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Author Topic: ALC and power  (Read 9159 times)
N7SMI
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« on: August 19, 2012, 06:24:59 PM »

I'm running a Signalink USB into an Icom 756 Pro 2. On digital modes, in order to avoid high levels of ALC, I have to turn the audio levels down so that the power meter shows only 10% (maybe 10 watts) output. My father-in-law also has a Signalink USB, an Icom 756 Pro 2, and the same antenna I have, yet he can crank up the audio levels to show 100% power and still has 0 ALC.

What would cause such a significant difference between identical systems? Is there something wrong with my rig? Is there any way (short of getting an amplifier) to increase power while minimizing ALC ?
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N4CR
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« Reply #1 on: August 19, 2012, 09:53:15 PM »

Is your RF power set to 100%?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K0YQ
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« Reply #2 on: August 20, 2012, 05:43:32 AM »

How about the line out level on your computer?  Is that okay?
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K0YQ
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« Reply #3 on: August 20, 2012, 07:09:43 AM »

Also, is the speech compressor off on the rig when using data modes?  Not sure if you're using the rear acc data connection and ssb-d mode or regular ssb mode via the front mic jack?
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N7SMI
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« Reply #4 on: August 20, 2012, 07:56:28 AM »

Quote
Is your RF power set to 100%?

Yes, on both rigs.

Quote
How about the line out level on your computer?  Is that okay?

Both computers are set to 100% for speakers and 50% for the application (both DM780) per the Signalink instructions.

Quote
Also, is the speech compressor off on the rig when using data modes?

I'm using USB-D mode via the rear acc data connection, so the compressor is always off in that mode (I believe).


I just tested again to verify these settings, and in order to get 0 ALC on my rig (no green on the meter), I have to turn the TX knob down so far that the power meter shows about 2% (maybe a couple watts at most) output. In regular SSB, I regularly show peaks at 100% plus power with nominal ALC - as is normal. I made many contacts this weekend in RTTY contest with it cranked up a bit (ALC and Power both at about 10%), but I'd really like more than 10 watts for working DX digitally.

Any thoughts?
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W8JI
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« Reply #5 on: August 20, 2012, 08:18:51 AM »

It might help if we understand how ALC works.

ALC compares a reference DC voltage in the radio to the directional coupler voltage on the radio RF output.

If the voltage from the RF power detector on the radio output exceed the internal threshold, ALC voltage is developed. This voltage shows on the ALC meter in nearly all radios as "ALC".

When we think about it this way, it is easy to understand that too much gain or level anyplace in the system before the final PA stage causes the comparator to develop ALC voltage. Too little power setting also increases indicated ALC, because the power setting is what determines the reference threshold for ALC activation.  SWR is usually included in the ALC system by looking at the directional coupler reverse power.

This means ALC is generated by too much of any of these:

Audio input level
Audio stage gain
transmitter IF stage gain
transmitter RF gain
reflected power on TX antenna port

It is also generated when power level threshold is set too low for any of the above, because power is reduced by decreasing ALC threshold.

Some radios have an internal TX IF gain control that can be programmed, but the most likely guess is you are overdriving the radio's audio input line from the computer.  My **GUESS** is signal link is giving you bad advice on setting levels, or you are not reading the manual completely. I would either add an attenuator pad on the audio line or turn the gain down on the computer. Matching radio and input audio levels is important.
 

 
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K0YQ
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« Reply #6 on: August 20, 2012, 08:35:15 AM »

Yes, it sounds like your computer line out level is too high into the Signalink.  Reduce that line out level and bet you'll be okay.
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N7SMI
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« Reply #7 on: August 20, 2012, 08:45:03 AM »

Thank you for the explanation of ALC. I understand this, but cannot accept that it's normal to be limited to 2 watts output on a 100 watt rig to avoid all ALC.

I have tried about every possible combination of computer gain and Signalink TX gain. I've tried multiple computers. No matter what the audio settings, the most output I can get is a couple watts before I start getting ALC.

The only other thing on your list I can control is reflected power on TX antenna port, and I get the same results with 2 antennas (one of which is the same as the other rig with no ALC issues) with no indication of reflected RF, so I don't suspect issues there.

I just watched several YouTube videos of different Icom rigs in digital modes and they ALL showed significant ALC (to about the 50% mark, or just short of where it drifts into the voice distortion zone). Perhaps I just need to abandon the idea of ZERO ALC and just keep it within acceptable ranges. Still, even with ALC in the red, I still can only get about 15% output.

