How do you properly interpret SSB signals shown on the waterfall?

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The waterfall display appears to show a lot of good information concerning the SSB signal I may be tuned to or those I see being displayed up or down the band.  Some signals appear to vary a lot in color and intensity.  Some sideband signals appear to have very sharp upper and lower occupied band edges while others appear to show audio peaks extending far beyond the filter bandwidth. Interpreting the displayed information properly seems a bit challenging.  I am just curious to know if anyone has done a write-up on this subject (with illustrations perhaps) they could share with the Flex user community?
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Bob KW4CQ

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Posted 2 weeks ago

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John - AF3K

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Bob, Great question.  While not specific to Flex you might want to take a look at this book (available on Kindle or Google books) "Software Defined Radio for Amateur radio operators and shortwave listeners" by Andrew Barron ZL3DW https://www.goodreads.com/book/show/33236364-software-defined-radio 

GL! John, AF3K
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Bob KW4CQ

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Thanks, John. I’ll pick up a copy.
Bob
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wayne suite

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Just quickly on the screen you can zoom in on the signal and visually see on ssb for example how wide it is on khz and in even more detailed zoom you can see the actual audio frequency response from lo frqs to high freqs detailing how the persons equalization affects the audio transmitted too much low or too little high or lack of width filtering etc. All in realtime.
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Bill -VA3WTB

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This is one of the best question I have seen in some time.

I have discovered the waterfall is not such a good tool to determine if a station is transmitting outside the band pass. Any signal stronger then s9 will start to show out past the band pass. As the signals are higher, perhaps s10 we see much more. But this does not mean that station is transmitting dirty.

Let me explain. Most radios have an IMD of around 36 down. So if the their signal is s9 they look clean. But lets say their signal is s10 or 20 over and there IMD is still 36db down, we see a mess in their signal because we see things below 36db as the signal rises to s10 +

If a signal is s9 and is transmitting out past the band pass, good chance they are transmitting dirty.

Keep in mind, that what we see on the Flex is also related to the display gain control slider.I set my slider to 27, this is a setting that works close to seeing an s9 signal. If I turn the slider to say 34 then even at s9 I can see things in a signal that should not be a problem but it looks like it is.
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Martin Ewing AA6E

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Waterfall colors are under your control. Adjust the display gain and color palette to your taste.

Signals that don't have well defined edges in the spectrum are often distorted. Read up about intermodulation distortion. Often linears are overdriven so they're not as linear as they should be.

73 Martin AA6E
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Paul

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Hi Bob, this is a summary - it might save you buying the book. Apologies if it's too much waffle (I am a retired university lecturer ;-)

The waterfall intensity (and sometimes colour) corresponds to the amplitude of every frequency component in a transmission. Frequency is displayed along the horizontal axis. The vertical axis corresponds to time, where the top is the present time and the bottom is in the past. The display rate controls this axis and the display gain sets the colour palette.

CW: ideally a cw signal would be a very narrow series of dots and dashes running down the display. In practise the signal may spread horizontally a little.

On SSB, the waterfall shows the continually changing range of audio frequencies being transmitted and their corresponding amplitudes. Ideally, an ssb transmission should have a good spread of frequency components within a well defined range. Their intensity will vary depending on many things, eg: the response if the mic, processing, equalisation, tx passband filter width. The aim is to get a good "power spectral density". Which implies that the available bandwidth is being used to its fullest extent. The edges of a transmission should be straight. Lines (or whiskers) outside the main body of the tx passband show splatter. DX stations can be considerably narrower than ESSB stations. This lets the receiving station reduce the rx filter to reduce noise without reducing intelligibility.

USB: the lower edge of the signal on the waterfall corresponds to the lowest frequency contained in the transmitted passband. The upper edge corresponds to the highest transmitted frequency.

LSB: the inverse of LSB, ie, the transmitted spectrum is flipped horizontally about the transmission frequency compared with USB.

AM: you will see both of the above sidebands together with a central carrier. You can get an idea of the modulation depth by comparing the amplitude of the sidebands with the carrier. The power in each sideband should be no greater than 0.25 the carrier power. Which corresponds to 100% modulation.

Good luck and thanks if you didn't TLDR this. 73.
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Bill -VA3WTB

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That is correct, what I mean is that if we turn the gain to high, we see things that should not be visible. S9 is the sweet spot because that is a 36db IMD. What I do is tune to a known clean signal at s9 I adjust the gain slider to show clean skirts on the signal. Now any signal over s9 should show a distorted signal to me if they have an IMD of 36db