The AM Forum

Any idea why the modulation sidebands would not be symmetrical? The Valiant has a matched pair of 6146's for the modulator. The upper sideband is decidedly lower in strength than the lower sideband. Only on 40m have I noticed this. Should I swap the tube or B+ lead positions, or do I have something else at work here? The radio is buttoned up and running nicely on 80m and 20m. I understand that on 40m the VFO is prone to issues as it is at the fundamental of the transmitter. Everything is screwed down tight like its supposed to be.

One thinks they can enjoy the scenery for awhile and then something else pops up. Whichever one of you bozos convinced me to get a tube transmitter is on the FIRST KILL list.  :o

Seriously though, having fun and it looks great in the shack.

Might be FMing,  especially if the VFO's fundamental in on 40M.

How does the sidebands look like on other bands.

Looks fine everywhere else. I found a posting right after I posted this. Apparently, someone's Ranger 2 did it as well, but not with a crystal.

If it's doing what I think it's doing, there's some kind of phase shifting going on that is causing the lower sideband to somewhat cancel out, I've heard of some others saying they've had the same problem with other kinds of transmitters, they all had one sideband that was either weaker or in one case, not even there (USB with carrier from a plate modulated rig). Usually it seems like it always happens on a band where the VFO is running straight through. It's kinda like the concept behind how the old Kahn AM stereo method worked, something must be causing a slight amount of phase modulation, combined with a slight phase shift of the audio, which is causing one of the sidebands to cancel (in Kahn's system, by giving the proper phase shifts to the stereo sum and stereo difference audio feeds, then phase modulating the L-R onto the carrier, and feeding the L+R into the transmitter's modulator, it was possible to get the right side audio to appear on the upper sideband and the left side audio to appear on the lower sideband).

Most likely it's FMing.  OK with a crystal, OK on other bands.  FMing is the only thing left.  It would make the upper and lower sidebands look different on a AM trace.

Yep, look at the link below, then scroll down to "Detecting Incidental FM":

http://www.cliftonlaboratories.com/am_modulation.htm

W8JI has looked into the Johnson VFO's, and here is what he has to say:

http://www.w8ji.com/johnson_vfo_chirp_jump.htm

With the advent of SDR receivers, some of the highly desired BA AM transmitters are showing varying amounts of side-band imbalance. My Gonset G-76 on 15M with the internal VFO has a 30db side-band imbalance.  ???

Jim
WD5JKO

Interesting. Talked with some guys on the upper end of 40 last night, and didn't think it had any issues. Its just something, like your wisdom teeth, that you don't really think about, I would venture to guess.

I've cured the chirp issue "JI" talks about. Very simply fix and mine had it.

FM that follows the voice, not the FM that results by a modulator PS being shared with the VFO that has no regulation.  You will not hear that "pulling effect" because it is following the voice resulting in incidental FM.  That results in uneven sidebands. Here a paper from Agilent that explains it.

Al

However, I did put the Beast in its cabinet last night as well as enough cabinet screws to establish a Faraday cage. Since I did this, I figure that any latent major problems will surface shortly. I term this act of brazen defiance "Tempting The Fates".

I find the WAAAA-UUUUMPHHH!!! that the transmitter makes when keyed very, very satisfying. So, I can understand the almost visceral appeal of these old AM warhorses.

Have had several 40m qsos with the Beast, and no one has complained. Someone with a pro setup said I was seeing the unequal sidebands correctly. So this will be relegated to the "things to look at in the future" tasker list.

Running around 390 mils to take some strain off the screens. It modulates 100% with the D104.

Philip

That characteristic whomp makes the Valiant seem a lot more burly than it really is  ;D  My GPT-750 barely makes a click when keyed.

My postings may have made me somewhat of a minor ham household name, eg. the time and a half I'd had fixing the Valiant. Seems like every other QSO I have on AM lately, I get asked..."Hey, are you the guy who's been posting on AMfone about the Valiant"?

Almost afraid to answer in the affirmative, but the replies have always been nice.

