AM characteristics on SSB

KF4HR

When watching the spectrum display on my 6700 I've noticed that most SSB signals show their side band signals on the proper side of the suppressed carrier line, but some SSB signals have characteristics of double side band (particularly older equipment), meaning their side bands can be seen extending on both sides of the suppressed carrier.  Obviously these transmitters are not functioning properly.  I'm curious what's going on with these double side band-like SSB signals?     

Bill Roberts

I've noticed the same thing, too...  particularly on lower priced rigs like the Yaesu FTDX 1200, IC 718, etc.  I suppose that in the most basic sense, those rigs exhibit poor suppression of the unwanted sideband.  Others might be able to add insights into other spurious emissions coming out of those "offending" "rigs."

Jim Gilliam

Most over driven linear amplifiers show this characteristic. Many times you will notice the signals stay within their desired passband until a certain level is reached and then starts spreading out on both sides. The amplifier is driven too had and is operating outside its design parameters and starts generating non-linear products of which you observe on your receiver.

Ken - NM9P

THis also can happen when people use external audio devices to process and equalize their audio, but have them improperly adjusted to that the higher levels of bass "**** y" the filter and show up on the other sideband.  Properly adjusted, they can sound wonderful, but improperly used, they can cause problems.

Martin AA6E

It is probably the result of intermodulation distortion.  Various components of the SSB signal are mixing with each other (and their harmonics) because their "linear" is not so linear.  That produces audible distortion inside the normal sideband - making your voice sound muddy.  But it also spreads the signal outside the sideband both on the high side and the low side.  It gets worse rapidly as you increase power.  If you care about this, it's best practice to dial back your power to maybe 1/2 of the maximum that your amp is capable of.

It's also possible that the transmitter has poor opposite sideband suppression in its sideband generator.  That used to be a problem, but modern equipment is usually good on that score.

One of the neat features of Flex SDR is that you get to see this clearly.  Think how popular you can be if you let everybody know what their signal looks like on your screen!

73 Martin AA6E

Rick WN2C

sometime it is the all knobs to the right mentality. You know more must be better and if the audio setting at 50% is getting me 100 watts then 90% will at least make me louder and give me MORE POWER.

Bill -VA3WTB

If a radio's IMD is around 28 down and you hearing them at S7, they should look pretty good. But when you start seeing that rig at S9 they spread out accordingly, you see more.

Lets say they have an IMD of 32 down and you see them at S9 and they look good till their signal increases to S10, now you would start seeing problems in the transmit.

On a Flex with an IMD of 38 to 40 down you won't see anything till you see a signal of S15 over 9

On an Anan with pure signal at 70db down you would need to see a signal of say 40 over 9 before seeing anything.

So bottom line is. A radio's IMD combined with signal strength has a lot to do with what wee see.
Now, I am talking about the radio it self. But as many of you have mentioned, out board audio devices, over driving an amp, Or setting mic gain to high all effects the final IMD transmit of the system. In these cases a radio with an IMD of 32 after an amp is over driven for example the IMD falls to 25, this means that a signal could look messy at only S7 or 8.

On the flex panadapter look at a signal and notice the vertical line on their signal. look at how long the line is straight up and down, count the horizontal lines on the panandapter and count them from the top of their signal down to were is starts to spread out. That will tell you their IMD db down. You need a signal at least S9 to get a good reading.

In most cases their lines may go all the way to the noise floor. At S9 they should have an IMD of 36db down

Bill Roberts

Yeah, that's for sure.  I hold off on such comments   It's a sure way to make enemies and still not solve technical problems.

Bill -VA3WTB

Think of it, before Gerald in vented the amature radio panadapter, who knew?

KF4HR

There is a group that occasionally meets on 3.600.000 USB.  I've noticed no member of this group has side bands that extend even the least bit below 3.6 mhz.  Interesting operating, although I'm not sure I'd take that chance.       

Martin AA6E

Before good SDRs, IIRC most ham panadapters had low res, slow sweep rate, or poor IMD so they were not very useful to accurately check signal quality.