To swing or not to swing

[Moleculo]

The real significant factor is the detector used in the AM receiver. Even if negative peak limiting were used on the transmitter, anything over 150% positive modulation peaks are likely to sound distorted in the typical diode AM detector. Beyond that point requires a syncronous detector. Commercial AM radio stations have figured out how to modulate at 125% positive peaks without adjacent channel interference.

Many noise blankers also introduce distortion when used on a signal that has an extreme modulation to carrier swing ratio.

Someone who knows what they're talking about . A lot of people (maybe most) forget that there is no point at producing a transmitted signal that the intended receiver is incapable of handling.

 

[JoeDirt]

IF, the word speaks for itself... Do you use a scope, an RMS reading meter, or a Peak reading meter to accomplish this one?



Yes, when the negatives hit zero distortion starts because you are at 100% modulation regardless of what the positive peaks are doing. Think of it this way, when your negatives hit bottom and stay there for any length of time it is the same thing as your positive peaks flat topping, it is distortion in either case because, positive or negative peak, you have reached the electrical limit of your amplifier. This means your positive peaks can swing to high heaven but if the negatives hit zero you are at or over 100%. You can see this in just about any 29LTD with a swing kit and the diode clipped, well if you look at it on a scope that is…



The negatives still tell the tale regardless of what the positive peaks are doing.

Your talking about a swing kit, and for the most part this means a resistor and a capacitor in parallel, there's no magic here. These two parts installed as a swing kit do nothing to prevent your radio from going over 100% modulation. If your radio can sound distorted and go over 100% without the swing kit, its going to do the same thing or worse with the swing kit.

The comment about the carrier moving with modulation making the scope an inaccurate way of determining modulation percentage......is wrong. Modulation is the movement of the carrier between the unmodulated base line and fully modulated peaks. The scope is the most accurate way of instantaneously measuring modulation and is why they build them as "Station Monitors".

The 1 watt swinging to 30 watts has some people saying it sounds good and others saying it sounds like crap. Who is right? The answer is probably both of you since it depends a lot on the receiver and signal strength being received. The weaker the signal, the less noticeable the distortion may be.

The real significant factor is the detector used in the AM receiver. Even if negative peak limiting were used on the transmitter, anything over 150% positive modulation peaks are likely to sound distorted in the typical diode AM detector. Beyond that point requires a syncronous detector. Commercial AM radio stations have figured out how to modulate at 125% positive peaks without adjacent channel interference.

Many noise blankers also introduce distortion when used on a signal that has an extreme modulation to carrier swing ratio. Even with the mic gain turned down, at 1 watt carrier and 30 watts PEP that's too much swing to sound good in most receivers. Futhermore, without negative peak limiting the adjacent channel interference from a transmitter set like that would be horrendous.

I use a scope..... On a 29 or most radios with a modulation transformer you are going to be limited by your negatives. I was talking about a swing kit on other radios like a Galaxy with a transistorized modulation section. There swingkits work a lot different than a 29 style radio. On the Galaxy style radios the carrier increases with the modulation... a 29 is different. I didn't clarify what i was talking about.

Anyways a radio that's modulated with transistors i can get %200+ percent positive peaks and still keep the negatives around %95 without flat-topping. Since the swing kit on that style of radio the carrier raises with the modulation you can show %200+ modulation on the scope but its still only really doing a %100. Even though the scope shows more! Since the scope it not telling you everything that's going on... you can get a radio the dead-keys 1 watt and swings 30+ watts that sounds just fine because its still %100 mod. The rest its just carrier no audio!

Here is a Gate of me.... Im deadkeying 2 watts swinging 30+ and on a scope its showing %300+ modulation on the positive peaks and %95 negatives.... But if you listen in all reality i am at a %100 in audio, the other %200 is just carrier!

Too Files - Free File Hosting - No limit - Joe Dirt2-14-10 - 3828_joedirt2-14-10.wav - Too Files

JUST BECAUSE A SCOPE TELLS YOU YOUR DOING MORE THAT A %100 POSITIVES DOSENT MEAN ITS TRUE!!!!

If your radio has a swingkit and it is setup properly does not mean your going to sound like crap swinging a lot.

BTW the radio is a Galaxy DX88

Here some scope pictures

The carrier

1K tone

Now thats right around %250 positive peaks

Im not claiming what Motormouth Maul is claiming.... Im just saying if you radios setup right you can get it to sound just fine with a lot of swing!!!!

