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So far my experiences with PSK-31 and PSK-62

Discussion in 'Working Different Modes' started by PA1EJO, Apr 27, 2016.

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  1. PA1EJO

    PA1EJO Ham Member QRZ Page

    On my blog I describe how I recently established some QSOs in BPSK but I'm not sure about certain issues that I've run into. One of the is, how do you check the quality of the BPSK signal that you transmit, and the other is, what does the ALC meter on a FT-991 really show? I would be grateful if someone has some answers for both issues.
     
  2. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I don't have an FT991, but in general for PSK and other digital modes, you want to adjust the RF output to maximum, then adjust the computer audio output/mic gain to where there is no ALC indication at all. That will help ensure a good signal. You can also ask the station you're working for a report.
     
  3. PA1EJO

    PA1EJO Ham Member QRZ Page

    THX for the advice, so far the BPSK QSO's seems to work, I'm setting now the audio level such that it flickers between half power and full power when the SSB transmitter is on. You don't want is off (HI) and you don't want it full all the time when transmitting. The ALC indicator on the FT-991 is somewhat weird and not configurable.
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The ALC meter on any rig shows the amount of feedback being applied to the transmit signal to soft limit the total power. In other words, if you drive a rig up to about 95% of its rated output power there will be no ALC feedback applied and the ALC meter will read zero. But if you apply enough drive signal, whether voice peaks through a microphone, CW gain or digimode signals coupled into the audio chain such that the rig gets up to that last 5% or so of its rated power the ALC circuitry will begin to apply negative feedback to the gain stages to begin limiting the total power out of the rig and the ALC meter will rise above zero. As you drive the rig harder more negative feedback is applied to further limit the signal power and the ALC meter reads even higher.

    The net effect of ALC action is to provide soft limiting of the transmit signal that limits total power output but does so in a way that preserves signal quality better than hard limiting with a sharp cutoff power. But it also has the effect of compressing transmit signals that are strong enough to get into the ALC operating range and signal compression like that can cause distortion of a signal. On SSB or even CW the amount of ALC induced distortion is minimal and generally acceptable if you stay in the ALC safe range indicated on the rig's meter. But in PSK and other digital modes you really want to avoid any signal compression and the resulting distortion products so you turn off any speech processing and adjust signal levels below the point where the ALC meter rises above zero.

    -Dave
     
  5. KF5FEI

    KF5FEI Ham Member QRZ Page

    For all the Yaesu radios I have, setting the radio to the maximum power, then adjusting the PSK audio to show 25% to 33% of max power on the radio power meter usually works well for PSK. This would give you 25-33 watts on a 100-watt radio.
     
    PA1EJO likes this.
  6. N0IU

    N0IU Ham Member QRZ Page

    I don't have an FT-991 so I don't know how to hook it up for the digital modes, but the other thing to keep in mind with any radio is that if you have speech processing or compression, it MUST be turned OFF to prevent overdriving your signal.

    And if you have a really crappy signal that is splattering all over the place, people are usually not shy about saying something to you over the air! Personally, I usually send a very politely worded email if it is listed on QRZ (or somewhere else)
     
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  7. PA1EJO

    PA1EJO Ham Member QRZ Page

    True, all microphone equalizers and everything else that might change the audio your PC intended to send out has to be off.
     
  8. PA1EJO

    PA1EJO Ham Member QRZ Page

    If the ALC meter on the FT-991 does as you say then I don't understand what it really shows when we regularly transmit SSB or FM or any other signal. Whenever I transmit the ALC meter on the FT-991 comes up, when it starts to throttle the transmitter output the ALC meter goes lower, so could it be that the ALC meter on the Ft-991 shows just the opposite of what you are describing, an inverted value of the feedback signal?
     
  9. K6BRN

    K6BRN XML Subscriber QRZ Page

    Not all ALC systems are created equal. Quite a few older systems have a simple limiting curve that phases into hard limiting fairly quickly. But on the Yaesu FTDX3000 (and likely the very similar FTDX1200), I've found that some ALC indication during JT65 is beneficial and results in no discernable increase in splatter. I suspect Yaesu may be taking advantage of their transmit DSP processing to shape the limiting function and minimize harmful distortion. Have not tried this with PSK-31 yet.

    Brian - K6BRN
     
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Agreed, the specifics of ALC can vary but the premise is the same which is intentional signal compression as you approach the maximum rated power of the transmitter. Whether the compression curve is steeper or shallower or has a complex curve as a function of amplitude it is still signal compression and if you look at it as a time domain windowing function it has impact in the frequency domain. That impact may be subtle enough to ignore for many modulation modes but if you compress a transmitter's output there will be frequency domain impacts.

    Your JT65 may benefit or at least not see much negative impact from subtle ALC compression but remember JT-65 is based on FSK and not PSK and FSK is somewhat more tolerant of non-linearities in the signal processing chain.

    That's highly unlikely and would be different than virtually every other ALC meter on the market. I don't have the rig in question but if they called their meter 'ALC' and used that to represent the opposite of traditional ALC that would cause a lot of confusion regarding mic gain setting and other applications of ALC.

    Here's the relevant excerpt from the 991's manual and it's description of using ALC is the same as every other rig on the market that supports an ALC function:
    The behavior described above is exactly what you'd expect for a normal ALC function which is to supply negative gain feedback as a transmitter approaches full output power. Yes, as Brian mentions above not all ALC functions have the same response curve and their exact gain compression function may differ but in my experience they all do the same basic thing which is to soft limit transmit power (regardless of the specific curve) and that means signal compression whether hard, soft, multi-slope, etc. Signal compression is not a good thing for PSK signals.

    -Dave
     
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