? An inexpensive modulation monitor for Part 15 AM use ?
Tha Dood in Poca, West Virginia, recommends finding one of the old boom-boxes that had VU meters built in... calibrate the VU meter according to an average of local licensed stations, and set your modulation by way of that VU meter.
I am guessing that such a method would work fairly well.
Maybe we can even add a VU meter to an existing AM radio and achieve the same thing. Maybe I'll try it.
I kind of liked the REA software/hardware solution that TimInBovey had previously, but was wondering what hardware only solutions existed. It doesn't have to be THAT inexpensive.
Audio VU (Volume Unit) meters respond to a defined average value of an audio waveform present in a circuit having a given impedance value (see http://en.wikipedia.org/wiki/VU_meter).
A VU meter is unsuitable to accurately measure/set the percentage of modulation of AM/FM transmitters.
Those modulation limit percentages are defined by the +/- modulation peaks of that r-f waveform -- which a VU meter does not accurately account for, or display.
Many years ago a radio engineer explained to me the difference between a VU meter and some other type of meter, one for reading average level the other for watching leading-edges of peaks or something like that, and I'd completely forgotten about it.
What type of meter is suited for watching modulation level type values?
What type of meter is suited for watching modulation level type values?'
A modulation meter.
They aren't real cheap, though.
A good way to watch the modulation is to use an oscilloscope. Almost any scope will work but if you can, get one with the XY display capability.
Used scopes can be had for little more than the shipping costs.
Basically, all one has to do is sample the RF signal from the transmitter. This can be as easy as placing a wire near the antenna and connecting it to the scope input. With a bit of experience it is easy to spot the zero bound peaks of the envelope and adjust the audio accordingly. The scope can be used to continuously monitor the signal or, as I do, use it to set the calibration of a fast responding LED bar graph indicator which is connected to the audio input of the transmitter. This obviates the need to be near the transmit antenna.
The XY display is a bit more complicated to set up but it produces a trapezoidal display. This is even better than the time based display mentioned above.
Neil
I've got a scope that was used for my tape studio and can be used for AM modulation.
It has vertical and horizontal inputs good from 10Hz to 100kHz and was used with the Big Talker Shortwave transmitter to check modulation.
My only problem is that each of my several part 15 transmitters is in a different location and there are no shelves or surfaces near any of them to set the scope.
A roll-around cart would speed things along, until the outdoor towers start going up... then it's in the mud with the scope.
Having a modulation meter would work safely indoors from a single location.
There are three ways that I know of to actually, acurately measure Part 15 AM modulation. 1. A scope. Scopes are indeed available very inexpensive - however they require more user knowledge to operate and if you want to leave something monitoring 24-7 they're not exactly handy. 2. The REA unit, which requires a laptop and will end up costing you about $450 or thereabouts, plus you need to connect it to a computer for the display. I bought this outfit and it worked perfectly and readings were spot on with mod monitors used at commercial stations I work for. 3. Buy a real, actual, radio station off air modulation monitor. The most expensive, but most accurate, easiest, and convenient. You can occasionally find a used Belar outfit, or a used Inovonics unit on ebay, etc. Still won't be cheap, several hundred dollars if you're lucky. I finally gave in and just bought one of these a few months ago: http://www.bswusa.com/Modulation-Monitors-Inovonics-525-AM-Reference-Modulation-Monitor-P2982.aspx I was lucky and got a demo unit at a reduced price through my regular contact at BSW (decades in the business I've developed some regular contacts). In addition it can be programmed for several stations so I can monitor stations I work for as well, so it counts as a work expense too.
Far as I know there is no other way to actually, acurately, realistically monitor Part 15 modulation on AM. You could use a cheaper mod monitor, like a Belar that doesn't work with signal off the air but is driven by the transmitter, but you'll lose so much signal driving the monitor (if you even HAVE enough signal to drive it) that it makes it impracticle. NO sort of audio VU meter, or anything else that comes before the output of the transmitter itself is going to give you actual modulation readings. The calibrate VU to other stations levels in the area doesn't work because the variability in radio receivers and their AVC etc makes this futile at best. You could possibly get a local engineer or someone with a real mod monitor to monitor your signal and you could set your VU levels to match when 100%, or whatever modulation you desire hits at the 0 on your VU. We do this is "real" radio so the DJ's have a clue where there audio is when they're on the air, but even then the processors are set up to prevent overdriving the transmitters, and also bring up low levels to normal. In effect, how you set whatever processing you use is what actually determines your audio drive to the transmitter and the processor output is set up so it will only give you as much as you want for whatever maximum modulation you want. Even if the studio cranks it, the processor limits the output to a maximum preset output that results in the modulation maximum you want. Same thing on the low side. A low signal is boosted to the mod level you want. Modulation is set using the output and other controls on the processing. But -- this is turning into a processor discussion !
I think when you really get down to it, you don't REALLY need to know. The "go to distortion and back off" plan gets you pretty darn close, but I just HAD to know that I was pushing it as far as I dared to get maximum range and still good sound.
