I installed an Inovonics 525 Modulation Monitor and Reference Receiver in the station last week. This is, I believe, currently a state of the art device. It replaces their earlier model 520. It can display modulation from a signal received over the air or from a direct line from a commercial transmitter. It's incredibly easy to set up. This is probably not something a typical Part 15'er would buy, as it's a current, commercial grade unit. But I stumbled into one that was "B" stock and heavily discounted and since I can use it to monitor the commercial AM I work for, as well as our competition, all from the convenience of my studio, I can pass it off as a work investment as well.
It comes with a loop antenna designed for mounting outside which I don't need as a simple hunk of wire is more than enough to give me the top limit of signal strength being located maybe 25-30 feet from my Procaster, although the signal does have to go through three plaster and lath walls.
I was going to write a review and decided that was far too much typing for me, so I made a video which is NEVER expected to run 10 mintes plus. But anyway, if you're interested my quick video showing how it works, etc is at:
I no longer have doubts or questions about the modulation of my transmitter, and have confirmed that the processing in the Procaster gives me asymetrical modulation. I'm peaking just at 100% negative and 125% or more on positive peaks. I'm pushing it as far as I can because I've demonstrated to myself how much of a dramatic increase in coverage I get if I'm modulating as hard as I dare. I believe the Procaster has settings that will let me push it even more, but the further you go the less dynamic range I get and I like to have SOME variance in volume! Anyway, have a look if you're interested.
I purchased it from here: http://www.bswusa.com/Modulation-Monitors-Inovonics-525-AM-Reference-Modulation-Monitor-P2982.aspx
And if you really want the skinny on how it works, etc you can download tha manual here:
http://www.inovonicsbroadcast.com/wp-content/uploads/catablog/downloads/525ManualRev1.pdf
assuming my links work, sometimes I have issues pasting them in right!
Tim in Bovey
i'm trading my delta asm-1 to a inovonics 530 off air fm mod monitor. i will hopefully be acquiring some belar mod monitor set i can leverage to a invonics 520 or 525 then i will have a complete studio.
http://pira.cz/fm_broadcast_analyzer/
Tunable across the FM broadcast band; needs an r-f voltage of about 70 µV across its 50 ohm input connector to measure most everything one needs to know about the modulation of an FM broadcast station (probably more, in the case of Part 15 FM).
Cost is 225 Euros, or about $255 at the present exchange rate (plus shipping).
There are many more options for off the air FM Modulation monitoring. Not so much for accurate AM modulating. AM is more critical as maximum modulation increases audible range. Although overmodulating on FM can make you illegal!
Tim in Bovey
...overmodulating on FM can make you illegal!
And the same for overmodulating on AM.
There is a difference in what constitutes "overmodulation" of an FM waveform from that of an AM waveform, though.
Negative-going AM peaks cannot exceed -100% modulation -- a physical limit.
FM peaks of either polarity have no physical limits -- just administrative (FCC), and hardware limits.
Are their modulation limits for FM and AM Part 15 broadcasting? I've never seen any published in any rules anywhere.
I know in commercial AM it's -100 and +125, and FM it's 100 with a smidge extra if you're using a subcarrier -- e.g. at work we get to go to 102% as we're injecting 4% for the RDS on a subcarrier.
I'm hitting 130 quite regularly here on AM. If there are limits for Part 15 use can someone point me to them?
Tim in Bovey
Are their modulation limits for FM and AM Part 15 broadcasting?
For conventional DSB, full-carrier AM -- there is the natural limit imposed by the fact that negative-going alternations of the modulating waveform cannot exceed 100% modulation. Such attempted operation will cause distortion in AM broadcast receivers tuned to that Part 15 AM station, and possibly interfere with the reception of other stations above and below the carrier frequency of that station.
There is no such natural restriction when modulating a Part 15 FM transmitter. However exceeding the ~ distortion-free deviation capability of the transmitter and/or the linear range of the circuits in the receiver tuned to that signal can produce a high amount of audio distortion at the output of the receiver, as well as interfering with FM stations above and below the carrier frequency of that Part 15 FM station.
I should have phrased that more completely. Are there any LEGAL limits to Part 15 broadcasting. e.g. the rules clearly state a licensed broadcast AM is legally limited to -100 and +125.
I'm aware of the technical limitations, but are there any legal limitations?
Tim in Bovey
...are there any legal limitations?
Indirectly, as given in FCC §§15.209(a), 15.215(c), 15.219(c), and 15.239(a)(b)(c).
In general they require the emissions from intentional radiators in the AM/FM broadcast bands to produce lower fields outside those bands than are permitted inside those bands. Such out-of-band emissions can be the result of the modulation of such transmit systems.
For example, a transmit system operating at/near 1700 kHz and modulated to +/-100% with audio frequencies exceeding 5 kHz would radiate energy on its upper sideband that would not meet the 20 dB suppression required for out-of-band emissions at and above 1705 kHz, per §15.219(c). [With +/-100% AM, the upper and lower sidebands are only 6 dB below the unmodulated carrier, regardless of the modulating frequency.]
If §15.215(b) applies, then the emissions at 1705 kHz and above would be limited to 30 µV/m at a distance of 30 meters. This is a much lower field than a system otherwise compliant with §15.219 typically produces at that distance from its antenna system, when using normal modulating bandwidths.
Modulating the carrier to greater than +/-100% would increase out-of-band emissions from the example above.
It is specified to be confined within 200 KHZ
The "out-of-band" interference from overmodulation happened to me as follows...
The audio feeding my AMT5000 transmitter was being taken from an FM stereo tuner, tuned to the frequency of my C.Crane FM transmitter, a home-brew STL (Studio Transmitter Link).
The audio quality heard on AM radio from my setup was fine, but two channels above and below my main AM frequency, a howling whistle was audible. As I tried to figure out if I was causing the whistle, I think it was Neil Radio8Z who suggested it might be the 19kHz pilot frequency passing unfiltered from my FM tuner.
Indeed that's what it was. Luckily the tuner provided a second audio output which filtered the pilot tone, for the benefit of persons wanting to record from their tuner into an analog tape recorder, where the pilot tone would beat against the bias frequency in the record electronics.
A lingering question for part 15 stations might be whether we can take advantage of the NRSC 10kHz bandwidth allowance, as opposed to a 5kHz bandwidth.
for Part 15 FM is 200 kHz as John stated. It is found in Part 15.239 a).
One often overlooked source of out of band emissions from transmitters is caused by the transmitter going into self oscillation. This can happen if the power supply is "soft" or when, in multistage units, the DC power to each stage is not properly isolated. A check on this is to use a scope to look at the DC supply voltages. Oscillations, if present, are usually readily seen.
Some designs require proper neutralization which must be adjusted to be effective in preventing this.
Neil
I just knew I had read the 200 kHz limit for FM someplace, but when I went back to the rules I couldn't find it.
Tim in Bovey