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Author Topic: Noise blanker intermod on IC-765: is there a fix?  (Read 8497 times)
W0BTU
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« Reply #15 on: February 04, 2011, 09:02:12 PM »

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, ...

Thanks, Chris.

Did you try this with noise present? The key is whether we can reduce a given level of noise in the presence of a certain level of nearby signals.
« Last Edit: February 04, 2011, 09:04:31 PM by W0BTU » Logged

W0BTU
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« Reply #16 on: February 04, 2011, 09:23:02 PM »

If the voltage applied to the stock NB diodes was wrong (so that they were in a "limbo" state, neither fully on nor fully off), could that contribute to intermod?   Undecided
« Last Edit: February 04, 2011, 09:30:55 PM by W0BTU » Logged

W0BTU
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« Reply #17 on: February 04, 2011, 09:40:31 PM »

... 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.

I must be going nuts. I do not understand this at all. I thought the diodes switched the blanker in or out depending on the voltage applied to the diodes.

I've got to find that NB schematic.
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W8JI
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« Reply #18 on: February 05, 2011, 04:58:21 AM »

If the voltage applied to the stock NB diodes was wrong (so that they were in a "limbo" state, neither fully on nor fully off), could that contribute to intermod?   Undecided

NO.

As I keep repeating, the signal path is through the same blanker switch diodes with the blanker ON or OFF. The bias on the diodes in the ON state, whether blanker is on or off, is exactly the same unless the trigger triggers the diodes. When the triggering system fires, the four diodes are reversed biased. It is a hard switch.

There are several strong clues that the diodes are NOT the real problem, and the real problem is accidentally getting corrected when the diodes are changed.

The clues are:

1.) Any PIN diode will work, even when the frequency range of the diode means it does not even act like a PIN.

2.) Bias currents are being readjusted.

3.) You don't have a problem with the blanker off, despite the bias and signal path being the same (only the switching system from the AM detector is off).

This should tell anyone the diodes are being blamed when they are really no part of the problem. If it was the diodes, or even diode bias, the problem would show equally when the NB was off or on.

If it a gate depth, waveform, or gate timing issue that issue certainly could be accidentally fixed by dicking around with anything that loads the switch that drives the diodes. When the load impedance on switch that reverses voltage on the diodes is changed, the slope and depth of reverse bias will change.

There are four diodes in a full wave center tapped switch.  This design would be a very low IMD switch regardless of the type of diode, provided the diodes are on. This is because one side would always be trying to stay on while the other side was trying to turn on. I've used similar switches before myself to reduce distortion.

73 Tom

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W0BTU
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« Reply #19 on: February 05, 2011, 07:22:44 AM »

If the voltage applied to the stock NB diodes was wrong (so that they were in a "limbo" state, neither fully on nor fully off), could that contribute to intermod?

NO.

As I keep repeating, the signal path is through the same blanker switch diodes with the blanker ON or OFF. The bias on the diodes in the ON state, whether blanker is on or off, is exactly the same unless the trigger triggers the diodes. When the triggering system fires, the four diodes are reversed biased. It is a hard switch.

Thanks. I didn't understand this. I thought the diodes switched the blanker in and out of the circuit. I still haven't found the schematic.
« Last Edit: February 05, 2011, 07:24:30 AM by W0BTU » Logged

N0SYA
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« Reply #20 on: February 05, 2011, 08:18:54 AM »

Mike, the 765 nb circuit looks the same as the nb in the 751, it's an h or balanced attenuator. Just look for the 4 diodes with a resistor being the crossbar in the H in the 9MHz IF.

By the way, another home brew rig in qex has info pertaining to nb diodes:

"Note thet I used 1N4148's for the gate diodes. I made comparative measurments between 1N4148's and HP 5082-2810 Hot Carrier Pin diodes. The differences between the two diode types in regards to loss, attentuation, switching time and Intermodualtion Distortion were so nominal that I feel it is not necssary to use the more expensive HP Pin diodes for this application. The measured loss through the gate is only 0.1 dB and when baised off, the attenuation is 78 dB. This equates to 13 S units (6 dB per S unit) of noise pulse attenuation!"

http://www.ve7ca.net/Hbr200.htm
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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 #21 on: February 05, 2011, 08:23:30 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, ...

Thanks, Chris.

Did you try this with noise present? The key is whether we can reduce a given level of noise in the presence of a certain level of nearby signals.

