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Author Topic: Noise blanker intermod on IC-765: is there a fix?  (Read 8283 times)
W0BTU
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« on: January 27, 2011, 01:29:24 PM »

I started a thread on January 8 at http://forums.qrz.com/showthread.php?279566-Noise-blanker-intermod-on-IC-765-is-there-a-fix about this, and I thought I would share it here.

There were many responses to the thread, which is too long to post here.  I would appreciate any input the Icom experts here would have on this. :-)
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WB2WIK
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« Reply #1 on: January 27, 2011, 05:44:23 PM »

It's funny, I had an IC-765 and its NB worked about the same as most other rigs of the era (pre-DSP), i.e., it would create issues when strong signals were in the first IF passband, but otherwise did okay.

The only RX I have that does this much less than all the others is the NB-7 blanker in my TR-7.

That it has a roofing filter might be helping that out.

When I had the 765 I read a lot of stuff about improvements but don't recall seeing anything much about improving the NB performance: Assume you already checked all the Icom blogs, etc.
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W0BTU
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« Reply #2 on: January 28, 2011, 08:09:54 AM »

It's funny, I had an IC-765 and its NB worked about the same as most other rigs of the era (pre-DSP), i.e., it would create issues when strong signals were in the first IF passband, but otherwise did okay.

The only RX I have that does this much less than all the others is the NB-7 blanker in my TR-7.

That it has a roofing filter might be helping that out.

When I had the 765 I read a lot of stuff about improvements but don't recall seeing anything much about improving the NB performance: Assume you already checked all the Icom blogs, etc.

Hi Steve,

Yes, before I posted the QRZ thread, I checked all the Icom mods,  groups, etc. until I was sick of it.

I think what is in order next, is an A-B test: switching between my IC-765 and the IC-751A while playing with the blankers. (Then, swap my R-4C with the 751A & do the same comparison.) I thought I would post this here on eHam before I did.

Thanks for sharing your experience. Perhaps something is wrong with my 765's blanker.
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WA0ZZG
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« Reply #3 on: January 28, 2011, 04:37:35 PM »

Gentlemen;
All noise blankers in modern upconverting radios have this problem.  Consider the noise blanker as nothing but a small receiver that taps off right after the roofing filter.  No additional filtering added and using an AM detector.  Any strong signal that gets by the roofing filter will hammer it.  Only cure is a better roofing filter.
Collins Radio tried to beat this by using a seperate 40MHz receiver with its own antenna as a noise blanker.  Expensive, and required a second antenna, but did a good job.
Dave...
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W0BTU
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« Reply #4 on: January 28, 2011, 07:18:19 PM »

Dave,

I realize that. My point was: Compared to the noise blanker in my IC-751A or my R-4C, the intermod in the presence of strong signals using the IC-765's blanker is far worse.

When no strong signals are present, the IC-765's noise blanker works great.

I don't need to use the NB very often, but when you need it, you need it.
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N0SYA
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« Reply #5 on: January 29, 2011, 07:51:29 AM »

Gentlemen;
All noise blankers in modern upconverting radios have this problem.  Consider the noise blanker as nothing but a small receiver that taps off right after the roofing filter.  No additional filtering added and using an AM detector.  Any strong signal that gets by the roofing filter will hammer it.  Only cure is a better roofing filter.
Collins Radio tried to beat this by using a seperate 40MHz receiver with its own antenna as a noise blanker.  Expensive, and required a second antenna, but did a good job.
Dave...


Well that's neat and all but noise at 7MHz may not be the same or even present at 40MHz, or am I crazy?
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
W0BTU
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« Reply #6 on: January 29, 2011, 09:28:51 AM »

Chris, did you ever do any more experimenting with the NB PIN diodes?
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N0SYA
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« Reply #7 on: January 29, 2011, 11:20:04 AM »

Hiya Mikey

I have some BA282s installed in an R71 nb circuit but need a few more parts to make the 71 operational, they're on the way and I should have the rig together this coming week. I expect the same results I had with the 751. Doesn't mean that's what I'll get but my money is on sweet sweet victory. If you're impatient you could easily remove/replace the nb diodes in your 765, it'd take more time to disrobe the main board than to remove/install the diodes. If you have some diodes that is.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
W0BTU
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« Reply #8 on: January 29, 2011, 12:01:23 PM »

Not impatient at all.  Smiley
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N0SYA
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« Reply #9 on: February 04, 2011, 06:37:01 AM »

I should have an answer for you in an hour or so.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
N0SYA
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« Reply #10 on: February 04, 2011, 09:09:04 AM »

Ok so this is in the case of an Icom R-71A hf rx, with the 1SS53 stock PN junction noise blanker diodes being replaced with BA282 Siemens PIN diodes. What I find is that intermod is only apparent when the nb control is full clockwise, if there is a (or multiple) 60 over 9 sig you get imd only if you open the control full clockwise when the nb is enabled. Before the 1SS53s were replaced with the BA282s it didn't matter if the nb control was full clockwise or no - if you had enough enabled nb level to be effective you also got intermod, if there were plenty of high level sigs the stock diode nb would imd even at low nb control levels. So if you wanna try this, it should be an improvement over pn nb diodes. Please keep in mind that this is just my opinion from what I have seen.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
W0BTU
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« Reply #11 on: February 04, 2011, 01:46:13 PM »

Thank you, Chris!

I am way behind on work here, but I'm going to add those diodes to my shopping list.

