Interference in FM radio reception.

Dramatic license.

If you want to sell this script, we're going to have to change some things. If you won't cooperate, I'm not going to let you know when the meeting is.

I'll bet!

Thanks. I'll let you know how it goes. Though I've lost some interest in finding the interfernce, and I'm turning my attention to finishing the script and shopping it around.

I prefer poetic license:

If you're going to sell this script, it will need drama, suspense, action, intrigue, politics, explosions, violence, a chase scene, sex, and display the advertisers products. However, the one thing that must never appear in the script is something that requires the viewer to think. If there's even the slightest hint of "how does that work?", the script won't sell. Science fiction and defective physics are perfectly acceptable, as long as the actors all pretend that it's real, and that nobody in the story questions the technology. To many people, microprocessor controlled parking lot lights, triggered by sophisticated PIR detectors, are more like magic than science.

I forgot to mention that the lights were all LED lights and VERY bright. Something like this:

The real question is whether LED lights generate RF interference. There's no consensus and plenty of opinions. My experience has been that some do, and some don't. I have several consumer LED house lights. Only one belches RFI. I haven't bothered to check which ones are boost, buck, or both.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

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I live in a small town in New Hampshire. About twenty years ago, (before we got cable into town) I suddenly started to get numerous calls from customers on one side of town complaining that TV channel 5 had just simply disappeared. My house was not affected and so this was news to me. I visited one affected house and found a grey screen with no audio. It appeared as though something was completely swamping the entire channel. We contacted the FCC in Boston and what a waste of time that was. So I drove around town visiting different homes and asking if they were also affected by this. Eventually I was able to draw a sort of "lobe" of the pattern. The pattern was somewhat directional and the radiating point appeared to be a communications tower set on private property. The gentleman that owned the tower was very proud of it and took me on a short tour of the facility. He would lease parts of this tower to different commercial services. Among these services was a radio data link to Massachusetts which operated on a frequency 200KHZ below channel five's video carrier. While in the shack I noticed a bandpass filter sitting on the shelf which was marked with his operating frequency. I asked if that shouldn't have been in line with the antenna and at that point the meeting became adversarial. I often wondered if perhaps the filter was tuned incorrectly and it's insertion into the line caused problems, so that was why he removed it, or perhaps he was overmodulating, creating excessive sidebands which poked into channel five. In any case I found it interesting though that a few days after my visit the problem mysteriously disappeared , never to return again. When I was fifteen I built my first kit, an Eico CB radio. The receiver was as wide as a barn door but it had an excellent transmitter section. We lived in an apartment building in the Bronx and most television sets of the day were built with 21MHZ IF strips. So when I keyed that transmitter no one for blocks around was able to watch channel two on their TV sets. I installed a low pass fiIter on my rig and I lost track of how many high pass filters I installed for my neighbors. I would also think about your radio's IF frequency, although on second thought that would not be limited to only certain channels. I'd really like to know if you find this thing. Please keep us informed. Lenny

Many such TV's had AFC (automagic frequency control). Give it a strong nearby carrier, and it will lock on the carrier instead of the TV signal carrier.

Nicely done. That sounds like quite a bit of work.

It had to be more than 200Khz. Channel 5 video is at 77.25MHz. The

75MHz "telemetry and radio control" band is roughly from 75 to 76MHz. It would need to be more like 2MHz below the CH5 carrier. Still, for a 6MHz wide TV signal, that's quite close if running high power. However, as I recall (and am too lazy to lookup), the highest power allowed at 75MHz is something like 1 watt. I don't think it was a filter, but rather far too much power.

Nice detective work. I once gave a visiting FCC inspector an "unofficial" tour of our mountain top radio site. I explained everything and answered many questions. In gratitude, he sent about

100 "failure to post licenses" and other administrivia violations, not to the service company, but directly to the customers. That didn't cost much in fines, but as a result, we lost a few customers. I don't give tours any more.

My guess is that he was running too much power trying to span the distance between NH and Boston. Adding the filter probably increased the loss to the point where the link failed. My guess is that it was taken off the air when the site owner discovered what was happening.

