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How I would like to change the cell phone industry [was AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency]
DTC hath wroth:
Jeff Liebermann wrote: If you're thinking of impulse noise, you're mostly correct. However, there are plenty of other sources of AM noise available. For example, the typical VHF aircraft radio requires substantial filtering of the magneto to avoid hash. Same with any onboard motor. If you've ever tried to install a TV (VSB is a form of AM) in a vehicle, you'll also find that ignition and motor noise can be a problem. Dang it...my memory is slipping. What did Motorola call their noise filtering circuit on their old low band Motracs. Extenders[tm]? As I recall it was a simple noise blanker. "Extender". It was a 2nd almost identical receiver, tuned to a nearby empty frequency. If there was any impulse (ignition) noise, both receivers would detect the pulses. The 2nd (Extender) receiver would block the IF signal in the main receiver for the duration of the pulse. This resulted in a "hole" in the receive IF and audio, but it was far less noticeable than if the pulse were allowed to be heard. The tricky part of the design was getting the delays nearly identical in the two receivers. It also made the 80D/140D/Motrac/Motran radios rather huge and heavy. Extenders were considered a "standard option" on most Low Band (30-50Mhz) radios as this is where the ignition noise is the worst. The more generic term "noise blanker" applies to this scheme, as well as a mess of others that detect in a single receiver or blank in the IF or audio. GE decided that "extender" was a good name for their mobile repeater, and called it a "mobile extender" or more commonly just "extender". http://www.mbay.net/~wb6nvh/chpradio.htm Egads. I'm cleaning house and found a large box of 40 year old Motrash control heads and cables. Want some junk? There was also a scheme to eliminate ignition noise that involved running a wire to the points on the distributor. The assumption was that there was a substantial delay between when the points opened, and when the spark jumped in the spark plug. This allowed the receiver to be blanked before the noise pulse arrived, which really improved the noise blanker performance. I was working on the design when marketing decided that it should tilt at other windmills. Only a few prototypes were built, were never patented or produced, and worked really quite well. Cheaper too. That was all just fine because cars were begining to use electronic ignition systems, which didn't have easily accessible ignition points. -- Jeff Liebermann 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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