FYI, here's what Tom had to say about this. http://forums2.qrz.com/showthread.php?279566-Noise-blanker-intermod-on-IC-765-is-there-a-fix&p=2157867#post2157867

I think I could have communicated my points in that thread a little better than I did, because it appears that Tom might think that I don't expect ANY intermod. (See http://forums2.qrz.com/showthread.php?279566-Noise-blanker-intermod-on-IC-765-is-there-a-fix&p=2158640#post2158640 .) Not so; my point is the difference between the way that the NB works in the IC-765 versus the other rigs. The 765's NB ought to be usable to a much greater degree than it is, that is, nearly as good as my other radios under the same conditions! I have been using noise blankers for years, and I know how the others worked.

I had the opportunity to do one A-B test (instantly switched between both radios on the same antenna) last night, since I was getting S9 line noise on my NW Beverage from about 2 miles away. Tuned to the same 160 meter frequency, the R4-C's NB had little intermod while the IC-765's NB had so much that I couldn't hear the station on that rig.

Before I make any mods though, I'm going to do some more A-B tests between the IC-751A and the IC-765.
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W8JI
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« Reply #12 on: February 04, 2011, 03:16:44 PM »

Mike,

Let me restate what I said. You said you had NO problem with the blanker off.

That closes the case totally on the diodes being an issue. Here is why:

The signal routes through the diodes when the blanker is off or on. The bias in the diodes is the SAME whether the blanker is off or on. Thus we can very safely conclude any notion the diodes are causing your problem is wrong, because the diodes in the "on" state between triggers of the blanker are biased exactly the same, blanker on or off.

Now we come to the part where some people report a diode change improves the blanker.

The only possible thing changing diodes can do is change the "off" threshold of the diodes. If the diode has a different turn-off point, it will be like reducing the blanker trigger point. In other words, it is possible different diodes can increase the trigger point and reduce "off" time. We could do the same thing by altering the slope of the pulse from the blanker switch, or even blanker threshold.

I've seen a lot of mods over the years that don't do anything like the assumption behind the mods dictates, but make a change based on accidentally changing something entirely different than what the basic idea behind the mod is.

A few others have pointed out the problem with blankers, and they are right. The blanker is a switch, just like a hard driven mixer is a switch. When the blanker is turned off or on, it totally drops the receiver off. If it turns on and off fast, it is very effective at sharply rising and falling pulses with a minimum possible effect on the desired signal. That effect is determined by the length of the noise pulse. Ideally we want minimum unnecessary off time (so we have maximum window to watch the signal), and the most rapid on and on transition time.

Anything we do is a compromise in some way. If we make the transition sharp, it can click. If we make the transition soft we lose blanking effectiveness because we have to turn off earlier and finish later which increases the window where the receiver is muted.

This is why the same blanker will get different reviews, based on how the user operates and what type of noise he has.

Since the 765 (at least to you) has no issues when off (I doubt this is totally true based on the circuitry), the diodes are absolutely excluded as being the root problem. The problem you notice cannot be caused by the diodes, it has to be a threshold and slope problem if it is really worse than any other blanker in the world. But like others say, it can't be a blanker without causing a problem when strong signals are in the passband of the roofing filters. This is because the receiver can never tell the difference between a good signal and a noise. Every blanker in the world sums ALL of the signals inside the roofing filter in an AM detector. The AM detector then feeds a triggering circuit that drives a switch that disables the IF path.

We can control the threshold where a trigger occurs at a certain total signal level threshold appearing inside the passband of filters, but that is all.

I'm not saying the diodes can't have an effect, but clearly the diodes are NOT the root cause of any problem with IMD that you observe. If they were the root cause, you would have the same problem with the diodes on or off.

My bet is when diodes are getting changed, people are accidentally "dicking around" with something else that sets the threshold or slope. They have a luck accident, setting the threshold or slope to something OK for them, and blame the diodes that clearly cannot be the problem you see.

This is like the nichrome parasitic fix, where someone has a bad tube or a bad component, fixes the bad component or bad tube at the same time the nichrome is added, and thinks the problem was cured by the nichrome. It's really very common in "home modifications" to not really improve things by what we think we did.

I wager we can get the very same improvement by changing the right resistor(s) or capacitor(s) and not messing with diodes at all. If someone is altering boas on the diodes, it is absolutely certain they are altering the threshold and perhaps even the slope of the trigger circuit, because the finite current and voltage of the trigger system drives the dc impedance of the diode switch system. That dc impedance is lowered by any increase in diode current.

When I fix a problem, I like to know what the problem is.

73 Tom
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N0SYA
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« Reply #13 on: February 04, 2011, 03:20:51 PM »

Mod one Icom and test it against an unmodded Icom and see what ya think. And I don't think it matters what exact PIN you use, just as long as it's a real PIN, that being said some PINs may be better than others for nb use, as you may recall I said my modded 751 could have the nb full clockwise and no imd with PINs. I used the 282s this time since I had 4 new ones, although I am going to try the hp3168s in the next R71. Also, no fair using the steam powered rig to compare to the solid staters, but you knew that. You can always reinstall the stock diodes if you don't like what results you get.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
W0BTU
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« Reply #14 on: February 04, 2011, 08:52:22 PM »

That closes the case totally on the diodes being an issue.

Did I say that I completely disagreed with that (even though I'm buying the diodes)? :-)

Anyway, I decided that I was going to compare the 751A and 765 NB schematics. But I had a frustrating time this evening trying to find the @!$&% NB circuit in the IC-765 service manual.  Angry  

Another day.

Thanks so much Tom for your input!
« Last Edit: February 04, 2011, 08:59:49 PM by W0BTU » Logged

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