I had posted a link to a video I did of the Inovonics unit a while back. Crummy video, but you get to see the unit and how it works. I think this is the link:
Tim in Bovey
Consider a perfect audio sine wave as the modulating source for an AM transmitter (graphic below).
The polarity of this waveform changes from positive to negative once per complete cycle of the sine wave.
An AM transmit system can be adjusted so that this waveform modulates the carrier to exactly 100% at the positive-going and negative-going peaks.
An AM transmitter cannot be modulated greater than 100% by negative-going peaks because the carrier cannot be reduced by more than 100% of its unmodulated value. Attempting to do so creates very high distortion in the modulated waveform, and r-f "splatter" around the carrier frequency that can interfere with other radio services.
An AM transmitter may be modulated to greater than 100% on positive-going peaks without producing splatter, but not without adding distortion to the modulated waveform.
Whichever one of those audio peaks produces negative-going modulation of the transmitter cannot exceed the value producing 100% negative modulation. The other audio peak can be differentially amplified so as to produce greater than 100% positive modulation, that is, the modulating waveform is then asymmetric.
But that asymmetric waveform no longer has the audio spectrum of the original sine wave. This, by definition, is distortion.
Asymmetric modulation of an AM transmitter does increase the average value of the demodulated waveform at the output of an AM receiver, which in effect improves the signal-to-noise ratio of that program channel. It also can increase the coverage radius of an AM station, for a given signal-to-noise ratio at the receiver.
However these improvements come at the cost of increased distortion of the transmitted waveform(s).
Rich's step-by-step tutorial in the preceeding post describing the nature of a modulated wave in its symetric and asymetric instances is very good to print and save with your station engineering documentation.
Right now I use Tim's cheap and easy suggestion of "just backing off until it's not distorted," and that has worked for years.
Also, I am not trying to extend range nor be extra loud on the dial, but having the knowledge to advise others on the subject is worthwhile.
It is tempting to have a dedicated modulation monitor at a few hundred dollars per Tim's approach, but can one monitor be used to serve four transmitters, or do we need four monitors?
A medium wave modulation monitor won't do any good for the shortwave or FM transmitters.
Is there a way of connecting a scope to an AM receiver in such a way that the modulation of each station can be viewed?
John WDCX that "How to Scope Your Modulation" link gave me the missing knowledge that limited my scope's usefulness.
You see, when a dear lady friend's dad died he left a B&K Oscilloscope behind which the family didn't know what to do with, so she gave it to me and it worked "as was" to set azimuth of tape heads which marked me as a professional.
But, the scope had lost its probes and cables, so doing RF work has been kind of trial with error.
Then one day a guy named John posted the details of a perfect probe and that old professionaility has returned.
Sailing the seas of success.
For a trapezoidal display useful to show amplitude modulation, the oscilloscope input connected to the r-f output sample of an AM transmitter will need to have a frequency response extending at least a bit beyond the AM carrier frequency.
Some older/cheaper scopes were spec'd only to a few hundred kilohertz, so you might want to check this out, Carl.
Luckily the manual was included with this gift (BK Precision 1403A) and we're looking closely at the specs...
My earlier quote on specifications may not be correct, given that the printed specs cover many aspects...
Looking for reference to frequency span...
VERTICAL AMPLIFIER
: C312P31B or 75AV31
: 10 mV/DIV or better
: DC to 5 MHz (-3dB)
: 2 Hz to 5 MHz (-3dB)
HORIZONTAL AMPLIFIER
: 300 mV/DIV or better
: DC to 250 kHz with EXT. GAIN Control set at maximum. DC to approx 40 kHz with EXT GAIN Control set at mid-range.
SWEEP CHARACTERISTICS
10 Hz to 100 kHz continuously variable in all ranges.
etc.
I do hope the quoted portions of the specs describe the features under discussion.
The scope would seem to be the best way of all, if you have a scope that's up to it. What about the mini handheld scopes, it should show waveform clipping as well as any scope, and be small size like an LED mod monitor box.
In my transmitter travels I've seen a manual for a cinema transmitter, meant for outdoor movie theater projection booths, as a rack mount unit, with an oscilloscope built into the front panel, now how's that for luxury! It's a Cini-Fi or similar named unit, and shoots a signal through the ground that reaches cars at the outdoor theater through the old speaker wiring.
I can see what you mean ArtisanRadio, the posts at Engineering Radio all are talking about the loudness of the sound, not about the range, I saw no mention of range the stations were getting, or if it improved with the 222 or not, something to test.
My theory is, you can process to fill up your carrier and keep it relatively clean, but will a station really benefit by heavy clipping to chase listeners on the fringes, when their close by listeners might not enjoy the sound quality?
A part of me has disgust for blasting, commercial stations trying to outdo each other in modulation wars, and I'm kind of sick of the square waves to tell the truth. If you're a kid broadcaster with no audio control, and finally get your DBX compressor/limiter, it can be so cool to feel like a big beefy station with loud audio like other stations, and strut your coolness in front of your friends, but like other special effects, like reverb on the mic, that can wear off after a while.