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Author Topic: Excited about digital HF modes - how to monitor transmission?  (Read 2117 times)
KC9QQM
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Posts: 207




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« on: September 25, 2017, 12:40:44 PM »

I am having a lot of fun on Hf digital on the various modes but I am concerned that I am not over driving but not under driving the output. While I have made a number contacts here and abroad, what is a good way to monitor my transmission for modulation so there is no platter or anything?

I run the computer to a RigBlaster Advanatge to a ICOM 706MKii. I have the computer level controls at 50 and the physical controls on the RigBlaster for both transmit and receive at about 10 o'clock.

Jeff
KC9QQM
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KC4ZGP
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Posts: 1637




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« Reply #1 on: September 25, 2017, 01:41:11 PM »


Jeff,

FL Digi, PSK-31, Kenwood TS-450S.

I feed my computer's speaker output into the transceiver's microphone input.

I set the transceiver's VOX control to 'on'.

I make the computer send something.

I increase the MIC gain just until the ALC indicator begins to deflect. Any higher and splatter occurs.

I listen to my transmit signal on another radio.

Collapse the antenna and set the receiver's RF gain to minimum so you don't to overload the receiver.

Kraus

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K0UA
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Posts: 1368




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« Reply #2 on: September 25, 2017, 01:45:41 PM »

I am having a lot of fun on Hf digital on the various modes but I am concerned that I am not over driving but not under driving the output. While I have made a number contacts here and abroad, what is a good way to monitor my transmission for modulation so there is no platter or anything?

I run the computer to a RigBlaster Advanatge to a ICOM 706MKii. I have the computer level controls at 50 and the physical controls on the RigBlaster for both transmit and receive at about 10 o'clock.

Jeff
KC9QQM

Make sure there is no ALC showing on the ALC meter.  Run your audio drive up until the ALC shows some indication then start backing off until it just disappears.  You can also listen to your audio with the monitor function of your radio or a second receiver.  On my 7300 with its built in soundcard, I run the transmit level at 23% and the receive audio at 11%  But that has nothing to do with your rig.  On WSJTx I want to be able to run the transmit audio out control at wide open and still just have no ALC showing on the rig.  This is the 23 percent reading.  If I set it to 25 percent the ALC meter will start to move.  You have by now probably seen some overdriven FT8 signals.  Some are really really nasty.

With my audio out slider control on the WSJTx program wide open, I can run just about 100 watts out on the Icom Ic7300 on all bands with no visible ALC.

Make sure your microphone is dead when you are transmitting, you don't want to be like others on the band with every curse word, tv blaring, dog barking, "hi honey I am home" noise injected on your transmission.  USB-Data should kill you mic.  Make sure it does.

Obviously if you had a station monitor scope, and transmitted a two tone test and look at the trapezoidal pattern and all of that, would help ensure your signal was good, but most of us don't have any of that.  But if you will back off until you see no ALC, most rigs will be fine.
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K6BRN
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Posts: 456




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« Reply #3 on: September 30, 2017, 07:51:13 PM »

Gentlemen:

Not all ALCs are created equal.

In many modern rigs built for digital modes via direct USB port connection, significant ALC indication is not only OK but preferred, for modes like FT-8, JT-65 and JT-9.

I've tested AGC level tolerances on the Yaesu FTDX-3000, FTDX-1200, FT-991 and Icom IC-7300, with a spectrum analyzer, an oscilloscope and the help of fellow hams who were monitoring their waterfalls at the time as a sanity check.  The scope and spectrum analyzer were connected to the coax close to the radio via an in-line RF coupler and attenuators - looking directly at the transmitted signal.

Findings?  All of these (newer, digital-ready) radios behave just fine at 50% ALC indication on FT-8, JT-65 and JT-9, with no significant splatter (spectral growth).  In fact, loading them at this level tended to stabilize their output levels, which otherwise drifts as the radios warm up and cool down, requiring a constant hand on the WSJT-X power slider.  So, S.O.P. for me and my buddies who use these radios is to set the audio input level to 50% ALC indication and go with it.  Zenki has not complained.  So far.