 

[15minigrass]

Yes BM, its basically a Texas Star 350HDV. Firecracker CB shop repaired it for me. My cousin keyed 100+ watts into it w/his General Grant Gold Face. I know what was he thinking? A newbie w/all his Pop's old toys we go KEY happy.

Anyways you can see it on CB Tricks Amp section. The owner of CB Trick asked if he could use it. I get roughly 200-300+ out of it w/20+ input. But thats just e Rat Shack meter.

Thanx for the info. Jim....

you didnt start a war , LOL .

so you took out the swing kit and have about a 3 to 1 ratio of swing to carrier and got a lot more good reports on your radio .

cool beans


sounds like a plan jim . is the black widow 350 basically a texas star 350 ?
always good to see you on the forums

 

[15minigrass]

hello jim,
my comments were not aimed at anything you said nor did i mean to offend you, it was what the guy gating you said and imho he should not have said, he did not even tune his receiver to your frequency before recording you on ssb,

increasing am swing over what you have at the moment is unlikely to make it sound better,
there are far too many people with big swing into crappy sounding class c amplifiers on am and people using class c on ssb telling each other how great they sound, that in no way makes it true,
a topgun distorter is not the way forwards, its audio jim but not as we know it,

good to hear you are getting a jackson, i just picked up an early blackfront jacko myself for the car.

Hey no pro, thanx for the comment thou. Ill ask my tech to give it a proper tune and do some (AUDIO) magic.

Thanx Bob. Aloha, Jim...

 

[15minigrass]

Nice JD, really nice gate. A motormoth Maul Mic????

Aloha, Jim....

I use a scope..... On a 29 or most radios with a modulation transformer you are going to be limited by your negatives. I was talking about a swing kit on other radios like a Galaxy with a transistorized modulation section. There swingkits work a lot different than a 29 style radio. On the Galaxy style radios the carrier increases with the modulation... a 29 is different. I didn't clarify what i was talking about.

Anyways a radio that's modulated with transistors i can get %200+ percent positive peaks and still keep the negatives around %95 without flat-topping. Since the swing kit on that style of radio the carrier raises with the modulation you can show %200+ modulation on the scope but its still only really doing a %100. Even though the scope shows more! Since the scope it not telling you everything that's going on... you can get a radio the dead-keys 1 watt and swings 30+ watts that sounds just fine because its still %100 mod. The rest its just carrier no audio!

Here is a Gate of me.... Im deadkeying 2 watts swinging 30+ and on a scope its showing %300+ modulation on the positive peaks and %95 negatives.... But if you listen in all reality i am at a %100 in audio, the other %200 is just carrier!

Too Files - Free File Hosting - No limit - Joe Dirt2-14-10 - 3828_joedirt2-14-10.wav - Too Files

JUST BECAUSE A SCOPE TELLS YOU YOUR DOING MORE THAT A %100 POSITIVES DOSENT MEAN ITS TRUE!!!!

If your radio has a swingkit and it is setup properly does not mean your going to sound like crap swinging a lot.

BTW the radio is a Galaxy DX88

Here some scope pictures

The carrier

1K tone

Now thats right around %250 positive peaks

Im not claiming what Motormouth Maul is claiming.... Im just saying if you radios setup right you can get it to sound just fine with a lot of swing!!!!

 

[Shockwave]

I use a scope..... On a 29 or most radios with a modulation transformer you are going to be limited by your negatives. I was talking about a swing kit on other radios like a Galaxy with a transistorized modulation section. There swingkits work a lot different than a 29 style radio. On the Galaxy style radios the carrier increases with the modulation... a 29 is different. I didn't clarify what i was talking about.

Anyways a radio that's modulated with transistors i can get %200+ percent positive peaks and still keep the negatives around %95 without flat-topping. Since the swing kit on that style of radio the carrier raises with the modulation you can show %200+ modulation on the scope but its still only really doing a %100. Even though the scope shows more! Since the scope it not telling you everything that's going on... you can get a radio the dead-keys 1 watt and swings 30+ watts that sounds just fine because its still %100 mod. The rest its just carrier no audio!

Here is a Gate of me.... Im deadkeying 2 watts swinging 30+ and on a scope its showing %300+ modulation on the positive peaks and %95 negatives.... But if you listen in all reality i am at a %100 in audio, the other %200 is just carrier!

Too Files - Free File Hosting - No limit - Joe Dirt2-14-10 - 3828_joedirt2-14-10.wav - Too Files

JUST BECAUSE A SCOPE TELLS YOU YOUR DOING MORE THAT A %100 POSITIVES DOSENT MEAN ITS TRUE!!!!

