IMD and S/N

Discussion in 'Working Different Modes' started by K9AAN, Jun 15, 2010.

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  1. K9AAN

    K9AAN XML Subscriber QRZ Page

    From what I have read, you should have NO ALC action.

    I always set it my FT-950 for no ALC action and get great reports even with QSB.

    ALC clips the audio and creates squared off waves which create bunches of sidebands.

    It could be that you are lucky and your sidebands don't have a lot of energy.
     
  2. W1MSG

    W1MSG Ham Member QRZ Page

    When I was using my FT-857D the manual for PSK stated to have the meter set to ALC with almost full scale. I guess it depends on what you read. I dont think anyone can agree on anything, and every system is different.
    To get no ALC on my IC-7000 I have to have a volume level of 0, then I get no signal output.. Beats me, would love to see a screen capture of my signal. I may have to take my other laptop out to the truck and see if I can get one with my current settings.
     
  3. K4RKY

    K4RKY Ham Member QRZ Page

    I missed it! ...

    It is always funny to see (smart) people strut their stuff.
    I have always been told and read the same comments in various guides it is (recommended) 50% of rated PEP of radio. I use a older Kenwood ts140s and my settings are 40watts with approx. 20% mic gain (no more!) and sound card is at 47%. I pay close attention to the ALC, if it moves I am driving too hard. UNder xtreme conditions I could see more power and I have inadvertantly forgot to lower my power level before and no one noticed it. (at least they didn't complain) You can see a difference in your waterfall though.
    I noticed your Avatar ( I salute you!)
     
    Last edited: Sep 1, 2010
  4. W1MSG

    W1MSG Ham Member QRZ Page

    Yup, I am not one of those smart people, thats why I ask alot of questions, I dont want to be THAT guy thats wiping out the waterfall.

    Ok, now I was looking at my set up and no mater what I get 3 bars on the ALC scale, doesnt matter happens every time the radio keys up so that is probably a IC-7000 quirk. The one thing I have learned is that you have to come up with your own settings. I tried yours and there is no way I can get 47% on the sound volume, it pegs out my ALC big time.
    So my set up is kind of weird, and like you no one complains about my signal.
    RF Level is at 100 watts, Mic Gain 50%, Sound Card Volume about 14 but I need to adjust it a bit depending where I am on the water fall, and I adjust it to between 25 and 50 % on my Power meter on the radio so I shouldnt be putting out more than 30 to 35 watts, if someone says they arent getting it all I just bump the volume up just a hair and then they seem to get the 100% copy.
    So I guess you just need to understand how to get a good clean signal on your gear, and I am learning all the time.
    If its cooler in the morning I'll set up PSK out on my mobile rig and see if I can get a screen capture of my main stations signal, I'll post it up if I can get it to work.

    Thanks for the Salute, 13 months till retirement and counting, its been a very long 30 years ..

    Craig
     
  5. W8JI

    W8JI Ham Member QRZ Page

    I don't see the "splatter" problem you are referring to on the display. The next signal up in tone does not look like a repeat of his signal because the peaks and valleys and length of bits are different. The IMD might be "poor", but it could be from your own gear. Your own AGC can add IMD, as can the audio stages in your receiver. So we really don't know where the problem is.

    The reason all of this is a problem is because digital modes are generated in a sound card, processed through the transmitter audio system, and run through the SSB transmitter stages to the antenna.

    When the signal gets to the receiver, the signal not only goes through all the normal stages (often using a wide filter), it also goes through with multiple signals and a wide slice of noise. Then after going through the audio stages it is processed in a computer sound card.

    It is a system full of potential problems all the way through, including your own receiver and sound card, and no "meter" at the receiver sound card can tell you where the distortion is. It might be anywhere in that entire system, including your own gear. In order to really know if his signal was a problem, you would have to swicth to a narrow filter and look only at his signal, use a slow AGC, and maybe even use an attenuator pad. Otherwise you have no real indication of where the IMD report really comes from, and whose fault it is.

    Power level itself is not an issue, the real issue is maintaining linearity through all those stages from the sender's computer to your computer. You cannot have harmonic distortion (which is all audio, and has nothing to do with ALC or RF sections of the system) or intermodulation distortion (which does relate to RF parts of the system, but also includes the audio parts).

    The entire reason there is such a focus on power levels is the system itself is a poor type of system and has horrible dynamic range. Because there are so many things to go wrong, signals have to be kept down near the noise to mask the inherent problems.

    Many of the problems are caused by how the receiver is used, and most people use the receiver as a wide IF stage that is at baseband audio frequencies. The computer provides all the selectivity in most setups. This means it is common for strong signals to cause problems in multiple receivers, not through any fault of the transmitter...but through fault of how the receiver behaves.

    Using a narrow filter in the receiver will greatly help, because you depend less on linearity of the audio and AGC systems in the receiver.

