roofing filter questions
Locked
 
|
Hi,
I'm looking for articles on how the new roofing filter designs work. I've seen the PowerPoint on Rob Sherwood's site, but it doesn't answer all my questions. Questions came up in a discussion with a friend of mine who is considering the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates almost 100% CW, and doesn't hear any IMD products. His beef is noise from stations that are just outside the passband of his cascaded IF crystal filters. He's hoping the roofing filter will increase the selectivity of the radio. My sense is that it will help, but not by providing additional selectivity. My understanding is that it will improve the dynamic range within the passband by keeping the hardware AGC in the downstream IF stages from being triggered by loud adjacent signals. Is this the correct way to think about it? My friend maintains that the roofing filter would provide selectivity as well, but my sense is that unless it's narrower and/or has steeper skirts than the IF filters, it won't. I believe the Kenwood roofing filter mod is only on the order of 2K-3K wide, so the selectivity shouldn't improve. Even if the roofing filter was the same or narrower than his IF filters, would it improve selectivity? Is there an analogy with cascaded IF crystals? Typically, a filter in the final IF stage (e.g., in the 450KHz range) outperforms a filter in an earlier stage (e.g in the 8 MHz range.) Is this because it's easier to make better filters at lower frequencies, or because it's more effective to filter after any amplification by previous IF stages? Obviously, I don't know squat about this. 73, Dick WC1M _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
|
|
Tell him he should buy a K-3.
---- Dick Green WC1M <[hidden email]> wrote: > Hi, > > > > I'm looking for articles on how the new roofing filter designs work. I've > seen the PowerPoint on Rob Sherwood's site, but it doesn't answer all my > questions. > > > > Questions came up in a discussion with a friend of mine who is considering > the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates almost > 100% CW, and doesn't hear any IMD products. His beef is noise from stations > that are just outside the passband of his cascaded IF crystal filters. He's > hoping the roofing filter will increase the selectivity of the radio. > > > > My sense is that it will help, but not by providing additional selectivity. > My understanding is that it will improve the dynamic range within the > passband by keeping the hardware AGC in the downstream IF stages from being > triggered by loud adjacent signals. Is this the correct way to think about > it? > > > > My friend maintains that the roofing filter would provide selectivity as > well, but my sense is that unless it's narrower and/or has steeper skirts > than the IF filters, it won't. I believe the Kenwood roofing filter mod is > only on the order of 2K-3K wide, so the selectivity shouldn't improve. > > > > Even if the roofing filter was the same or narrower than his IF filters, > would it improve selectivity? Is there an analogy with cascaded IF crystals? > Typically, a filter in the final IF stage (e.g., in the 450KHz range) > outperforms a filter in an earlier stage (e.g in the 8 MHz range.) Is this > because it's easier to make better filters at lower frequencies, or because > it's more effective to filter after any amplification by previous IF stages? > > > > Obviously, I don't know squat about this. > > > > 73, Dick WC1M > > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
|
|
In reply to this post by Dick Green WC1M
> > > Questions came up in a discussion with a friend of mine who is considering > the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates almost > 100% CW, and doesn't hear any IMD products. His beef is noise from stations > that are just outside the passband of his cascaded IF crystal filters. He's > hoping the roofing filter will increase the selectivity of the radio. > > > > > 73, Dick WC1M > > > > The question is the source of this noise - it could well be reciprocal mixing from the 1st LO in which case no post-mixer filtering is going to fix the problem. This is exactly how reciprocal mixing appears to the user. Until the noise origin is pinned down, it's not possible to state with certainty that a narrower high IF filter will do anything. Jack K8ZOA www.cliftonlaboratories.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
|
|
In reply to this post by Dick Green WC1M
From Inrad regarding the 950 roofing filter: http://www.inrad.net/product.php?productid=88 ######################################################### The TS-950 roofing filter mod consists of a 6-pole, 4 to 5 kHz wide filter followed by a high dynamic range feedback amplifier. The amplifier provides enough gain to override the filter insertion loss. The IMD dynamic range will be improved 5 to 15 dB for some signal spacings. Also, the blocking dynamic range will be improved up to 10 dB for close in signals. The roofing filter mod inserts a narrow band crystal filter between the RF assembly and the IF board. Reducing the bandwidth at this point in the radio helps to keep strong off-frequency signals out of the second mixer, where they can cause intermodulation. Transmission is not changed, as it does not pass through the roofing filter. ############################################### If he's being bothered by signals "just outside the passband of his cascaded IF crystal filters" (presumably 500 Hz each), a 4-5 kHz roofing filter won't help at all since the roofing filter is ~10X the bandwidth of his IF filters and will continue to pass the signals bothering him. If he had a narrow roofing filter like the K3 (200-500 Hz), that would help IMD (intermod) and BDR (desense) issues but would *not* reduce transmitted issues from a dirty interfering TX signal. Jack K8ZOA is right that it depends on the exact nature of the interference (i.e. is it transmitted [phase noise or key clicks] or internal to the receiver [IMD, BDR or LO phase noise]?) 73, Bill W4ZV |
|
|
In reply to this post by Dick Green WC1M
Dick
The conditions you describe are convincing evidence most if not all his noise is internally generated in the 950. As Jack points out, Where is the question, not Whether. Since it is very unlikely the noise comes from outside the radio, no roofing filter will eliminate it. What a narrower roofing filter might possibly do is reduce the distance (in Hz) between the current dial freq and a station which causes unwanted interference in the radio. Then again, if the internal interference is generated in front of the filter, a narrower filter may not help a bit. More testing required. In all probability the fellow who suggested replacing the 950 has a good point. Its front end is similar to, and a good deal older than, many made today that suffer similar problems in the face of strong nearby (but outside the filter) signals. While the numbers posted at sites like ARRL and Sherwood Engineering tell the technical story well, they don't address what defects actually sound like, such as (probably) your 950. I too operate almost 100% CW. A case in point is my two rigs, Icom IC-765 and TenTec Orion. Both delightful rigs but for different reasons. The (upconverted) IC-765 has the IR narrow roofing filter 4KHz. Strong signals inside the roofing filter, but outside of the 2nd IF filter produce interference products that I bet sound similar to or exactly like your 950. The Orion produces no such products regardless of filter. BTW my shameless plug, the Orion is for sale because I have a K3 on order, ad on eham.net. 73 Art k6xt at arrl dot net Message: 2 Date: Tue, 13 May 2008 13:18:40 -0400 From: Jack Smith <[hidden email]> Subject: Re: [Elecraft] roofing filter questions > > > > Questions came up in a discussion with a friend of mine who is considering > > the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates almost > > 100% CW, and doesn't hear any IMD products. His beef is noise from stations > > that are just outside the passband of his cascaded IF crystal filters. He's > > hoping the roofing filter will increase the selectivity of the radio. > > > > > > > > > > 73, Dick WC1M > > > > > > > > Dick: The question is the source of this noise - it could well be reciprocal mixing from the 1st LO in which case no post-mixer filtering is going to fix the problem. This is exactly how reciprocal mixing appears to the user. Until the noise origin is pinned down, it's not possible to state with certainty that a narrower high IF filter will do anything. Jack K8ZOA www.cliftonlaboratories.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
«
Return to Elecraft
|
1 view|%1 views
| Free forum by Nabble | Edit this page |