Right. FM with modulation index lower than the modulation frequency, for instance modulating with 1 kHz audio and 300 Hz delta f produces a spectrum that is similar to AM but sidebands (at 1 kHz from the carrier in this case) have opposite polarity. AM has sidebands with the same polarity. Therefore the right amount of FM over an AM modulated signal can produce AM with unequal sidebands. It is nearly impossible to see this effect on the envelope.
The envelope of single sideband AM looks identical to the envelope of DSB AM.
Technically SSB AM is better than DSB AM. It requires half the spectrum and sounds the same after a traditional AM demodulator.
Now it may require a SSB like generation chain (remeber the old Collins KWM2 AM mod).
Maybe combined AM plate modulated generation and a "touch" of FM could become a new topic for us to reduce frequency occupation.

Giorgio

This doesn't really sound applicable, and I believe simultaneous AM/FM is illegal. In any case, the FMing is a technical fault on 40m only, and may or may not have related to one of the big HV filter caps cooking off.

In related news, replaced the HV filter caps (blew up one as I had it in backwards) with a pair of Xicon 450V, 100 uF. No effect on the unequal sidebands. Swapped tubes. Also no effect.

Other than RF circulating currents, I have no other ideas on how to solve this issue, or whether I should even bother. On air reports on the 40M band are favorable, ie "good audio".

Should we move on? One doesn't really notice the lopsided sidebands unless you are listening/looking at me with a Flex or other radio with a panadapter.

In regards to Johnson VFO's...I had the VFO in a Ranger Chirping on 40m CW
for years. After extensive rebuilding of the rig, still the Chirp, ONLY WHEN IN THE CABINET!
out of the cabinet it was clean...finally I was working on a friend's Valiant one
day and noticed the coupling shaft between the exciter & the cap was non-conductive...
I pondered this and looking on the spectrum analyzer, the Valiant was clean on all bands...

In short order, I pulled my Ranger, replaced the shaft between both caps with non-conductive
material...put it back in the cabinet...no more chirp...clean on all bands...
go figure...
73
Steve

Speaking of Johnson VFOs and shafts, how is the "hot melt glue" coupler fix holding up?

Thanks & 73,
Jack

Steve,
Check out W8JI's analysis on the Johnson VFO issue in relationship to ground loops.

http://www.w8ji.com/johnson_vfo_chirp_jump.htm

73,
Joe, W3GMS

The repair seems to be holding. I'll likely end up redoing it with some plastic tubing. The reduction mechanism needs reworking with new grease. It seems to be holding calibration reasonably well.

AME (SSB with carrier) has around 35% distortion on a diode detector at 100% modulation.

Which stands to reason: it would be identical to a two-tone SSB test with equal tones. The envelope is not a sine wave; it is extremely pinched at the negative peak.

With AME, you also lose the diversity advantage of having two sidebands. That can be a very great advantage at times, as sync detector and PowerSDR users well know.

73,

Kevin, WB4AIO.

I am wondering if there is a way to harness some incidental FM in a plate modulated transmitter? For example, if you are in a QSO on 3880 AM, and then a sideband QSO starts up on 3878. All of a sudden the receiving station has to tighten up the receiver bandwidth, and tune to your upper sideband. So in this situation, if the transmitting AM station could move more audio power to the USB, then communication capability is enhanced. Conversely, going on the offense, if the AM station boosts his LSB energy enough, maybe the 3878 sideband station will just go away, or perhaps move down to 3875... :D

Perhaps the RF PA neutralizing capacitor could be tweaked either side of a feedthrough power null to create the desired effect since this would change the phase between a lead and a lag of the RF feedback. Then there is the audio phase variable.

To recap, if a plate modulated transmitter can have unequal sidebands due to incidental FM, is there a way to control the depth of the sideband energy lower to upper ratio, and is there a way to select which sideband, lower or higher, is the strongest? If so, this could be a valuable tool, perhaps a highly refined version of SBE.

Jim
WD5JKO

First a clarification:

FMing, for the reasons already discussed here and elsewhere, will cause the sidebands to be unequal. However, to get partial cancellation/enhancement of one of the two sidebands, over the range of audio frequencies associated with the modulation, one would have to modulate the phase of the RF signal with the Hilbert transform of the audio signal that is already modulating the amplitude.

At any particular audio modulation frequency (e.g. when the modulation is a sine wave) FMing will result in the enhancement of one of the sidebands and the reduction of the other sideband. However the effect is smaller at higher modulating frequencies (actually inversely proportional to the modulating frequency), and therefore not useful for approximating SSB + carrier.

