavities can be a bit narrower. Setting the loops for minimum loss can create a more narrow, but shallower, response.

 

[gmclam]

I've done some searching on this topic, and the DB4140 cavity filter, and (lol) most of the search results led me back here. I realize that this is the type of filter necessary to do the job I need, but I also realize that all cavity filters are not created equal.

Yeah, without the original paperwork, it's difficult to tell from here. But in general, a single notch cavity is going to create a dent maybe about 250 KHz wide, and maybe 25 db deep. It'll taper off to lower loss as you get further from the center frequency, and the response is not going to be symmetrical. It'll be a bit lossier on one side of the notch than it is on the other, by maybe a couple of db - which you'll probably never notice.

The signal I want to filter is only 10dB hotter than the typical frequencies I scan, so I am willing to give up some depth in the notch to have less loss on adjacent desired frequencies. I can certainly set the notch center point to one side or the other of the undesired frequency, to minimize loss on a desired adjacent.

250kHz wide and 25dB deep would be PERFECT! I'll keep my eye open for one of these. Thank you very much for all your help.