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563nb [2007/04/20 22:42] n5na |
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| Have you ever noticed how strong CW signals, >20db over S9, seem to clip? Yes? Well that's because the blanker has been suddenly activated. The gain of it's 2 stage I.F. amp suddenly jumps sky high and it produces a plus, opening the noise gate and clipping the CW. It's the sudden amplitude change that causes this. I have observed this on four Omni VI's. The work around is to kick on the -20db attenuator, turn down the RF gain or turn up the N.B. level to stabilize the blanker's reaction to the agc thus reducing the pulse level. This is fine but maybe you are trying to hear a weak signal and a 30db over 9 guy is 15khz up band banging away. Kicking on the attn. kills the weak guy and turning up the N.B. invites all kinds of unfriendly visitors to your passband. Somehow we must kill the blanker. The cure is easy: | Have you ever noticed how strong CW signals, >20db over S9, seem to clip? Yes? Well that's because the blanker has been suddenly activated. The gain of it's 2 stage I.F. amp suddenly jumps sky high and it produces a plus, opening the noise gate and clipping the CW. It's the sudden amplitude change that causes this. I have observed this on four Omni VI's. The work around is to kick on the -20db attenuator, turn down the RF gain or turn up the N.B. level to stabilize the blanker's reaction to the agc thus reducing the pulse level. This is fine but maybe you are trying to hear a weak signal and a 30db over 9 guy is 15khz up band banging away. Kicking on the attn. kills the weak guy and turning up the N.B. invites all kinds of unfriendly visitors to your passband. Somehow we must kill the blanker. The cure is easy: | ||
| - | Remove the top lid. Locate the Low Level Driver/N.B. Board. From the front of the rig, it's the left center board. Remove the board. Locate the 2 I.F. transformers. Next to each you will see a small IC. These are U1 and U2. Next to each of these you should see a 33k resistor. These are R39 ((Corrected from R30 to R39 in post dated March 12, 2005)) and R42. R39 connects to pin 5 of U1. R42 connects to pin 5 of U2. Orange, Orange, Orange is the color code. Remove them and replace them with 16K resistors. Reinstall the board. | + | Remove the top lid. Locate the Low Level Driver/N.B. Board. From the front of the rig, it's the left center board. Remove the board. Locate the 2 I.F. transformers. Next to each you will see a small IC. These are U1 and U2. Next to each of these you should see a 33k resistor. These are R39 ((Reference to R30 corrected to R39 in post dated March 12, 2005)) and R42. R39 connects to pin 5 of U1. R42 connects to pin 5 of U2. Orange, Orange, Orange is the color code. Remove them and replace them with 16K resistors. Reinstall the board. |
| After replacing these resistors and turning the N.B. level max counterclockwise, I was able to apply a needle pegging CW signal from another rig with no side effects. The resistor value was chosen by gradually decreasing the value so that the N.B. amplifiers had zero gain, even when presented with a strong signal. I then checked for proper N.B. operation. My friendly power company supplies me with a constant test signal on 10m. Rotating the N.B. control clockwise quickly and totally eliminated the power line noise. The N.B. knob needed to be advanced about 5 degrees more than before to overcome the extra bias I had created. Mission accomplished. | After replacing these resistors and turning the N.B. level max counterclockwise, I was able to apply a needle pegging CW signal from another rig with no side effects. The resistor value was chosen by gradually decreasing the value so that the N.B. amplifiers had zero gain, even when presented with a strong signal. I then checked for proper N.B. operation. My friendly power company supplies me with a constant test signal on 10m. Rotating the N.B. control clockwise quickly and totally eliminated the power line noise. The N.B. knob needed to be advanced about 5 degrees more than before to overcome the extra bias I had created. Mission accomplished. | ||
| I believe the typical lab test overlooks problems like this. Labs use steady state signals to measure IMD, not pulses and having a blanker run full time is certainly an invitation for Mr. IMD. | I believe the typical lab test overlooks problems like this. Labs use steady state signals to measure IMD, not pulses and having a blanker run full time is certainly an invitation for Mr. IMD. | ||
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| Steve Ellington N4LQ | Steve Ellington N4LQ | ||
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| + | ---- | ||
| + | **March 14, 2005** | ||
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| + | A couple of days ago, Steve, N4LQ, described a modification to the Omni VI noise blanker to allow it to be more completely turned off when the NB control is turned fully CCW. He changed the values of R39 and R42 from 33K to 16K. | ||
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| + | I just wanted to mention that I have incorporated this mod and it works for me. Thanks, Steve. | ||
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| + | 73, | ||
| + | Chuck NI0C | ||