A way to see this is to note that using the modulating waveform, m(t), to modulate the frequency is the same as using the integral of m(u)du to modulate the phase. But the integral of m(u)du is not the same as the Hilbert transform of m(t). In the frequency domain, the integral of m(u)du is 1/(j x 2pi x f)) x M(f); but the Hilbert transform of m(u)du is j x M(f). The sign (I.e. j = -1/j) is not a problem, but the extra term: 1/(2pi x f) is a problem.


This is well known, and easy to prove using algebra and trigonometric identities, plus the approximations for sin(x) and cos(x) when x is small compared to 1.

The good news is that it is easy to introduce small amounts of phase modulation in the path between the VFO and the AM modulated RF output stage. Performing the Hilbert transform on the modulating signal, to obtain the input to the phase modulator, can be done with a classical 90 degree phase shifting network or by using modern DSP methods.

The bad news is that this approach (simultaneous AM and phase modulation) to enhancing one of the sidebands with respect to the other sideband will introduce large distortion products (unless the modulation index is very small, which would defeat the purpose intended here).

Whether this is a useful way of fighting QRM... versus the alternatives... including switching to another frequency... is not clear.

Stu

When one finds unequal sidebands in an AM broadcast op, it usually means the antenna system is too narrow-banded, henceforth overly reactive either going up or down in frequency.

In other words the sidebands check in, but they can never leave :P.

73DG

DG:

Find out how many geigers you have with that counter?

Also, I note that most of the unequal-ness is in the bandwidth below about 300-400Hz in the audio. After that, it starts to pick up and get more or less even. So, perhaps, for whatever reason it is, we gets lots of FMing in the low end. In which case, its NOT that big a deal, since most of the audio power for intelligibility is around 2 KC. Now, why it does it only on 40m, is anyone's guess.

I can't believe you're quoting math I haven't seen in years. I'll have to pull out my network analysis book and relearn Laplace transforms. Of course, Abe Lincoln's favorite equation was taking the indefinite integral of the reciprocal of cabin. The answer is log (cabin).

The waaa-uumph is probably the power supply / transformer recovering from the overload to go into transmit.
The table top transmitters were nice in their day. And had serious limitations, except for the Collins TX's. 32V1 was a brute transmitter!!! And no waaa-uuumph.......hi
Fred
Fred

 


You are right. I made a mistake to recall the old mod to kwm2 without checking the mathematics.
Anyway by locally demodulating the rf output of an AM transmitter and using this signal to fm or to pm the vfo it is clearly possible to enhance one sideband or the other. This is an interesting starting point for reducing band occupation of an old tube transmitter. Still not very convinced but it looks interesting...

http://en.m.wikipedia.org/wiki/Beat_(acoustics) (http://en.m.wikipedia.org/wiki/Beat_(acoustics))

Here we have the pspice model of a phase modulated sine generator:
http://pspiceprograms.blogspot.it/2008/07/single-frequency-frequency-modulation.html
They say frequency modulation, but the model describes a phase modulation.

With PSPICE I simulated the AM modulation (100%) of a phase modulated carrier: with M=0.8 we have a good suppression of one sideband if the two modulating signals are 90 degrees out of phase.

As FM and PM are related by a derivative of the modulating signal:
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part12/page1.html

AM modulating an FM modulated signal may produce the approximate suppression of one sideband with the two modulating signals in phase or out of phase, without 90 degrees phasing filters.
With PM, with the 90 deg shifter, and with constant PM modulation index, the unwanted sideband suppression does not depend on the modulating frequency (that is good but the 90 deg audio filter is required).
Instead with FM, without the 90 deg audio shifter and with constant FM modulation index, the unwanted sideband suppression depends on the modulating frequency.

In addition we have residual sidebands from FMing the carrier.
In conclusion this system of sideband control is very rough, does not provide a clean spectrum, etc, but seems to work. It is not perfect indeed...
I have not studied in detail. No time to spend for it.

Therefore we may have a perfect AM (100%) envelope and one sideband reduction (or suppression), but we need more spectrum (FM related spectrum) to compensate for it. Interesting but useless, I suppose ;-)