    Because of the limited dynamic range, the signal level people accept is greatly limited. The signal level limit hurts the range of using this method of transmission and reception, and causes a lot of fighting and arguing. The real issue is the silly idea of expecting every stage from the transmitting station's sound card to the the receiving station's sound card to be perfectly linear and distortion free.

    Since that will never happen, it forces people to have weak signals. The weak signals limit useful range.

    73 Tom
     
  6. W1MSG

    W1MSG Ham Member QRZ Page

    Good points Tom, now I am even more confused, so the echoing around his signal, and the fact that he wiped out 4 or 5 other signals and painted the entire waterfall a dark blue and the -9db IMD could be on my end ??

    I guess I'll just have to deal with the few signals that disrupt the waterfall, there arent that many, only a certain few and I just dont try to make contacts while they are on.
     
  7. K9AAN

    K9AAN XML Subscriber QRZ Page

    Yes, it could be on your end. Your RF gear is high quality. What are you using for an interface? If you don't have a high quality sound card OR if RF is getting into your audio cables... Do you have a torroid around your audio cables before they go into the computer?
     
  8. AI3V

    AI3V Ham Member QRZ Page

    Lets go over some basics.

    While it is possible for receiver troubles to degrade IMD, it's unlikly that just ONE signal in the receiver passband will be affected.

    The transmitter final is in-fact the part that usually overloads first- it's far and away the most expen$ive transistor in the signal chain, and sets sold to a price point are sensitive to that.

    Adjust your transmitter for ZERO ALC. The alc is feedback to prevent the final from being driven to clipping (saturation), the alc does this by reducing the gain of one or more stages in the transmitter. This gain reduction also degrades the linearity of the transmitter, and because we are dealing with a linear signal, any degradation is bad news.

    A typical 100 watt transmitter (2 x 50 watt transistors) will usually run at best IMD when run at 50% of that power. (look up the manufacturers data sheet for your transistors- there is a lot of info there!)

    The psk signal has a 2:1 peak to average ratio.

    Therefore, a ordinary average reading wattmeter should show about 25 watts when set for best imd.

    A peak reading meter will of course show 50.

    Try these settings:

    Set the rf power to the rated power of your transmitter. It's ok to have the sound card and mic gain up, and a BIT of alc.

    Set the sound card level to 90%

    Set the mic gain for 25 watts avg, with NO ALC.

    Go back and see if the rf power follows the sound card level, lowering the S/C 3 db MUST cut your power in 1/2.

    For receive, the single best improvement is a narrow filter, signals that can't get to the waterfall, can't interfere with the waterfall ;)

    Rege
     
  9. AI3V

    AI3V Ham Member QRZ Page



    There are a couple different thing here you are seeing.

    **

    If his signal has good imd and the strongest signal in the passband, and he makes another signals imd degrade, that is a problem caused by "agc pumping" in your rx.

    The strongest signal in the receiver passband will "capture" the receiver agc, and reduce the receiver gain for ALL of the signals in the passband, in order to keep the volume at the speaker more or less constant.

    This reduction will tend to cross modulate all the signals but the strongest one with a signal that is a combination of agc time constant. and ionospheric fading.

    **

    If ALL the signals in the passband get distorted, this would indicate receiver blocking, this is similar to agc pumping, except it is caused by a very strong signal outside of the receiver passband.

    It can also be caused by having your RX agc set to off, and having his signal outside the linear range of your receiver.

    **

    On the other hand, if ONLY his signal is bad, well, he's misadjusted his transmitter.

    **

    Also, if his signal, and the signals next to him are tore up, but the other signals in the passband are ok, then, again, he has misadjusted his transmitter.

    ***************************************

    For best results with any receiver:

    Use as narrow as possible a receive filter.

    If you must view multiple signals in the waterfall, try turning your receiver agc OFF, This is a POOR substitute for filtering the other signals out in the first place, but, depending on the relative signal strengths, your receivers dynamic range, and your sound card sensitivity , may work a bit better.

    Turn off the noise blanker- these are notorious for causing "cross modulation" on a received signal.

    Keep attenuator/preamp and rf gain set so there is no movement, or perhaps a s-1 reading of the background noise. In general, for HF, and especially 160,80 and 40M and a fullsized antenna, this means preamp off and/or attenuator on.

    If your receiver has provisions for a fixed level audio signal to feed to the computer, as opposed to one that varies with the volume control, use it. That will be one less place to get out of adjustment.

    Rege
     
  10. K6CLS

    K6CLS Ham Member QRZ Page

    one time, saw a signal on the waterfall I thought was splattering, but it turned out that my receiver was misconfigured (ACG was on, preamp was on) and the soundcard input level was too high. I fixed all that and the other ham's signal was real pretty!

    the episode was very embarassing to me because I emailed a screenshot to him, then figured out the issue was on my end.

    all that said ... I sure appreciate getting honest signal reports. some times it is on my end, some times theirs. it's fun to work together and figure it out.
     
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