Any other suggestions?
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K0YQ
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« Reply #8 on: August 20, 2012, 09:09:18 AM »

With correctly set audio levels I can run AFSK RTTY at 100 watts on my 756Pro3 with zero alc indication.  Same with psk at 50+ watts.
   
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AA4PB
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« Reply #9 on: August 20, 2012, 09:52:18 AM »

I use a SignaLink-USB and an IC756PRO. On PSK31 I can get up to 75W output on the average reading power meter before I start to see any ALC indication. On a tune signal (steady carrier, no modulation) I can get nearly 100W before seeing an ALC indication.

I'd first put the radio on a dummy load to make sure high SWR is not affecting the ALC. Make sure that the RF power control is fully clockwise for maximum power output. Use the SignaLink Tx control to adjust the power output.

If you are on a dummy load, have the tx power control set fully clockwise, have the compressor turned off (which should be automatic in USB-D mode) and still get an ALC indication at only 10W output then you probably have a radio problem.
« Last Edit: August 20, 2012, 09:57:28 AM by AA4PB » Logged
W8JI
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« Reply #10 on: August 20, 2012, 01:08:16 PM »

Thank you for the explanation of ALC. I understand this, but cannot accept that it's normal to be limited to 2 watts output on a 100 watt rig to avoid all ALC.

It is not normal to have ALC at a few watts if the drive levels are correct.

Did you check the output on another receiver for hum or noise?

I see it has isolation transformers, which is great. But if a jumper is in the wrong spot you can still get a ground loop.

Maybe you can borrow another 756 rig and see if a different rig has the problem. It is not normal for ALC to be present unless drive levels are high.


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N7SMI
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« Reply #11 on: August 20, 2012, 01:27:48 PM »

Did you check the output on another receiver for hum or noise?

When I monitor the output, it seems and sounds fine. I've made 100's of digital contacts with my few watts and always get excellent signal reports (even when I push it and have higher ALC).

Quote
Maybe you can borrow another 756 rig and see if a different rig has the problem. It is not normal for ALC to be present unless drive levels are high.

I may have to try this. I'm going to try the dummy load first and isolate any possible antenna reflected power issues.

My next experiment will be if I have the same ALC issues when in USB and not USB-D modes. If so, this should tell me if it's an audio drive level (or perhaps Signalink or ACC port) issue because ALC and power currently work fine in standard USB/LSB mode with voice via my microphone.

Thank you all for your help. I'll do some experimenting and report back. I've very open to any other ideas.
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N4CR
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« Reply #12 on: August 20, 2012, 03:50:37 PM »

First, try your SignaLink USB on his rig to eliminate that.

Then try your rig on his setup to further divide where the problem is.

You need to understand which component is causing the problems.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N7SMI
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Posts: 341




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« Reply #13 on: August 20, 2012, 08:10:13 PM »

I just did some more testing and now see I'm only getting about 20% max output in all modes - even built-in RTTY, FM, and when tuning. I'll try it on a dummy load (as soon as I track one down), but barring a miracle, it seems to be an issue with the finals or something else internal.

Thank you all for your help.
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AA4PB
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« Reply #14 on: August 21, 2012, 05:37:33 AM »

I can pretty much assure you that it is not a SignaLink problem. If there were hum, noise generated by the SignaLink, or too much drive from the SignaLink then the symptom would be that you couldn't decrease the power output even when turning the Tx level all the way down on the SignaLink. Since your problem is that you have too much ALC with low power output that indicates a transmitter problem to me. There is also a possibility that the wattmeter is wrong. If you go key down on CW and get significantly less than 100W output then check the current draw from the power supply. If it's close to 20A then the wattmeter is suspect.

As I said, the first thing to do is to get it on a dummy load in order to eliminate variables in the antenna like high SWR. I'm thinking that if the load impedance were very high then it may be possible to generate too much ALC voltage at low power levels. You might also try working the RF power control back and forth a few times to wipe the contacts clean. A poor connection in this control could cause it to be at a very low level even though it is physically set fully clockwise. You might try turning it while transmitting to see what the ALC reading does.

If the youtube pictures are showing ALC with PSK31 then they are wrong! You don't want to have any ALC while running PSK31 or any digital mode that has an amplitude component. Most ALC will compress the peaks of the signal which causes distortion and poor IMD. Constant output modes like RTTY (only frequency shifts, no amplitude changes) will be unaffected by ALC.
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