HF Digital Error correcting? Also, what's up with PSK31?

Discussion in 'General Technical Questions and Answers' started by N0NS, Oct 9, 2008.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: abrind-2
ad: L-MFJ
ad: Subscribe
  1. AB0WR

    AB0WR Ham Member QRZ Page

    The problem is that you get exactly the same IMD performance from the driver/final system for both CW *and* psk31 -- which is what was being discussed. That's why talking about IMD in the driver/final is a smoke screen. Jeesh, even if you created the psk signal directly at RF in a DDS system -- YOU WOULD STILL NEED AN AMPLIFIER CHAIN to make it into a usable signal on the ham bands. At least I have yet to see a DDS chip that can operate at the 20watt level let alone the kilowatt level. So you will have exactly the same IMD problem as you have in a SSB transmitter.

    The *narrow*-band data attribute is not the issue in the audio system. Heck, a single audio tone would be as narrow as it gets -- yet the audio chain should be able to handle a single audio signal perfectly well!

    If the psk31 signal were a square-wave being fed into the audio chain then speaking to the bandwidth of the audio chain would be appropriate. Yet the psk31 signal is specifically shaped to avoid this issue. It was *designed* to work through the audio channel of a typical SSB system.

    psk31 user are *actually* driving amateur radio to pay attention to Sec. 97.313 Transmitter power standards:

    "(a) An amateur station must use the minimum transmitter power necessary to carry out the desired communications."

    psk31 doesn't require the power output that many modes do in order to provide good contacts. Those that holler about psk31 users complaining about overly strong signals in the psk31 waterhole don't get much attention from me. You can only work what you can hear. It doesn't make much sense to run 1.5kW PEP on psk31 when everyone else is running 20watts. Everyone may be able to hear *YOU* but that won't help you hear anyone else. Running that kind of power just violates 97.313 and just turns you into a pariah.

    If each setup uses the same kind of driver/amplifier chain then the IMD products contributed by that chain will be the same regardless. Everything else is just so much smoke.

    I agree we should see better IMD performance from our equipment. That's a different argument though.

    tim ab0wr
     
  2. AB0WR

    AB0WR Ham Member QRZ Page

    Tom,

    You are still blowing smoke. You start off talking about audio distortion being the problem and yet you end up talking about RECEIVER. dynamic range

    You speak about dumping "narrow data modes into a mic jack" but that is meaningless. That narrow mode doesn't cause *ANY* of the degradations you are speaking of.

    Sideband suppression and the "other" issues are just as much of a problem with CW as it is with psk31. Of course carrier balance is meaningless for CW since you many times generate CW by unbalancing the carrier balance in order to feed the LO signal through. And hum? That's not caused by the mode, that's caused by the physical implementation. You can get just as much hum on a SSB signal as you can on a psk31 signal.

    The original question was why psk31 is so popular compared to the other digital modes. It is because of its utility -- pure and plain. Trying to run down the mode by complaining about the IMD levels in transmitter driver/final stages is just blowing smoke. Complaining about the "effective range" is blowing smoke as well -- lots of DX gets worked on psk31 *every day* of the year.

    Your directly generated psk31 signal will *still* have to run through a driver/final amplifier chain in order to get it up to a power level that provides an "effective range". You'll get just as much IMD out of that driver/final chain using your method as those using computer generated psk31 today.

    For those that are *really* concerned about hum and harmonic distortion in the audio chain it is easy in many transmitters to just bypass the audio chain and connect directly into the first mixer, just like lots of people have done in FM transmitters to get higher data rates.

    You *still* won't be able to run any more power because of the IMD limits of the driver/final amplifier stages.

    tim ab0wr
     
  3. KC4YLV

    KC4YLV Ham Member QRZ Page

    IF DSP? Pah, I work PSK31 regularly with a PBT knob and a notch filter.

    PSK31 gives you great flexibility in tuning and filtering - you can combine your tuning knob, filter passbands and notches to narrow down to just about any signal out there. TX and RX frequency is set with a mouse click, you aren't stuck to a certain CW tone.

    As stated earlier, you want to turn off AGC on the RX side, back your RF gain down. And make sure you are using a rear panel audio in and out. They are usually more or less line level and are way easier than a microphone input to interface to.
     
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    Tim,
    There is no IMD in a CW transmitter that does not use the audio chain.

    There is only one tone!

    73,
    Harry WB3BEL
     
  5. W8JI

    W8JI Ham Member QRZ Page


    There actually are sidebands when the signal rises and falls on CW Harry. It is an AM signal while being modulated with the lowest order sidebands spaced out accoring to the slope of the rise or fall at any instant of time.

    Those sidebands can generate harmonics if the stages that follow have harmonic distortion.

    What Tim is missing is with a slow rise and fall time the sidebands are very close to the carrier, there are no audio chain noise or distrortion issues, nor are there sideband suppression or carrier suppression issues on CW.

    Generating a narrow mode signal in a computer sound card, piping it to the transmitter audio input through cables, amplifying that signal through an audio system never intended to be extremely low distortion, and then modulating it and amplfying it through dozens of stages that have to be perfectly linear AND have very high dynamic range is a whole lot different than having a stage a few stages back from the PA generating a filtered rise and fall time on an essentially pure carrier.