Yes, I tried it on the ambc band where I often have 30 over 9 noise and it got rid of most of the noise yet did not cause imd unless I set the control to over say 3/4 cw. I also employed it on hf in the swbc bands on am and ssb.
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N0SYA
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« Reply #22 on: February 05, 2011, 03:34:01 PM »

Just noticed that the diodes mentioned here: "Note thet I used 1N4148's for the gate diodes. I made comparative measurments between 1N4148's and HP 5082-2810 Hot Carrier Pin diodes." specifically the HP diodes, are not PIN types but passivated Schottky types, the wrong kind to use when you can employ PINs in a nb. These Schottkys would make great mixer or detector diodes but no improvement in a nb I bet.



http://www.datasheetcatalog.org/datasheet/hp/1N5712.pdf
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WA0ZZG
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« Reply #23 on: February 06, 2011, 02:45:29 PM »

Gentlemen;

One quick diversion back to the Collins 40MHz noise blanker.  It was designed for a mobile system where the AM whip was used as the noise pickup...
Check here for a noise blanker education:

http://home.earthlink.net/~christrask/Noise%20Blanker.pdf

Dave....
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W0BTU
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« Reply #24 on: February 06, 2011, 03:35:32 PM »

Thanks, but the Collins NB used a scheme similar to the MFJ-1025. It's not the same.
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W8JI
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« Reply #25 on: February 06, 2011, 04:51:46 PM »

Thanks, but the Collins NB used a scheme similar to the MFJ-1025. It's not the same.

The Collins blanker is not the same as the MFJ at all. The Collins blanker is a blanker, not a phasing system.

The problem with the Collins blanker is it receives noise on lower VHF. Blanking depends on the same pulses being present on the NB frequency as on HF. Good luck with that Collins idea on 160 meters, or for noise on any band that is not impulse noise from very local sources.    :-) It is probably darned good for car ignitions. It would not work at all for OTHR (woodpecker), QRN from thunderstorms more than a mile or two away, distant power lines, and so on.

It would be interesting to run a pulse generator into the ICOM and see what the problem really is.

Mike, the diodes are in circuit and on with the same "on" bias all the time. This is true with the blanker on or off. The signal path is always through the four diodes. When power level inside the roofing filter passband abruptly increases, with the blanker on, an AM detector switches a source on that back-biases all four diodes. The diodes stay off for the length of the AM detector output plus some small time delay.

The slope of the transitions between on and off and from off back to on as well as the threshold of the AM detector and the amount of bias current and voltage on the switch will determine the blanking, IMD, and mixing.

73 Tom
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N9MXY
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« Reply #26 on: February 06, 2011, 07:28:46 PM »

Wouldn't the biggest factor in the diode performance in a NB circuit be the gate response time?
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N0SYA
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« Reply #27 on: February 07, 2011, 07:14:02 AM »

It would be nice for someone who has the scope and other needed gear to actually test these circuits with pn and pin diodes, or to at least determine where and why the imd often comes with nb activation.
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W8JI
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« Reply #28 on: February 08, 2011, 07:33:07 AM »

It would be nice for someone who has the scope and other needed gear to actually test these circuits with pn and pin diodes, or to at least determine where and why the imd often comes with nb activation.

This isn't new stuff that needs investigating. I built my first noise blanker of similar design with vacuum tubes in the early 1970's. Early QST articles from the 60's had similar blankers.

The principle of all blankers like this is a noise pulse has wide bandwidth, while a signal is narrow. An AM receiver samples a wide bandwidth of the band, and detects the accumulation of voltages in the passband. A trigger system looks for sudden  changes in level and outputs a gate voltage that turns a switch in series with the normal IF path off.

One of the three major factors in modifying distortion is correctly stated here:

Quote
Posted by: N9MXY 
Wouldn't the biggest factor in the diode performance in a NB circuit be the gate response time? 


The only other major factors are the bandwidth of the AM detector system, and the trigger threshold.

In the 765 Mike reports no IM problems (that bother him) with the blanker off. Since the signal passes through the gate diodes or switch diodes with the blanker on or off, this quite obviously means the diodes themselves cannot be the root source of problems.

Changing the diodes and bias can only modify the timing and depth of the trigger. It will change that because the loading on the finite impedance of the trigger is changed, ESPECIALLY if diode bias is changed.

This means the change to PIN diodes (or any other type of diode) if it improves something is nothing but an accidental "cure". The very same change could be implemented by simply changing some resistors or capacitors in the trigger system.

Why surprises me is why mods are published, or why mods are made, without understanding how the system being modified works and understanding what the mod really does. Accidents rarely produce optimum results.

The reason the 765 is different than other radios can only be:

1.) The IF bandwidth in front of the AM detector system is different

2.) The gate timing or threshold is different

If it really was the diodes, the problem would be there with the blanker off since the normal signal path is through the diodes with the blanker on or off, and when the diodes are on and the signal is passing the forward bias is exactly the same with the blanker off or on. The only time the diodes change state is when the trigger flips the bias around.

73 Tom
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WA0ZZG
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« Reply #29 on: February 08, 2011, 08:45:55 AM »

It is probably darned good for car ignitions


Yep, that's what the Collins noise blanker was for.  The module was bolted to the lid of the KWM-2.


Dave...
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