More FM02 learning
First ("Let me make this perfectly clear" [RMN]) - thanks to ukewarrior for all the work in hosting the group buys for the FM02 and mini-test-antenna and Shure power regulators. Nothing in it for him but the pleasure in helping others.
I've been struggling to get my FM transmission working as I wanted, and have been following the many threads about antenna design, antenna placement, power supply hum prevention, pin 19 220pF capacitor pilot frequency tuning, etc. My experiences are related, but different, so I'm starting this thread (sharing my learning and perhaps learning more from others) so as not to detract from the other ongoing discussions.
Antenna design and placement: I'm using the simple separated coax dipole design many others are using. I'm not getting the distance or sound quality I expected (expected=2-to-4 blocks; actual=1/2 block). The antenna is so simple I don't know how it could be bad. There is no dead short between sides as measured by my DVM (~10MOhm). I know the speed of light in a vacuum (and even adjusted for speed of light in atmosphere...) and at 92.9 MHz half-wavelength is about 65.5". I have my dipole in PVC pipe suspended horizontally inside the back side of the house above a window. From others' comments ("I have it leaning against the wall in my basement", "I have it tacked to the rafters in my shed", "I taped the wire to my window"...) I didn't think I needed to go through the trouble of finding a way to get the antenna outside the house. Is that foolish? Does putting it on the outside of the house make that much of a difference? Is it better if it's closer to the natural ground plane (the earth), stuck on a pole in the yard? Do I really need to put it on the roof? I've also intentionally kept the lead coax length to a minimum, about 5'. I thought that would be the next most valuable thing I could do to get all the 50mW of the FM02 out to the antenna. I don't have a SWR meter, and although I see that RS has them for about $50, I'm getting nickel-and-dimed to the poorhouse with this project. If the SWR reading was bad, that would tell me what the problem is, but not how to fix it. How to I fix, or simplify two wires heading in opposite directions?
Pilot frequency capacitor: From the schematic and board layout ukewarrior posted it is clear that pin 19 is not connected to anything. It is clear that the "near pad" for the unused SMD capacitor location near that pin is connected to some other trace, not clear what. However, the "far pad" for that location looks to be connected to ground. I'll be trying out adding the 220pF cap to between pin 19 (direct solder) and using the far pad as a ground point. I'll post results later. I am currently experiencing what I think others have called the "hiss" or "sssshhhhhh" symptom.
Audio connection: Well, this is the real surprise to me. I have spent at least 50 hours battling my audio connection. I originally had a wireless connection planned (more below) but had to give in and use a wired connection. What I found was that whenever I plugged the FM02 into my PC headphone jack (front panel) my PC sound system would go crazy. Looking at the PC audio control panel, I'd see that it was disabling/removing every audio recording source and then restoring it about every second. Not only did that render the audio system useless, but you can guess the impact on the rest of the machine. I tried lots of testing and found that it was specific to plugging the FM02 into that jack - nothing else caused that problem. I reloade drivers, finally forced myself to go through the dreaded OS upgrade from the much-maligned Windows Vista to Windows7(64), tried different jacks, connectors, cables... nothing mattered. The FM02 caused the problem no matter what. Finally, after reading one of the other threads (tried to find it again for reference but can't quite get the one post) I got an idea. Frankly, I thought all the concern about "RF traveling upstream" was a problem of small likelihood or impact. Nonetheless, when all else failed, I tried what someone else had recommended (what an amazing concept) and put a snape choke on the audio input line just before entering the FM02 enclosure. It's just a big magnet, and the audio line is shielded, and how could any RF (or other) energy get there anyway? Bottom line: the sound system disruptions stopped. I couldn't believe it, so took the choke off, and the problems returned. I tried reducing the number of turns of wire in the choke: one is not enough, two is marginal, three resulted in one lock-up, four have been running for about 5 hours with no problem. So that leads me back to my antenna design and placement. The dipole is so simple, I don't know how it could be bad. I understand that there is buffering/filtering missing from the FM02 design that might prevent this (but it doesn't have it) but why this issue? My sound system might be more sensitive than others, I guess. My antenna is about 3 feet from the FM02, and the audio cable is 3 feel long. Should I take the additional loss of a longer feed coax length and get the antenna away from the FM02?
Using bluetooth with synchronized audio sources: Finally, my "great idea" that I had to abandon was a design for untethered (wireless) connection to the FM02 FM transmitter. I don't want to have any cable from my show computer (in my home office) to any other room in the house, or the attic, or worst of all, out an open window to the outside. I found a Belkin Bluetooth audio receiver (meant for iPod/auto use), and a Bluetooth Class-1 USB dongle transmitter. The plan was to have the show computer send the audio out the USB Bluetooth dongle, to the Belkin Bluetooth audio receiver, and the output of that (audio) driving the audio input of the FM02. I know Bluetooth is limited range, somewhat line-of-sight, but thought it would work through one wall or 10' or so. I thought it was an elegant design to get the audio signal to the attic or perhaps just outside the window. As it turns out, the problem wasn't with distance. I have over 100 hours of effort fussing with settings, drivers, configuration, etc, and finally found a way to direct audio to the Bluetooth USB dongle AND to an internal line that my show software (home developed, not Vixen) could also listen to. After all that, I found that there is an apparent latency in the Bluetooth stack so that my video/light control was about 0.5. second ahead of the sound through the Bluetooth channel. That was very distracting and would not be acceptable. I imagine there is some audio stream buffering happening but can't find a way to reduce it. So for ErnieHorning (or Idunbenhad, or dirknerkle, I can't precisely remember) that used a similar setup to get Christmas music out to the yard during setup, it would work fine. However, with the sync issue, it's no good.
Finally ("That's all I have to say about that." [Forrest Gump]) that's what I have to share. If anyone takes the time to read through this all ("I would have written a shorter, but didn't have the time." [Blaise Pascal]) I hope there's value here for you. If anyone can educate me on the suspected, and dreaded "RF energy traveling upstream on the audio input" issue I'd be grateful. Do I need to get the antenna further from the FM02? Do I really need a SWR meter? Also, do I really need to string coax out the window and put my dipole in the yard to get signal out for a few blocks?
Thanks.
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