    You can read about the sidebands generated on CW at this link and also at several links on the page:

    http://www.w8ji.com/keyclicks.htm

    The bottom line and the proof of the problem is easily seen just by reading any PSK reflector or the white papers about PSK on Internet. PSK people whine and complain and yammer on and on about anyone running a strong signal in the PSK area of the band. It is extremely common behavior. They want to Lynch anyone running more than a few watts. The reason is simple. The dynmic range on PSK and other through the audio channel narrow modes, regardless of what some might feel, is generally terrible compared to other modes. That's why they fight against having good signal levels so much.

    73 Tom
     
    Last edited: Oct 16, 2008
  6. KI4NGN

    KI4NGN Premium Subscriber QRZ Page

    Actually CW is just an on-off, unmoduated carrier: there is no tone.

    Mike
     
  7. KI4NGN

    KI4NGN Premium Subscriber QRZ Page

    The problem is that people don't know how to use their receivers. The dynamic range has nothing to do with anything.

    Their receiver's AGC kicks in on strong signals within the passband, reducing the RF gain, which reduces the gain on ALL of the signals within the passband. The solution is to get that strong signal out of their passband via tuning, passband shifting, and filtering. The dynamic range of their receiver doesn't have doodle to do with it.

    Mike
     
  8. AG3Y

    AG3Y Guest

    Mike is absolutely correct. But I want to pick a nit or two here.

    This discussion has been going on for many pages, and a basic issue has been incorrectly defined for almost the same number of pages.

    IMD (intermodulation distortion) IS CAUSED by a non-linearity in the amplifier chain caused by the INTERACTION of TWO or MORE tones being passed through that amplifier chain.

    Harmonic distortion is caused by a non-linearity in the amplifier chain caused by a changing of the shape of a SINGLE tone. Each tone that is passed through that amplifier chain may have a harmonic distortion product as a result of the non-linearity of the chain, but a second tone is not necessary to produce harmonic distortion.

    When testing for harmonic distortion, only one pure sine wave needs to be injected into the amplifier chain. Any signals that appear on the output that are NOT amplified versions of that single sine wave are harmonics caused by harmonic distortion.

    When testing for IMD, two pure sine wave tones are fed into the amplifier, and any output signals that are not harmonically related to the two pure tones are most likely to be considered intermodulation products.

    A non-linear amplifier chain is very likely to have BOTH IMD and harmonic distortion products, but unless two or more signals are introduced at the same time, and at nearly the same level, the harmonic distortion is very likely to be the major contributing factor.

    The function of the Automatic Gain Control is an entirely different matter, and as Mike and others have pointed out so well that it shouldn't need to be repeated, IF a narrow passband is used to allow the desired signal, and ONLY the desired signal to come through, a strong signal somewhere else on the band should NOT affect the reception of the desired signal. The problem has always been one of passing EVERY PSK31 signal on the band within 3.5Khz of the desired signal through the receiver at the same time! You have the same difficulty with anything including CW if you don't use a narrow filter. Yet we hardly ever hear anyone complaining that a strong CW signal 2Khz away from the one we are trying to copy is "ruining" our ability to copy the desired signal. We simply switch in the narrow filters and happily work away on the weak signal!

    Now, in practical terms, what does this mean? A station may beproducing "ghost" traces at harmonics of the main signal on the waterfall. Let's say he is using a 1khz modulating center frequency, but his track is appearing at 2 and 3 khz as well. This station is overdriving something in the amplifier chain of his rig. The ghost tracks are primarily a result of harmonic distortion.

    In another instance, a station is seen as being very wide, with many tracks overlapping one another and copy is impossible, even though the signal is quite strong enough, thank you very much! This station will probably have harmonics that will appear up the waterfall, but the wide, close in signals are a result of intermodulation as the modulating signal shifts phase from one side-tone to the other.


    Hopefully this explaination will be satisfactory. I am not a mathematician, so I cannot give you exact numbers, and do not intend to get into the real nitty-gritty of these two distortion types, but I thought it necessary to at least define the fundamental differences between the two.

    73, Jim
     
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    Yes. I agree 100% with all you said Tom regarding CW sidebands. I guess I oversimplified my response to keep it short.

    But I think that the point that a lot of folks are making on this thread is that they want a digital mode that is easy to use and has good performance. I don't want to start a war about CW vs other digital modes because some folks don't know CW or don't like it.

    However, some of the points that are being made that show some weaknesses in PSK31 and other modes that use the Audio interface to a SSB transmitter/receiver are valid. And looking toward the future many of these can be addressed by new digital modes and radio chains that are optimized for them.

    In my opinion RTTY has more general adoption and can support basically the same types of QSOs that PSK can support with similar equipment. Some radios even support direct FSK modulation for RTTY.

    The digital modes can be fun, but sometimes you need more than flea power and a crappy antenna to make a QSO. This is especially true for unusual propagation modes under conditions that can change very rapidly or polar routes that are between locations nearly antipodal. A mode that can not support these types of dynamic range extremes is rather limiting.

    Yes you can work a lot of DX on psk31. But it usually is not very long haul or very rare. It is not so much a confirmation of the excellence of psk31 as it is a validation that narrow bandwidth modes have an advantage over wide bandwidth modes when signal to noise ratios are weak.

    73, Harry
    WB3BEL
     
  10. WA9UAA

    WA9UAA Ham Member QRZ Page

    Jim,
    You just cleared up some theory for me, thanks! This thread has inspired me to dust off my old PK-232 and use the FSK function with my TS-480. I think some of the newer rigs will be better performers with Amtor and Pactor for example. To top things off, I am also using an old NEC laptop with it's own terminal program and 8 line display, in dumb terminal mode. :D
    73,
    Rob WA9UAA
     
Thread Status:
Not open for further replies.

Share This Page