AMT3000 & 5000 Modulation

This is an interesting way to compare or set the transmitters. I didn't quite follow the description of the display. Usually, the carrier remains constant in amplitude and the sidebands appear on each side of the carrier with modulation. I am guessing that the analyzer resolution was not narrow enough to separate the sidebands from the carrier so it appeared that the carrier amplitude was "jumping".

It is a surprise to many that in the AM spectrum the carrier amplitude is constant with modulation and in FM the carrier amplitude varies with modulation. This seems counter intuitive but this is how it works.

Neil

Yes, this particular spectrum instrument has two bandwidth settings: 20kHz and 400kHz.

Of course the 20kHz is suited for AM signal obervation, and the signal peaks show no side-to-side swing at all, only up and down.

If I switch to 400kHz on the medium wave band it's like zooming in too close to the picture and seeing about 20 stations in one glob.

Looking at an FM station with 400kHz bandwidth the signals shake back and forth like poplar trees in an earthquake.

On FM I can see the HD signals side-to-side of the carriers that have them, and they look like tall shrubs being shaked vigorously to loosen the fruit.

So what you're saying is, shaking your head "yes" is frequency modulation, and shaking your head "no" is amplitude modulation(?)

A fairly expensive spectrum analyzer is needed to resolve the sidebands of an AM signal separately from their carrier wave.

If the analyzer is capable of doing so, and is used as required to do so, then barring abnormalities and as Neil states, a double-sideband AM signal will show a constant carrier amplitude on that analyzer display screen, as it is modulated.

During modulation, two additional r-f signals will be present in the output spectrum of the transmitter.  One appears below the carrier frequency, and one above the carrier frequency.

The frequency separation of those sidebands from the carrier is a function of the modulating frequency.  The amplitude of each of those sidebands with respect to the amplitude of the carrier is a function of the percentage of amplitude modulation.

An example of this spectrum display is shown in Figure 18 at the link below.

http://www.anritsu.com/en-GB/Media-Room/Newsletters/files/guidetospectrumanalyzers.pdf

The link to a tour-guide of what a serious spectrum analyzer can show is very fascinating and appreciated.

At least this "economy" analyzer shows more than the part 15 operator has without it.

Thanks for the appreciation you posted, Carl.

The Part 15 hobby is interesting not only for playing radio with microphones and music, but also for the technical exploration.

One area that I really want to explore is the controlling of the RF signal in two ways: observing the performance characteristics through measurements and scope displays, and remote controlling as many functions as possible from a handy central location.

This can lead to endless building projects and experimentation.

This morning I am running a novel string quartet arrangement of a Bach organ work, and watching the spectrum analyzer show AM 1550 (AMT3000) and AM 1680 (AMT5000) jumping up and down in unison.

This is like radio on television, and stimulates thinking.

First, I did a re-run of my favorite imaginary conversation in which a beautiful woman sent by the FCC would tell me that I can't broadcast because I don't have a license.

Then I would say, "Ok then, I'd like to apply for a license!"

And she's say, "You can't get a license."

And I would trail off with, "But..."

The subject would change and I would ask, "Who do you work for?"

And she'd reply, "The government, of course."

And I'd say, "Ya but, which one?"

At this point the Bach ended and I started to make breakfast.

Bruce, The Dog Radio Group

Just me thinking as I type (while trying to do both) that there are a couple of approaches to remote monitoring. One is examination of the radiated signal. This can be as simple as listening to it on a receiver and could extend to measuring the FS (done here) and perhaps the frequency.

The other is to monitor the current into the transmitter (also done here)since the source is indoors and is easy to instrument. This monitoring, though gross, does indicate when something has changed and is sensitive enough to indicate rain detuning the antenna.

I had previously thought, and so did 12Vman if I recall correctly, that it would be nice to be able to constantly monitor the antenna tuning with the ability to remotely change it as needed but nine months' experience with the outdoor system shows that, except for transitory weather effects, there is no change in tuning so monitoring and/or adjustment is not worth the effort. Given that, it still would be neat to have the capability.

Neil

For the time being my main "window" into the AM medium wave band is a spectrum analyzer, and I keep an ongoing record of the readings of peak levels on my three AM frequencies.

Those peak levels do go up and down and I have yet to become sensitive enough to comprehend the cause of these changes.

Also, each transmitter has its own measurements: the AMT5000 can be fine-tuned for exactly 100mW to the final RF stage and "1-turn" past peak for E-Class effeciency; the AMT3000 for maximum Q into the loading coil by arriving at 13VDC output; and the LPB2-20 carrier current unit for SWR (minimum standing wave ratio) and the particular Wattage power being fed through the coupler to the electric line.

It's a project always growing and the programming is a whole other universe.