Older rigs with input through the audio port directly may not do so well and ALC implementation on older rigs can be pretty crude, so problems could exist.  But I've not checked any, either, so don't have any examples.

BTW.... one of the biggest causes of obvious "splatter" I've run into when working with WSJT-X is caused by setting the computer RX audio level too high.  ADC saturation driven distortion is generated in the sound card itself, resulting in splatter on the RX waterfall and multiple images of strong receive signals.  A similar phenomenon can happen if the TX level is set too high.

Best Regards,

Brian - K6BRN
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K6BRN
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Posts: 456




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« Reply #4 on: September 30, 2017, 08:23:42 PM »

On the Jeff's original question:

Quote
...how to monitor transmission?

There are a lot of less involved and inexpensive ways to monitor TX signal quality, including having buddies look at your output on the digital modes.  But the better and more certain ways require a little investment in time, learning and money.  Like the following ones....

Easiest, plug-and-play solution is to use a station monitor built for this purpose.  The Telepost LP-500 and LP-700 are digital watt-meters with built-in station/signal monitoring capability.  Not cheap, but they do it reasonably well.

Another approach is to build or buy an inline RF coupler that provides an attenuated sample of the transmitted RF to a BNC port that can be connected to an oscilloscope or spectrum analyzer, with most of the signal still going to the antenna.  They are not too expensive and quite a few are available.  If you are looking to buy a decent one, go to the Wavenode site and look at their "RF Current Sensor" the CS-1 ($65) - its an inline RF coupler. 

The monitor output from a coupler is usually attenuated by 30-40 db relative to the input signal (-30db means that if you TX 100 watts, the actual signal at the monitor output port will be 100 mW) and can be fed directly into an oscilloscope.  If you have a spectrum analyzer - they are way more sensitive/delicate (easy to blow up), and you'll usually need to add an additional attenuator to keep their input power level at about 1 mW max.

Oscilloscopes are easy to find these days.  You'll need one with 50-100 MHz bandwidth.  They are available used or new for $100-400.  The Rigol DS1054Z is an especially attractive one, due to its features and price.  But old CRT based Tektronix scopes work well, too - any ham swap meet will have a half-dozen for sale, used.  Oscilloscopes can display your TX waveform exactly as it is and will reveal any gross distortions.  But subtle ones are hard to see on an oscilloscope, and they can cause "splatter" the 'scope will not detect.

Which leaves the spectrum analyzer.  These are frequency-domain instruments that do not attempt to show the TX signal at all.  Instead, they show the spectral content of the TX signal, by frequency and magnitude.  Splatter will generally show up very clearly as a significant widening of the TX signal spectrum, as additional "spurs" to either side of the main signal, or both.  Unfortunately, even though spectrum analyzers have become much cheaper over time (used to be $5K-$50K), most are still over $1k, at least for a decent one.

So... what is the BEST way to monitor your TX signal quality?  Always a tough and subjective question to answer.  But at least you now have some possibilities,

Best Regards,

Brian - K6BRN
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N4MQ
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Posts: 139




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« Reply #5 on: October 01, 2017, 03:58:08 PM »

Have you considered using a scope to monitor the output - this works for am ssb and cw, so it would give you a feel for the waveform. Woody Enjoy
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K0UA
Member

Posts: 1368




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« Reply #6 on: October 01, 2017, 04:24:15 PM »

Gentlemen:

Not all ALCs are created equal.

In many modern rigs built for digital modes via direct USB port connection, significant ALC indication is not only OK but preferred, for modes like FT-8, JT-65 and JT-9.

I've tested AGC level tolerances on the Yaesu FTDX-3000, FTDX-1200, FT-991 and Icom IC-7300, with a spectrum analyzer, an oscilloscope and the help of fellow hams who were monitoring their waterfalls at the time as a sanity check.  The scope and spectrum analyzer were connected to the coax close to the radio via an in-line RF coupler and attenuators - looking directly at the transmitted signal.