Ummm... you have it somewhat wrong. The hi pass filter goes on the 27 MHz xmitter, in order to remove any spurious rubbish from the transmit signal at 21.4MHz. A low pass filter would NOT work on the TV set as the frequency range is 54 to 800MHz and a low pass filter would block all of it. However, a 21MHz notch filter on the antenna would work wonders.

Incidentally, I helped an older friend build the same EICO CB radio. The tunable receiver was horrible, but at the time, I didn't know quality when I saw it.

Unless the front end is broadband and crude, the IF rejection of modern receivers is quite good.

For 2.4GHz, the last device that used an IF frequency was the ancient Lucent chipset and possible the early IBM wireless oddities. Literally everything for the last 10 years or so has been direct conversion, with no IF frequency. Google for "802.11 direct conversion receiver" for hundreds of examples. IF feedthrough is unlikely if there's no IF.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Argh. That's all wrong. I somehow merged the 21.4IF feedthru problem with the CB harmonic filter. The lo-pass filter should be on the CB xmitter, to reduce the 2nd harmonic that trashed Channel 2. The 54MHz high pass filter goes on TV, to keep the CB radio from generating harmonics in the tuner section.

Remind me not to post anything before my morning coffee fix.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Thanks for your comments Jeff. You're correct about the filters. I had them reversed. And the Radio museum page is close. I think the 770 was four or five channels of crystal control transmit. My rig was the 760 and I built two of them. Initially it had only one crystal for transmit. I had to cut a small window out of the back near the crystal socket so that I could change crystals. It was a pain and truly kind of short sighted on Eico's part. They also offered (I think the model was a 775) with a vibrator power supply. I eventually installed 23 crystals and a switch in a box underneath the radio to accommodate all channels. I also bought the special transformer, vibrator and other necessary components to convert my rig for mobile use. Ultimately the vibrator proved to be much too noisy and I replaced it with a blocking oscillator circuit built around a 400HZ power transformer. Probably wasn't very efficient but It It worked well, it was QUIET and it just happened to put out the 275V B+ that the radio required. I have great memories of bombing around The Bronx in 1966 with my CB and 102 inch whip antenna in my 1953 Desoto. Incidentally I went on to work for Eico as a QC technician a few years later for a short period too. Lenny

Thanks for your comments Jeff. You're correct about the filters. I had them reversed. And the Radio museum page is close. I think the 770 was four or five channels of crystal control transmit. My rig was the 760 and I built two of them. Initially it had only one crystal for transmit. I had to cut a small window out of the back near the crystal socket so that I could change crystals. It was a pain and truly kind of short sighted on Eico's part. They also offered (I think the model was a 775) with a vibrator power supply. I eventually installed 23 crystals and a switch in a box underneath the radio to accommodate all channels. I also bought the special transformer, vibrator and other necessary components to convert my rig for mobile use. Ultimately the vibrator proved to be much too noisy and I replaced it with a blocking oscillator circuit built around a 400HZ power transformer. Probably wasn't very efficient but It It worked well, it was QUIET and it just happened to put out the 275V B+ that the radio required. I have great memories of bombing around The Bronx in 1966 with my CB and 102 inch whip antenna in my 1953 Desoto. Incidentally I went on to work for Eico as a QC technician a few years later for a short period too. Lenny

Y0u had me worried there.

?-)

7

em

Sometimes I worry myself. I thought they were right the first time but I guess I am a bit foggy sometimes. Save some coffee for me too, OK? Lenny

The effect is psychological. One sip of coffee has the same effect as drinking the whole cup. Whatever the mechanism, I find it difficult to think clearly before my morning fix. Never mind that I'm also reading email, replying to email, talking on the phone, planning my day, dealing with paperwork, yacking on the ham radio, cleaning house, reading a magazine, watching TV, and assembling lunch at the same time. Simultaneously doing 10 things badly, takes less time than sequentially doing each correctly.

To error is human, and I sometimes need to reassure myself that I'm still human.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558