Well, you're in somewhat over my head (and years doing this), but I believe the issue is somewhat clouded by terminology that really means the same thing... RF mixes can happen in things you'd NEVER expect them to, and it'll bite you sooner or later if you play with RF. I think I understand both your points about the terminology and Jim's "definition" of passive intermod. That puts me ahead of about 99.99999% of the population out there who just wants to know why they can't yet get HDTV at a reasonble price, why their cell phones drop unexpectedly for seemingly "no reason", and gives me just enough knowledge to be dangerous so I can do the math and figure out if I'm causing horrible mixes with all the transmitters I play with! (GRIN) As I mentioned, I don't do RF stuff professionally but you and the other "RF Jim" can certainly continue the terminology discussion off-line... (Jim's not on the PICList.) Looking forward to more when you have time, though... I'm a sucker for learning new things! (Then I always find out there's more to learn... dang! When will I know it all?! hahaha... yeah right!) Nate On Wed, 2002-09-04 at 11:02, Jim wrote: > ----- Original Message ----- > From: "Nate Duehr" > To: > Sent: Tuesday, September 03, 2002 4:55 PM > Subject: Re: [EE:] What am I doing wrong? (RF amp) > > > > Hey Jim... that was a REALLY good question! (I sat and scratched my > > head on it for a minute and then asked someone smarter than I.) > > > > I e-mailed a friend that does RF interference work for a living and got > > this response, which seems to indicate that "passive intermod" is > > anything that produces intermod mixes that is not happening in an > > "active" RF device, like an amplifier, pre-amp, mixer, etc. > > I think we are coming full circle on something that at one > time was rather well-defined. That 'something' is this term > "intermod". Intermod was something one experienced in the > city or near paging sites or near TV and FM transmission > towers. It created terrible gibbersish in our receivers > and was, literally, the product of two or more carriers > overloading some stage of the receiver (could have been > the RF amp, the mixer stage *or* quite possibly one of the > IF stages, but usually not). > > And "Intermod", of course, involves mixing. > > The term "intermod" was meant to be a shortcut term for the > full term "inter-modulation", that is, the "inter-modulation" > of two carriers or signals one onto the other, usually > as a result of the usual distortion and/or maximum signal > handling capability of amplifiers. This is actualy a > well-known process, both in terms of the device physics and > mathematically - no 'magic' involved. > > But, it seems, the term "intermod" has now been transformed > into something *new*, with new meanings, meanings that are > covered by other what *used* to be well-known terms, like > 'mixing'. > > When it comes to "passive" devices mixing, their are devices > which are passive and *do* mix - it doesn't just take an > active device like a transistor to 'mix' - > > - and I am referring now to what are termed "single-ended > mixers" using a single diode (and this diode could be a tube, > a silicon or GaAs PN junction or a schotkty junction) up > to "balanced mixers" using a pair of devices all the way up > to diode ring mixers - "doubly-balanced mixers". > > > *NOW* we seem to be using the term "intermod" to descibe those > former situations where disimilar metal contact (rusted bolts > or tower sections) resulted in "the rectification of RF" and > there result is "mixing" (the so-called "detection" process > results in both a DC term plus a *rich* RF spectrum!!!!) and > noise generation ... but I would not be prone to term this a > case of 'passive intermod'. > > We used to call that a case of "poor bonding", allowing poorly > connected tower sections to creak and create noise! > > More on this later, time permitting ... > > RF Jim > > > > > > > He describes it better than I do... *and* he's another "RF Jim"! (GRIN) > > > > [I got the joy of hunting for four hours on a hot airport ramp for a > > squawking Aircraft ELT (Emergency Locator Transmitter, for those reading > > along, I'm sure Jim's up-to-speed on that TLA...)... Hangars make for > > lovely "RF chambers" for the RF to bounce around in and off of... we had > > the Airport folks open one hangar and nope... it was the hangar right > > straight across from it. The activation button for the ELT on the > > aircraft's panel is no more than an inch away from the headset jacks... > > and the instructor who gave the owner a checkride this morning punched > > the button with his thumb (no guard over the button either... GRRRR.) > > while pulling out his headset cord... thus making the emergency > > frequency (VHF GUARD, 121.5 AM) useless around Denver for four hours > > today...] > > > > Sorry, tangent there... here's Jim's reply... > > > > (Oh wait... another tangent!) By the way, his comments about "rusty > > bolts on a tower" isn't funny... we've got a repeater out here that > > crackles nicely and buzzes with a mix of a local TV transmitter that > > isn't very far away when the tower's wet or the wind's blowing... > > > > Been trying to find exactly where that mix is happening all summer... > > soon the snow will fall and we won't be able to get to the site without > > a Sno-Cat or snowmobile, but some changes to the TV transmitter (new > > tower, new antenna) have lowered the intensity of the mix to where it > > only opens the squelch circuit on the repeater now maybe once a month... > > and since the mix INCLUDES our own transmitter, even CTCSS tone > > requirements on the receiver don't help... RF can be such a pain > > sometimes... (GRIN) > > > > Nate > > > > ----------------- > > > > Your friend asks a good question. An active device is one that is just > > that active in the sense of RF generation. AN example of an active > > device is a power amplifier. Pas have specifications intermod testing. > > Obviously, multi-tone power amps must take this into account as they > > provide a wonderful platform to mix AND amplify the product at the same > > time. Another active device would be a pre-amp. > > > > Passive devices like filters, connectors and antennas, that do not > > amplify or change the fundamental carriers are passive devices. > > > > A mixer is an active device. As it takes two signal and produces a third > > product. This is why mixers have IP3 specifications, the third order > > intercept point, it is a pretty good indication of how much signal a > > mixer can take without producing big intermod products. > > > > Let me know if this stuff is not clear. Intermod can be created in any > > non-linear device that passes RF. From connectors, to filters, to rusty > > bolts on a tower. > > > > I love it! > > > > James A. Pierson Jr. > > Sales & Application Engineer > > Interference & Spectrum Management Products > > Summitek Instruments > > - A Smiths Group Company > > 12503 E. Euclid Dr. Suite 10 > > Englewood, CO. 80111 > > 303-768-8080 > > (F) 303-768-8181 > > www.Summitekinstruments.com > > > > > > On Tue, 2002-09-03 at 08:09, Jim wrote: > > > > "Passive intermod: > > > > > > What is the official definition of "Passive intermod" > > > and where is it normally experienced? > > > > > > I am familiar with 'intermodulation' in general, whether > > > it is seen in an Icom 2-AT 2-Meter handheld (it was a radio > > > that was especially prone to "I-mod") receiver or when two > > > nearby transmitters fire up and "intermod" is created in > > > one or the other's RF Power Amplifier stage by the prescence > > > of the two signals, or - > > > > > > - "intermodulation distortion" as seen in audio amplifiers (whether > > > they are power amps or not) due to the interaction of two or more > > > signals in a stage which is not precisely "linear" in operation. > > > > > > (This term 'passive intermod' seems to be a term that was birthed > > > of late.) > > > > > > RF Jim > > > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads