Rice Box AM Modulation

[W1QWT]

Hi,
I just got an early Christmas present from my wife. It is a Yaesu FT-897.
I was trying it out on 50.4 AM tonight with my regular 6 meter AM guys and I noticed that it has downward modulation (power goes down with modulation). Everyone said it sounded "ok fine" but
is this common with low level AM modulated rice boxes?
My TS-690 does the same thing but my old TS-140 has modulation where the power
meter goes up when I modulate it.
Usually I use my Clegg 99'er which ofcourse has upward modulation but I wanted to try this new rig. WHy do they do that?

Regards
Q

[Bacon, WA3WDR]

It could be happening because of transmitter ALC.  ALC can raise the amplifier gain when there is no modulation, so that the carrier goes to PEP level.  Then when you modulate, the ALC drops the gain so that the positive peaks are at PEP level.  In silent moments between syllables and words, the gain and carrier level go way up.

This can be cool.  In older, average-AGC receivers, it will cause the receiver AGC to drop receiver gain betwen syllables and words, which hides noise like a form of double-ended noise reduction.  On newer receivers with peak-AGC, it will cause gain to remain constant regardless of modulation, so the receiver does not compress received audio.

I call this "Reverse Carrier Control," and I think it might be a better way to run stations that use low-level AM with linear amplification.  There is an article on AM that WB5BXO John posted on his website.  I describe Reverse Carrier Control in the end part of the article.
http://www.qsl.net/wa5bxo/asyam/aam3.html

[W2VW]

Put on your Viking hat and it will clear up. But really, be sure that the rig is happy with the antenna load. ANY reflected power at all will start to dig into the power fold back loop and ruin your peaks. Only ask it to make about 20% of the ssb power AND if the microphone is peaky in any frequency range then you will overmodulate sooner. Audio polarity is another issue but you really have no way of dealing with that unless you want to futz around big time. Ask Dirk for a phase report.

[W2VW]

It could be happening because of transmitter ALC.  ALC can raise the amplifier gain when there is no modulation, so that the carrier goes to PEP level.  Then when you modulate, the ALC drops the gain so that the positive peaks are at PEP level.  In silent moments between syllables and words, the gain and carrier level go way up.

This can be cool.  In older, average-AGC receivers, it will cause the receiver AGC to drop receiver gain betwen syllables and words, which hides noise like a form of double-ended noise reduction.  On newer receivers with peak-AGC, it will cause gain to remain constant regardless of modulation, so the receiver does not compress received audio.

I call this "Reverse Carrier Control," and I think it might be a better way to run stations that use low-level AM with linear amplification.  There is an article on AM that WB5BXO John posted on his website.  I describe Reverse Carrier Control in the end part of the article.
http://www.qsl.net/wa5bxo/asyam/aam3.html

This would be great except that in practical applications the ALC has time delay and T.I.M.

[Bacon, WA3WDR]

If the TX audio is delayed by fifty milliseconds or so, leading-edge distortion can be eliminated, but the TX-RX dynamic tracking match is not going to be perfect.  I think that the toughest trick will be getting agreement on the time constant.  But given the fading and atmospheric distortion we tolerate now, I don't think people are really going to notice anything but the lower perceived noise.

I liked the noise reduction effect.  That's how I first noticed it when one station was using a newer rig on AM.

[Pete, WA2CWA]

Bacon pretty much nailed this one with his answer. We actually had the same discussion here about two years ago. The effect is controlled by the ALC circuitry and most of the current imported models use similar circuitry to do the same thing. Bacon's "reverse carrier control" phrase (or something similar) is something that should be coined by the manufacturers to describe their low level AM modulation.

Visual test: Put a SWR type meter in the antenna line and set the switch to read forward power. On AM, key the transmitter(dead carrier), meter reads upscale, as you talk, the meter will bounce downward and never jump above the dead carrier reading.

Better visual test: Hang a scope on the RF output line, i.e. Heath SB610, HO-10, any old scope with an RF detector; On AM, key the transmitter (dead carrier), adjust your vertical gain so that the upper and lower positions of the carrier fall on the scale lines(if you have them) or make a mental note where they are; Talk into the mike; the modulation envelope will dance between the upper and lower lines and not go beyond them.

This action of the modulator/ALC circuitry on the transmitted carrier is not detectable to a receiver's S-meter. It also provides much better control, with less distortion, when driving a linear amp.

The rigs that I've used over the last several years, IC-706, 706 MKIIG, 756 PRO II, 756 PRO III, and the new IC-7000 all use this modulation scheme.

[Bacon, WA3WDR]

I have called it CPAM ("Constant Peak Amplitude Modulation") and "Reverse Carrier Control."  Probably CPAM is a better term.

We always liked the idea of a big positive swing, because we thought it made us sound louder.  But in reality, the weaker signal between syllables makes us sound weaker.  The big negative swing of CPAM seems a lot better way to go, except that it does eat more supply power.  Not that much, though.

[Pete, WA2CWA]

I have called it CPAM ("Constant Peak Amplitude Modulation") and "Reverse Carrier Control."  Probably CPAM is a better term.

We always liked the idea of a big positive swing, because we thought it made us sound louder.  But in reality, the weaker signal between syllables makes us sound weaker.  The big negative swing of CPAM seems a lot better way to go, except that it does eat more supply power.  Not that much, though.

Yep. I remember your CPAM term. I think I used it when someone on eHam post a similar query on AM modulation using a 756 PRO II.

[Steve - WB3HUZ]

This action of the modulator/ALC circuitry on the transmitted carrier is not detectable to a receiver's S-meter. It also provides much better control, with less distortion, when driving a linear amp.

From what I've heard on the air from these rigs, I cannot agree with this statement.

[Pete, WA2CWA]

This action of the modulator/ALC circuitry on the transmitted carrier is not detectable to a receiver's S-meter. It also provides much better control, with less distortion, when driving a linear amp.

From what I've heard on the air from these rigs, I cannot agree with this statement.

Which rigs Steve?

[Steve - WB3HUZ]

Most notably the Icom 706. Many others seem to distort as the modulation nears 100%. I may be able to run some tests on a 706 soon. I'll post results here.

[W2VW]

Most notably the Icom 706. Many others seem to distort as the modulation nears 100%. I may be able to run some tests on a 706 soon. I'll post results here.

Bill KE1GF knows about the 706. The ones I've heard seem to be able to make the brand of AM that could be taken for an old time rig.
Some of the others sound like they need some more code written.

[Steve - WB3HUZ]

GF's 706 has been extensively modified. We were speaking of unmodified radios.

[W2VW]

GF's 706 has been extensively modified. We were speaking of unmodified radios.

I know. The point is that I don't think the IC706 uses bass ackwards carrier control from the start.

[Steve - WB3HUZ]

Cool. All I know is they sound like crap unless modified.

[Pete, WA2CWA]

Steve/Dave:
You're right; in their stock form they sound like crap on AM. And Dave, I can't remember  the modulator scheme in the 706 but it's probably not like the DSP stuff in the newer Icom's. I'll have to throw the 706 on a scope, haven't done that in several years, and see what the modulation (how the meter swings/how the waveform looks) and see if it's like the 756PRO II/III. Maybe I was dreaming.

Here's some info on AM in Icom DSP rigs:
http://www.qsl.net/ab4oj/icom/dsp_am.html