AM Transmit Question???

[X-Rated]

Greetings,

I have an FT-840 showing a peculiar problem and after
hours of studying the schematic and block diagram,
I seek your experience with a problem of AM transmission
*missing* the upper sideband...
That is, I have the lower sideband & carrier going out
but nothing else. This upper sideband is getting stripped
on the way out to the power stage at the 2nd IF filter.
AM RF power (at full setting) is roughly 25W but again,
only one sideband (as observed on the spectrum analyzer).
A few questions that might help me:

-This is a stock FT-840 (no CW or AM filters).
Can someone confirm proper AM xmit without
the optional AM filter (which seems to only
actuate if going into AM-narrow mode)?

-Are AM transmissions restricted to certain
bands only (I have never used AM on HF myself)?

-The upper PCB is missing D1032 (located close to
the central connector for the optional FM board).
Unlikely to have an effect but is this normal?

BTW, everything else appears to work fine with the unit.
I recently replaced a PIN diode on the receive path (just
after the input attenuator) - history of rig unknown and
therefore suspect static or improper setup. Also, I tried
to search past threads here as much as possible. Sorry if
I missed something related to this problem :)

Thanks for the wisdom!

From my Yahoo group. Seems like the transmitter would have to have filtering to take out a sideband and this is not easy to do. This guy seems to think it is a problem. I don't see it that way.

What do you guys know about SSB full carier AM on cheapie rigs? Does it happen?

[N2CHX]

From my Yahoo group. Seems like the transmitter would have to have filtering to take out a sideband and this is not easy to do. This guy seems to think it is a problem. I don't see it that way.

What do you guys know about SSB full carier AM on cheapie rigs? Does it happen?

Not very difficult and not uncommon. In broadcast, this actually almost became a standard. There was a thing called PowerSide by Kahn, which did exactly this. Because the information in both sidebands is identical, filtering one side out not only reduces the power consumed, it also saves occupied bandwidth with no loss in fidelity. Originally, PowerSide was designed to be a method of stereo transmission, with the upper and lower sidebands being modulated independently. Leonard Kahn was a genius, but unfortunately companies like Motorala had more lobbying power. PowerSide was never adopted as a standard (FWIW, Motorola's CQUAM was a better system for stereo). However, for mono applications such as ham radio, deleting a sideband has many benefits. Filtering it out is not nearly as difficult as you might think and I have no doubt many rigs employ this.

I'm guessing the method used in ham radio is simply to inject some carrier back into the already filtered SSB signal at low level. Quite practical AND efficient, and you lose nothing but gain efficiency.

[K7SGJ]

Not very difficult and not uncommon. In broadcast, this actually almost became a standard. There was a thing called PowerSide by Kahn, which did exactly this. Because the information in both sidebands is identical, filtering one side out not only reduces the power consumed, it also saves occupied bandwidth with no loss in fidelity. Originally, PowerSide was designed to be a method of stereo transmission, with the upper and lower sidebands being modulated independently. Leonard Kahn was a genius, but unfortunately companies like Motorala had more lobbying power. PowerSide was never adopted as a standard (FWIW, Motorola's CQUAM was a better system for stereo). However, for mono applications such as ham radio, deleting a sideband has many benefits. Filtering it out is not nearly as difficult as you might think and I have no doubt many rigs employ this.

I'm guessing the method used in ham radio is simply to inject some carrier back into the already filtered SSB signal at low level. Quite practical AND efficient, and you lose nothing but gain efficiency.

And you said you don't participate in the technical forums. Pffffffftttttt

[N8YX]

I'll expand on this later but most low-level-modulated, later model transceivers employ AME - exalted carrier AM - and are not noted for outstanding fidelity.

[N2CHX]

I'll expand on this later but most low-level-modulated, later model transceivers employ AME - exalted carrier AM - and are not noted for outstanding fidelity.

AME... Same as SSB with full carrier as I described above and probably use the same method I described to achieve it. Unless there's some bandwidth limiting other than suppressing one sideband or some interference to the remaining sideband, the fidelity shouldn't be affected. That said, distortion is a reality if the carrier level isn't carefully controlled, but frequency response shouldn't be affected.

