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Never seen anything like this before.....

guitar_199

Sr. Member
Mar 8, 2011
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Deer Park, TX
I'm not really looking to "press the crowd" for a solution.... it's just that I have never seen anything like this before... and I am just throwing it out there to see if it "rings a bell" with anybody.

A friend has (I believe) a "made for the US Navy" Harris UR-94.

Both sides of SSB seem to transmit fine and push up to decent power........
HERE comes the weird part....

When you throw it at AM.... and you look at it in the spectrum display of a nearby SDR on a computer.....

The carrier is there.....
Te UPPER side information is there.......and looks good.....

The LOWER side information... well... it is there... but only about half the amplitude of the UPPER side info.... AND it tends to "trail off" on the left end.... towards the noise floor.

It is almost like (this is just me trying to describe it!).... the carrier and upper side info are getting through.. but the lower side info is in the "skirts of a filter" and is being "shaved off" or "attenuated".

I stuck a screenshot of his photo... you can see that... to the right of the carrier... the upper side data looks good.... and the yellow in the waterfall on that side is nice and bright.

But if you look at the right... it starts out at about the mid-level.... then just fades (as the modulating frequency goes up). And note that the yellow in the waterfall on the LEFT side is weaker and falls away more quickly than the RIGHT side......

I double checked him.... and this IS AM...... just him modulating the mic.

It doesn't seem like an "audio EQ" problem or it would be attenuating on both sides of the carrier.
It just looks like a filter skirt is sitting JUST to the LEFT of the carrier and cutting the lower half off.

Like I say, I am just looking for a "ping" to see if anyone has ever seen anything like this.

Bob

1669223827502.png
 

Handy Andy

Do Your Research First, Then Decide...
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The SSB if the "centering" peaks are ok, then the suspect is the AM "shift" - so that might indicate a crystal lattice filter for the AM carrier is no longer centered on the IF passband used to mix the PLL side to the Carrier side - the shift can be from an aged cap or one of the passband crystals.

This sounds eerily similar to when a radio using AM sounds more like Dual sideband - because the carrier it's supposed to pass, is not mixing in to make the envelope for the audio from the carrier - so to me this AM signal and the passband it's using are not aligned.
 

guitar_199

Sr. Member
Mar 8, 2011
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Deer Park, TX
Mystery was solved.....by a knowledgeable member of a Harris radio specialty forum....

This radio does not do TRUE AM.

It does what is called AME
Amplitude Modulation Equivalent.

They essentially cut the lower sideband.... and suppress the carrier to a low level.....BUT ENOUGH for an AM receiver to lock on it.

I suppose that the idea is to cut the bandwidth in half...... but still be AM compatible.

So...... in short.... it is working as designed. I had just never heard of AME.

Live and learn.
 

guitar_199

Sr. Member
Mar 8, 2011
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Deer Park, TX
In fact.... my friend just found some reference text on the subject...
---------------
A Compatible sideband transmission, also known as amplitude modulation equivalent (AME) or Single sideband reduced-carrier (SSB-RC), is a type of single sideband RF modulation in which the carrier is deliberately reinserted at a lower level after its normal suppression to permit reception by conventional AM receivers. The general convention is to filter the lower-sideband, and communicate using only the upper-sideband and a partial carrier. [1]


The benefits of compatible-sideband over conventional AM are increased spectral efficiency due to a reduction in bandwidth of 50% as well as a decrease in wasted power. By using compatible sideband instead of AM, less RF power is required at the transmitter to transmit the same quality of signal the same distance. This results in compatible sideband being almost 100% power-efficient, where regular AM is comparably only 16% power-efficient (84% of RF power wasted). [2]

This modulation is currently mostly used in high frequency military communications.
 

TM86

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Jul 6, 2014
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This results in compatible sideband being almost 100% power-efficient, where regular AM is comparably only 16% power-efficient (84% of RF power wasted). [2]
Sounds like you can get the same signal strength at a given location for less power in and no loss in readability.
 

Captain Kilowatt

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Apr 6, 2005
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Mystery was solved.....by a knowledgeable member of a Harris radio specialty forum....

This radio does not do TRUE AM.

It does what is called AME
Amplitude Modulation Equivalent.

They essentially cut the lower sideband.... and suppress the carrier to a low level.....BUT ENOUGH for an AM receiver to lock on it.

I suppose that the idea is to cut the bandwidth in half...... but still be AM compatible.

So...... in short.... it is working as designed. I had just never heard of AME.

Live and learn.

I got into an argument several years ago on this forum with a member about this very subject. He INSISTED there was NO WAY a radio, either amateur nor military, would do that because it would result in severely distorted audio on AM and heterodyning on SSB. He insisted the problem was an unbalanced modulator in SSB mode. This is the first time I think I have heard anyone on this site that has experienced this since then.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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He INSISTED there was NO WAY a radio, either amateur nor military, would do that because it would result in severely distorted audio on AM and heterodyning on SSB.

In my further research, perhaps the way signals are Detected - is what is more for the fault of the "distortion".

Ok, as many of you already know, any "IF image" - the mixer produces two signals - one above as a Summation and one below the mixing frequency - as a Subtraction.

I originally thought we were dealing with a radio that needs repair, but in this instance - the radio is purposely designed this way to maximize the asymmetry.

Yes, you can modulate one sideband and AM carrier - you don't want this in SSB, but the carrier can be produced along with one sideband and transmitted to another receiver - designed to decode that signal - properly.

I mentioned mixing because it's the easiest way to show this - because of the conversion is similar to SSB, but you have the carrier that provides that "direction" the audio embedded in the sideband (Upper - non inverted or Lower as inverted) - some people seem to hear this as distortion, while others hear this as garbled garbage - the only reason is the way a radio is designed to decode that IF signal product in its detector.

So, if you simply twist the diode detector one way - can you hear it? Well, maybe, but the process begins (originates) knowing the transmitted signals offset from the carrier - USB or LSB - then your signal is selected from the corrected Images in your IF strips to decode such information.

Since the USB is easier to understand - because the carrier is needed to detect and the audio signal you want is in the frequency are above that carrier. You design the mixer / pll (or whatever you use) to shift the mixing products to meet the window of the filter.

So, the Mixers then use the upper side of the filter to "capture" the correctly non-inverted portion - and each stage then has to use this "side" to keep the correct aspect.

The transmitted side of the unused portion - is a mirror of the initial - so the combined bandwidth is sent but is much weaker so there is an example of where some may hear it as garbage or distortion - because their radio uses the wrong mixer images to decode the signal - so the mixer decodes it backwards - or inverted and so you have your correlation problem.
 

guitar_199

Sr. Member
Mar 8, 2011
715
959
103
Deer Park, TX
Just to add a little more.....

Through the Harris specialty forum he found ... he was able to come up with a mod that was very easy to implement to MAKE it full AM.

Apparently the filtering is switched on and off using diode switches ... much like signal routing is done in solid state radios these days.

The mod adds (I believe) diodes that somehow result in the lower sideband envelope being restored, and the carrier is higher than it was before.

So now, pending his checks with a local friend, it should be just plain old AM when he switches to AM.
 
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