Findings?  All of these (newer, digital-ready) radios behave just fine at 50% ALC indication on FT-8, JT-65 and JT-9, with no significant splatter (spectral growth).  In fact, loading them at this level tended to stabilize their output levels, which otherwise drifts as the radios warm up and cool down, requiring a constant hand on the WSJT-X power slider.  So, S.O.P. for me and my buddies who use these radios is to set the audio input level to 50% ALC indication and go with it.  Zenki has not complained.  So far.

Older rigs with input through the audio port directly may not do so well and ALC implementation on older rigs can be pretty crude, so problems could exist.  But I've not checked any, either, so don't have any examples.

BTW.... one of the biggest causes of obvious "splatter" I've run into when working with WSJT-X is caused by setting the computer RX audio level too high.  ADC saturation driven distortion is generated in the sound card itself, resulting in splatter on the RX waterfall and multiple images of strong receive signals.  A similar phenomenon can happen if the TX level is set too high.

Best Regards,

Brian - K6BRN

And yet I helped someone on the west coast, with setting his ALC level.  He said he was running at 50 percent on his ALC meter and his signal was horrible on FT8.  Harmonics of his audio frequency across the band. He set it down to 0 ALC and still had full power output, and no more spurious signals. This method doesn't require any instrumentation or much thought.   I am not doubting what you have observed and tested, it is just that it is easy for the average ham to remember "no ALC indication, and likely good to go".  We sure have a lot of dirty FT8 signals on the band, and I am sure most of the guys don't know they are doing it.  And I am not talking about receiver overload looking at wide flaring red traces.  I am talking about the multiple signals they are transmitting across the band.
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K6BRN
Member

Posts: 456




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« Reply #7 on: October 05, 2017, 07:56:15 PM »

James (K0UA):

No doubt you're right and some hams are not sharp enough to know how to operate their own computers and equipment, even with detailed instructions.  But should we should dumb it down for all hams to make it easier for the rest of us?

Some might know there are a second set of sound card controls on their computer that can be set to avoid distortion, outside of the WSJT-X application, if they are made aware of the option.

Also - per my post - not all AGCs are created equal, so the amount of tolerable ALC indication may vary by rig.  I just know 50% works fine on the FT-991, FTDX-1200, FTDX-3000 and IC-7300.

Woody (N4MQ):

Per my post above: 

Quote
Another approach is to build or buy an inline RF coupler that provides an attenuated sample of the transmitted RF to a BNC port that can be connected to an oscilloscope or spectrum analyzer, with most of the signal still going to the antenna.  They are not too expensive and quite a few are available.  If you are looking to buy a decent one, go to the Wavenode site and look at their "RF Current Sensor" the CS-1 ($65) - its an inline RF coupler. 

The monitor output from a coupler is usually attenuated by 30-40 db relative to the input signal (-30db means that if you TX 100 watts, the actual signal at the monitor output port will be 100 mW) and can be fed directly into an oscilloscope.Oscilloscopes are easy to find these days.  You'll need one with 50-100 MHz bandwidth.  They are available used or new for $100-400.  The Rigol DS1054Z is an especially attractive one, due to its features and price.  But old CRT based Tektronix scopes work well, too - any ham swap meet will have a half-dozen for sale, used.  Oscilloscopes can display your TX waveform exactly as it is and will reveal any gross distortions.  But subtle ones are hard to see on an oscilloscope, and they can cause "splatter" the 'scope will not detect.

Brian - K6BRN
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KG5AHC
Member

Posts: 76




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« Reply #8 on: October 06, 2017, 07:44:00 PM »

I don't have the specific web sites at hand but I know you can find listening stations and would be able to observe your own signal from a distance, and I believe they might have waterfalls to look at..

but I hate not offering SOMETHING... try this:

http://www.websdr.org/

73 and good luck
Jeff KG5AHC

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