If your radio has a swingkit and it is setup properly does not mean your going to sound like crap swinging a lot.

BTW the radio is a Galaxy DX88

Here some scope pictures

The carrier

1K tone

Now thats right around %250 positive peaks

Im not claiming what Motormouth Maul is claiming.... Im just saying if you radios setup right you can get it to sound just fine with a lot of swing!!!!

Unless that transmitter has negative peak limiting on it, those scope pictures have been doctored. The only way to exceed 100% positive peaks without RF cutoff on the negative peaks is with a negative peak limiter. All the scope pictures in the world won't change that fact. I know how to turn the carrier up to remove the RF cutoff too.

For some reason you don't seem to trust the scope as an accurate way of measuring modulation percentage. Have faith in the scope, it's your friend. If the scope says you're doing over 100% positive peaks you should start believing it. There is not a more accurate tool then the scope for measuring modulation. The two common ways are measuring the envelope or trapezoid pattern.

With respect to the differences between modulation transformers like the 29LTD uses and series pass modulator transistors like a 148GTL, the modulated pattern on the scope is identical. The only difference is carrier suppression is easier with the transistor modulator because you can adjust the unmodulated DC voltage without having to use a resistor and cap.

I'm sorry to have to disagree with you on so many points Joe but the goal here is to educate you, not pick on you. I understand you have found that you sound louder with this much swing. I use to think it was better too. Then I bought a spectrum analyzer and saw how dirty the transmitter really was set like that. The moral to the story is, too much of a good thing can easily turn into a bad swing

 

[W5LZ]

If a 'scope' doesn't tell you the percentage of modulation that your test radio is doing, then you are doing something wrong, or, you don't know what you are seeing. Has nothing to do with if the radio is solid state or not. Has nothing to do with the type of modulation (hi or low level).
Those facts tend to make me have doubts your whole post. Please explain how I'm wrong.
- 'Doc

 

[loosecannon]

i definitely have no business weighing in on the scope discussion, but i do feel the need to clarify a point so that everyone is on the same page.

in general, there are two types of "swing kits" used by CB shops and on CB radios these days.
one is the cap and resistor that is added to the 29LTD type chassis.

the other "swing kit", if im not mistaken (sorry if i am), is the type that uses the rectifier diode and is used in the galaxy style radios.
this particular type of swing kit is also called the NPC mod, and does give some negative peak compression.
how much negative peak compression and what value resistor to use in series with that diode is the subject of much debate.

i also think its important for everyone involved to know whether or not JoeDirt is using the motormouthmaul device when taking his scope measurements. if that has already been cleared up and i missed it; sorry.

just trying to help,
LC

 

[Shockwave]

Loosecannon, I do see where the NPC mod claims it does give negative peak compression and that is a step in the right direction. Keep in mind with Joe's 250% positive peaks and only 95% negative peaks, that's a good deal of compression for a simple diode to achieve. Even more remarkable is the fact when you look at the scope picture, the negative peaks show no trace of when that diode is conducting on them.

If the scope pictures really represent the values he claims then the NPC diode mod on the Galaxy is the perfect negative peak limiter. Something I highly doubt. It takes a little more then a diode to produce that carrier and modulated waveform. Otherwise the carrier was turned up to get rid of the RF cutoff. Perhaps you could post a video of the scope from carrier to full modulation several times.

This may help explain what you mean by "the swing kit raises the carrier with modulation". I've tried to figure out what you may have seen to make you think this. The only thing I can come up with is that after hard modulation you see the carrier at a higher point and then it settles back down.

If this swing kit really does raise the carrier with modulation in order to avoid RF cutoff, it also avoided any increase in audio normally associated with that carrier to PEP ratio. That may be why it's not distorted at those levels. Can't really tell what's going on from still pics of a scope. That would require video of various modulation conditions.

 

[ron103067]

Ok folks I've read this and the only thing I see is wether you can get a real high % of modulation which I understand to be nice and loud. But aren't you supposed to get the most modulation and be clear. so if loud and clear is what you want then why do you need a swing kit. Can't you just turn up the modulation or add a power mic and achieve the same effect with out using a swing kit. To me a swing kit is like sitting in a nascar and revving the engine then let it go back to idle and revving it again and so on why not just set it at the speed you want and leave it there. To me that makes more sense. But I am not a tech I am a hobbyist trying to learn. So does what I say make sense or am I just to dumb to understand

 

[JoeDirt]

15minigrass:

Thanks for the flowers!