[X-Rated]

I told the guy he was in luck. He was kinda pissed. He wanted both sidebands for the money. I was wondering if there was anything I was missing here. I knew CHU was USB with full carrier and they sound fine.

[WØTKX]

Tell him it doesn't matter, he'll still be heard quite well using AM on 11 meters.

[XE1/N5AL]

Tell him it doesn't matter, he'll still be heard quite well using AM on 11 meters.
But, his signal won't "swang both ways"!

[N8YX]

AME... Same as SSB with full carrier as I described above and probably use the same method I described to achieve it.
AME is partial-carrier AM, and can be derived using either the filtering or the phasing method...but it's a lot easier to do with filtering.


Unless there's some bandwidth limiting other than suppressing one sideband or some interference to the remaining sideband, the fidelity shouldn't be affected.

Pay attention to the bolded. We'll get to it in a second, as it's one of the two Roots of All Evil with this approach.


That said, distortion is a reality if the carrier level isn't carefully controlled, but frequency response shouldn't be affected.

And here's the other Root.

But let's discuss the whole Exalted vs True; low-level vs high-level bit and distill it into one easily understood word:

Cost.

High-level AM is to radiocommunications as Top Fuel is to drag racing: The more cubic dollars you pour into the machine, the faster you go...or the better you sound. Unlike racing, however, in the world of radio this approach comes at a weight or complexity penalty. Thus, the amateur equipment manufacturers adopted a couple of different approaches to the 'problem', as they saw it:

One is low-level modulation whereas the RF amplifier predriver device is modulated. You aren't going to get the "punchy" audio you will from a Class C exciter/Class B modulator but at least it's cheap - and transmits both sidebands. It also requires a somewhat expensive (relative) audio transformer.

The other is AME. The rig's existing SSB filter is used and the balanced modulator is unbalanced slightly when in "AM transmit".

Let's talk about the first of those two Roots...

Most current-day, non-DSP-IF multimode rigs which employ AME also have a fairly restrictive (~2.1KHz) SSB filter in their IF chain; one of the selling points of many of these sets is SSB receiver performance. Unfortunately, they almost always use the same filter for SSB TX...and thus, for AME TX.

Pinched audio. Pinched, communications quality audio. :yuck:

Drake did something neat with the TR7: It employed a filter board which always used the 2.3KHz SSB filter for all voice modes including its implementation of "AM". It is also very easy to make a TR7 use the accessory 6KHz AM RX filter on TX (a handful of components and two cut mainboard traces allows this to be done automagically). Carl's TR7 was thus modded and its AM carrier frequency aligned as close to "filter center" as was possible via the PBT/Ref Board adjustment. Sounds better, but...

Time for Root 2.

The unbalanced modulator scheme employs absolutely no mechanism whatsoever to ensure that applied carrier power tracks with modulation energy. The signal will be 100% modulated at one and only one carrier/sideband power regime; everywhere else it'll either sound muffled or distorted. It is left to the operator to set the mic gain and drive (carrier) power controls in a manner which doesn't result in distortion and the only true method of verifying proper operation - a station monitor scope - is probably absent from the desks of most amateurs.

I had looked into designing an analog of the Drake 'Controlled Carrier Modulation' setup for the TR7 as was utilized in the TR4 and T4X equipment but Carl ended up with the rig before I could twiddle my soldering iron. Maybe for one of the three remaining TR7s in the shack. :chin:

The other "fix" is to convert an AME transceiver to use low-level modulation. This is accomplished by installing an AF matching transformer whose secondary impedance matches that of the predriver device's collector (or plate) resistor then directing the Mic Amp output into its primary - rather than into the balanced modulator.

Since the "Monitor" and "Speech Processor" circuits of many convertible rigs are implemented somewhere in the IF stages you'll lose the functionality of both should the set be converted in this manner - and an outboard scope becomes a must-have for monitoring the modulation envelope to assure it doesn't clip. The result sounds a lot better than the AME implementation and to many people the effort is worth the trouble.

You can also directly feed the predriver AF matching transformer with an outboard EQ/compressor deck. Imagine the possibilities...

