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Author Topic: IMD and roofing-filters  (Read 1353 times)
KM3K
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Posts: 285




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« on: July 15, 2013, 05:01:34 AM »

W2VJN has a book "Managing Interstation Interference"; 2nd edition.
On page 89, he has this sentence "Narrowing the roofing filter has no effect on widely spaced signals, as the intermodulation takes place earlier in the signal chain of the receiver."
I'm struggling with that sentence; as I see it, a narrower roofing-filter would have an effect because it would attenuate those widely spaced signals.
Hopefully, some one could explain to me the point the author is trying to make and that I missed.
73 Jerry-KM3K
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KA4POL
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Posts: 1959




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« Reply #1 on: July 15, 2013, 05:09:38 AM »

You are not in contradiction to what VJN is saying. Yes, you can get away from widely spaced signals. However, the bad signal is still also inside the pass band as it is an intermodulation between desired and undesired signals.
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G8YMW
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« Reply #2 on: July 15, 2013, 05:30:36 AM »

I think  that what is happening is the first mixer (or less likely the RF pre-amp) is being driven into overload by the strong out-of-passband signals giving an in-band product.
I have seen this in a Mutek Yaesu FT221. The station was half a mile away from Lincoln Cathedral. This is the location of the GB3LM repeater, British Telecom put a pager in the same site on ~153MHz.
All was OK until someone opened the repeater. It gave products all over 2 metres (59+20 in places)
Mutek modded on of their passband filters which IIRC sorted it
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G3RZP
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Posts: 4442




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« Reply #3 on: July 16, 2013, 02:59:02 AM »

The crystal filter can cause intermods. One famous UK manufacturer selected filters for IMD: the best ones were for receivers and the less good ones went into exciters. They also determined which way round gave the best performance. It is usually considered that a crystal filter is a passive, linear, reciprocal,  two port network. Passive and two port - yes. Reciprocal and linear - not necessarily, although it does depend on the performance you're looking for. Mechanical filters  manufactured before about 1975 or so were quite poor, too, but they improved after that.

Many years back, I measured a number of KVG XF9B  9 MHz SSB filters, and with signals at 10 and 20 kHz away from 9 MHz, the third order IP was around 15 to 20 dBm, depending on filter and whether the signals were 9010 and 9020 or 8980 and 8990.
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WA3SKN
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Posts: 5453




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« Reply #4 on: July 16, 2013, 04:40:27 AM »

Why not ask the author what he meant?
However, narrowing the filter will affect the close-in signals on the skirt.  Signals farther away would be affected by the maximum losses of the skirt... controlled by the number of poles, not the filter bandwidth.
73s.

-Mike.
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KM3K
Member

Posts: 285




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« Reply #5 on: July 16, 2013, 05:06:08 AM »

The crystal filter can cause intermods.
One famous UK manufacturer selected filters for IMD: the best ones were for receivers and the less good ones went into exciters.
They also determined which way round gave the best performance.
It is usually considered that a crystal filter is a passive, linear, reciprocal,  two port network.
Passive and two port - yes.
Reciprocal and linear - not necessarily, although it does depend on the performance you're looking for.
Mechanical filters manufactured before about 1975 or so were quite poor, too, but they improved after that.
Many years back, I measured a number of KVG XF9B  9 MHz SSB filters, and with signals at 10 and 20 kHz away from 9 MHz, the third order IP was around 15 to 20 dBm, depending on filter and whether the signals were 9010 and 9020 or 8980 and 8990.
Selecting filters for IMD is really not the way to go and is not the method used by me when I designed roofing-filters.
About KVG, we were the only importer in the USA for them during the 70s and 80s.

Why not ask the author what he meant?
Simple answer....He does not answer emails.
Signals farther away would be affected by the maximum losses of the skirt... controlled by the number of poles, not the filter bandwidth.
Actually, it is a function of both poles and bandwidth.
But too many poles and a narrower bandwidth cause havoc with the group-delay at the band-edges.
73 Jerry KM3K
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G3RZP
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Posts: 4442




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« Reply #6 on: July 16, 2013, 07:58:06 AM »

I was talking about the early 1970s, when the technology wasn't that good on high intermod filters.  In the 30 - 40 MHz region, they were looking for +30dBm TOIP in a 6 pole filter.

I was told that a resonant SAW filter can offer good IMD, because there's more quartz than in a conventional crystal filter. I don't know how true that is, though.
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AA4HA
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Posts: 1384




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« Reply #7 on: July 18, 2013, 06:12:18 PM »

To my understanding an intermod filter works best between the RF and IF or even in between the IF stages.

If you want to get closer to the source of the interference wouldn't you increase the Q of the tuned RF stages for a narrower bandwidth or add a pre-selector?
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G3RZP
Member

Posts: 4442




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« Reply #8 on: July 19, 2013, 03:14:57 PM »

Trish,

it depends  from where the IMD is occurring.

If one is talking of IMD from stations 200 or 300 kHz away, front end selectivity may help - depending on frequency and available working Q. In terms of the effects of wide band noise in the receive channel caused by multiple IMD effects of stations well separated from the receiver frequency, then front end selectivity really helps. There are a number of references in professional literature to this.  Close in -  say from signals 10 and 20 kHz away, obviously front end selectivity cannot do anything to help because the Q required is impossibly high. There is a limit to how far it is worth going in a receiver  in terms of intermodulation performance because it's easy to be limited by reciprocal mixing before IMD is a problem. See my articles in QEX and NCJ and the 1986 RF EXpo proceedings.
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