Unless that transmitter has negative peak limiting on it, those scope pictures have been doctored. The only way to exceed 100% positive peaks without RF cutoff on the negative peaks is with a negative peak limiter. All the scope pictures in the world won't change that fact. I know how to turn the carrier up to remove the RF cutoff too.

For some reason you don't seem to trust the scope as an accurate way of measuring modulation percentage. Have faith in the scope, it's your friend. If the scope says you're doing over 100% positive peaks you should start believing it. There is not a more accurate tool then the scope for measuring modulation. The two common ways are measuring the envelope or trapezoid pattern.

With respect to the differences between modulation transformers like the 29LTD uses and series pass modulator transistors like a 148GTL, the modulated pattern on the scope is identical. The only difference is carrier suppression is easier with the transistor modulator because you can adjust the unmodulated DC voltage without having to use a resistor and cap.

I'm sorry to have to disagree with you on so many points Joe but the goal here is to educate you, not pick on you. I understand you have found that you sound louder with this much swing. I use to think it was better too. Then I bought a spectrum analyzer and saw how dirty the transmitter really was set like that. The moral to the story is, too much of a good thing can easily turn into a bad swing

If a 'scope' doesn't tell you the percentage of modulation that your test radio is doing, then you are doing something wrong, or, you don't know what you are seeing. Has nothing to do with if the radio is solid state or not. Has nothing to do with the type of modulation (hi or low level).
Those facts tend to make me have doubts your whole post. Please explain how I'm wrong.
- 'Doc

A scope reads voltage so it really cant tell the difference between carrier and audio power. Since on AM we modulate to carrier and can see the audio peaks, but since the NPC will raise the carrier with the modulation there is no real way of seeing what your audio is really doing. If you guys lisened to my gate you would see that my audio does not sound like its going %250+ like my scope shows!

i definitely have no business weighing in on the scope discussion, but i do feel the need to clarify a point so that everyone is on the same page.

in general, there are two types of "swing kits" used by CB shops and on CB radios these days.
one is the cap and resistor that is added to the 29LTD type chassis.

the other "swing kit", if im not mistaken (sorry if i am), is the type that uses the rectifier diode and is used in the galaxy style radios.
this particular type of swing kit is also called the NPC mod, and does give some negative peak compression.
how much negative peak compression and what value resistor to use in series with that diode is the subject of much debate.

i also think its important for everyone involved to know whether or not JoeDirt is using the motormouthmaul device when taking his scope measurements. if that has already been cleared up and i missed it; sorry.

just trying to help,
LC

Your on the right track and NO im not using anything Motormouth Maul except for a homemade Radio Shack mic i stole off his old website.

Loosecannon, I do see where the NPC mod claims it does give negative peak compression and that is a step in the right direction. Keep in mind with Joe's 250% positive peaks and only 95% negative peaks, that's a good deal of compression for a simple diode to achieve. Even more remarkable is the fact when you look at the scope picture, the negative peaks show no trace of when that diode is conducting on them.

If the scope pictures really represent the values he claims then the NPC diode mod on the Galaxy is the perfect negative peak limiter. Something I highly doubt. It takes a little more then a diode to produce that carrier and modulated waveform. Otherwise the carrier was turned up to get rid of the RF cutoff. Perhaps you could post a video of the scope from carrier to full modulation several times.

This may help explain what you mean by "the swing kit raises the carrier with modulation". I've tried to figure out what you may have seen to make you think this. The only thing I can come up with is that after hard modulation you see the carrier at a higher point and then it settles back down.

If this swing kit really does raise the carrier with modulation in order to avoid RF cutoff, it also avoided any increase in audio normally associated with that carrier to PEP ratio. That may be why it's not distorted at those levels. Can't really tell what's going on from still pics of a scope. That would require video of various modulation conditions.

Your last 3 paragraphs you hut the nail on the head!

First off im not doctoring any pictures!

Pretty much im using the NPC diode mod for my Galaxy. All im doing is adding a diode and a variable resistor to control the negatives. I also use the factory mod limiter to control the peaks and it also acts like a compressor and the same time. Since i use the mod limiter i can achieve loud audio with no bled over or distortion! I have put a lot of time and research into this subject and if you guys want to do an experiment yourselves on a Galaxy style radio I would be more than happy so share my project with you guys! Just give me the Model or board number off your radio and I will get you some numbers.