[N2CHX]

Yup, makes sense. I simplified it quite a bit, but what you're saying sounds right on. BTW, I have a double-balanced ring diode modulator I built for doing DSB and AM, which uses the same principle. It simply lacks the filtering to remove one of the sidebands. I originally built it to do DSB on the 22 meter Part 15 band. The pots control carrier balance and input level. The dip switches switch the inductors in the output network in and out for different loads and experimentation.8517

[N8YX]

...I have a double-balanced ring diode modulator I built for doing DSB and AM, which uses the same principle. It simply lacks the filtering to remove one of the sidebands. I originally built it to do DSB on the 22 meter Part 15 band. The pots control carrier balance and input level. The dip switches switch the inductors in the output network in and out for different loads and experimentation.8517

That belongs in the Homebrew and Mods forum, along with a thread detailing its construction...schematic...and encouragement for others to build it as-is modify it for use in the amateur service as a beacon transmitter.

[X-Rated]

That belongs in the Homebrew and Mods forum, along with a thread detailing its construction...schematic...and encouragement for others to build it as-is modify it for use in the amateur service as a beacon transmitter.

I remember a ham in Kansas on 75M with DSB supressed carrier when I was a kid. I think he had a HeathKit.

[KC2UGV]

That belongs in the Homebrew and Mods forum, along with a thread detailing its construction...schematic...and encouragement for others to build it as-is modify it for use in the amateur service as a beacon transmitter.

I agree. Even I could build that!

[N8YX]

I remember a ham in Kansas on 75M with DSB supressed carrier when I was a kid. I think he had a HeathKit.
I have a couple of Handbooks and at least one specialty ARRL publication which feature designs for a rig of that sort. IIRC, one was a complete transceiver.

A bit hard to find 1960s-era parts for, however. Coming up with a modern-day equivalent would be fun - but who wants to build?

[N2CHX]

That belongs in the Homebrew and Mods forum, along with a thread detailing its construction...schematic...and encouragement for others to build it as-is modify it for use in the amateur service as a beacon transmitter.

I built that a long time ago. Probably 8 years ago now. I had brought it upstairs from my old shop in the basement the other day, thinking about turning it into a project to do PSK with. If I do so, I will post the details.

[XE1/N5AL]

I remember a ham in Kansas on 75M with DSB supressed carrier when I was a kid. I think he had a HeathKit.
The SB-10 SSB adapter for the old Heathkit Apache transmitter had a switch setting for DSB.

[kb2vxa]

All very interesting, I had an SB-10 with the scratchy Apache but like the Gonset GSB-100 I had they were phasors that required nearly constant adjustment. Phil K2PG had/has a Kahn exciter in the audio rack at his West Creek, NJ site he planned to re-tune for 160M for use with the Collins 20V but never did. Being full carrier ISB the idea was suppressing a sideband to protect an adjacent frequency if need be and be the only ham with AM stereo. That would have been interesting because with two receivers one could receive stereo and get dizzy listening to it panning like Whole Lotta Love by Led Zeppelin.

Ring modulators are fun, they were the heart of the first police voice encryption, frequency inversion. A friend and I build a couple of demodulators to unscramble one when they went "yellow" which has a familiar ring from listening to the AM Gangstas on 3885. Some CB friends and I went one step further building modulators and demodulators. That drove the SSB crowd nuts, sideband with carrier we can't tune in? They went completely batty when we used SSB and OK, it's side band but we STILL can't tune it in. (;->)

[W7XF]

We, as hams, do not need broadcast quality AM. SSB with carrier sounds just fine. And, at the same time, has LESS bandwidth...isn't that the name of the game when it comes to our crappy band conditons this sunspot cycle? BTW Kelli.... nice breadboard project :)

[ka4dpo]

I'm assuming that is normal for that particular radio. One way to generate an AM signal in a Double Balanced Mixer is to unbalance one side. That explains the lower sideband and carrier. I suspect this is how Yaesu did it in that rig because it is inexpensive and doesn't require additional circuitry. It also sounds surprisingly good if the audio is not over driven. The signal from the (unbalanced) balance modulator is fed to the transmit amplifier chain. That's why it's only rated at 25 watts, more than that would kill the finals.

[K0RGR]

That belongs in the Homebrew and Mods forum, along with a thread detailing its construction...schematic...and encouragement for others to build it as-is modify it for use in the amateur service as a beacon transmitter.

It looks like that might be nice mated to the "under $50 all band SDR" in a recent QST.