I did a radio for a friend, he runs about 1500-2000 bird watts I can sit in his driveway and catch no bleed over on 28mhz. His radio dead keys 3-4 watts and swings 60 into a 4cx-1500.

I can post a video for you Shockwave!

 

[W5LZ]

I really am aware of what a 'scope' does. And if you know how to interpret those wave forms (can't spell 'lisiduous'), then you would be able to tell the percentage of modulation applied to a carrier. It isn't 'just' a matter of peak values as displayed.
- 'Doc

 

[Shockwave]

15minigrass:

Thanks for the flowers!





A scope reads voltage so it really cant tell the difference between carrier and audio power. Since on AM we modulate to carrier and can see the audio peaks, but since the NPC will raise the carrier with the modulation there is no real way of seeing what your audio is really doing. If you guys lisened to my gate you would see that my audio does not sound like its going %250+ like my scope shows!




Your on the right track and NO im not using anything Motormouth Maul except for a homemade Radio Shack mic i stole off his old website.




Your last 3 paragraphs you hut the nail on the head!

First off im not doctoring any pictures!

Pretty much im using the NPC diode mod for my Galaxy. All im doing is adding a diode and a variable resistor to control the negatives. I also use the factory mod limiter to control the peaks and it also acts like a compressor and the same time. Since i use the mod limiter i can achieve loud audio with no bled over or distortion! I have put a lot of time and research into this subject and if you guys want to do an experiment yourselves on a Galaxy style radio I would be more than happy so share my project with you guys! Just give me the Model or board number off your radio and I will get you some numbers.

I did a radio for a friend, he runs about 1500-2000 bird watts I can sit in his driveway and catch no bleed over on 28mhz. His radio dead keys 3-4 watts and swings 60 into a 4cx-1500.

I can post a video for you Shockwave!

Just because a scope measures RF voltage does not mean it can't tell the difference between carrier power and PEP audio power. If the PEP voltage doubles the RMS carrier into the same load impedance, power has gone up by four times on audio peaks. Simple ohms law supports this fact. You may need a little more experience working with the scope.

Can you show us video of the NPC raising the carrier with modulation? This part doesn't make sense. Modulation percentage is the difference between the carrier and audio peaks. If NPC raises the carrier with audio peaks, the difference between the two becomes less and you are not reaching 250% positive.

If that's the case then there is no real advantage to the modification since you could just set the carrier to the point that NPC raises it to in the first place. The end effect would be the same level of modulation.

 

[JoeDirt]

Just because a scope measures RF voltage does not mean it can't tell the difference between carrier power and PEP audio power. If the PEP voltage doubles the RMS carrier into the same load impedance, power has gone up by four times on audio peaks. Simple ohms law supports this fact. You may need a little more experience working with the scope.

Can you show us video of the NPC raising the carrier with modulation? This part doesn't make sense. Modulation percentage is the difference between the carrier and audio peaks. If NPC raises the carrier with audio peaks, the difference between the two becomes less and you are not reaching 250% positive.

If that's the case then there is no real advantage to the modification since you could just set the carrier to the point that NPC raises it to in the first place. The end effect would be the same level of modulation.

I cant show you the carrier raising on the scope because i don't know how? I have asked a few people and know one seems to know how? I know for a fact that my audio is nowhere near %250. I have had a few techs tell me that the NPC mods raises your carrier with your mod... how much I dont know?

Yes, your not going to get any more mod out of this modification! Swing kits get a bad name for being dirty but they don't have to be dirty! With what im doing with my radios you can get full power out and still sound like a radio that has no swing kit, or you can adjust it for that loud over mod sound!

Im working on a video on what im doing... I will have it up in a hour or so.

 

[loosecannon]

ron, yes, what you are saying does make sense.

the reason for the confusion is the terminology itself.
that is what i was trying to explain in my last post.

the ONLY reason to use the "swing mod" for the 29LTD type chassis (and most every other AM only transformer modulated CB radio) is to lower the deadkey to use a linear amp while maintaining a high swing to better modulate the amp.
(im certainly not offering an opinion on whether its a good or bad thing to do, just clarifying)

what these guys are talking about is called a "swing mod" sometimes, but is more appropriately called an NPC mod. (Negative Peak Compression)


this is a great discussion!
i hope everyone stays as cool as they have been so far, as this has been very informative so far, and i hope that continues.

the first person i know of that applied the "NPC mod" to CB radios was Billy Dean Ward who has been doing it for many many years.
maybe i can get him to come in here and offer a new viewpoint, as he certainly has a great amount of experience with this type of mod.

keep up the great posts!
LC