soundspider Posted April 6, 2011 Share Posted April 6, 2011 Hi, Had a strange thing today and couldn't find anything on the tinterweb about it so thought I'd just ask (apologies if this is stupid) - I had a cheapie Kam VHF radio mic on 175.0MHz that was picked up by a UHF receiver (also Kam, now that I think about it) on 864.3MHz, and made all sorts of funny noises, pop, buzzes etc. To put it in context, it also bled into other VHF mics at 174.1MHz and 174.5MHz, which I wasn't as surprised about it cos I figured the filtering was just rubbish, but was quite surprised about it reaching UHF receivers. Any thought? Thanks, Alan Link to comment Share on other sites More sharing options...
Brian Posted April 6, 2011 Share Posted April 6, 2011 864.3MHz is around the 5th harmonic of 175MHz so interference is very likely from a poorly transmitter. Harmonics from distortion usually occurs at odd multiples of the base frequency; the power generated decreasing as the frequency goes up. Link to comment Share on other sites More sharing options...
soundspider Posted April 6, 2011 Author Share Posted April 6, 2011 Is that intermodulation then? Ta for your help - it is a pretty poor transmitter! Alan Link to comment Share on other sites More sharing options...
Joe Mitchell Posted April 7, 2011 Share Posted April 7, 2011 Sounds like the KAM is labeled up wrong or something. There is absolutely no way any VHF radio mics should ever interfere with UHF. 175MHz to 863MHz is as far apart as the UK and Australia :-) Link to comment Share on other sites More sharing options...
themadhippy Posted April 7, 2011 Share Posted April 7, 2011 here is absolutely no way any VHF radio mics should ever interfere with UHF. 175MHz to 863MHz is as far apart as the UK and Australia realy? so 27MHz to 750MHZ would be like here to the moon,and yet a cb radio I owned years ago could be heard perfectly clearly on next doors telly Link to comment Share on other sites More sharing options...
Brian Posted April 7, 2011 Share Posted April 7, 2011 175MHz to 863MHz is as far apart as the UK and Australia :-)As I pointed out above it's only a factor of 5 which isn't much in harmonic terms. Many vhf radio mic start with a local oscillator based around a crystal, often running around 6-7Mhz. This then gets fed into a series of three triplers to end up with something around 175Mhz. A tripler is nothing more than something that distorts the incoming signal and then filtering the result to leave the desired third harmonic. However, as well as the wanted third harmonic you'll get odd-order harmonic all the way from DC to light. If your filtering is not up to the job then you'll be generating all sorts of frequencies. Link to comment Share on other sites More sharing options...
paulears Posted April 7, 2011 Share Posted April 7, 2011 Brian is quite right - and Joe missed it - VHF equipment has the capability to interfere with UHF, and in nearly every case I've come across, proximity and quality of filtering is the usual problem. When VHF was still popular, many people used the extended frequencies, up to about 216MHz - and there is significant output from a typical lav pack at 4 times the nominal frequency of the transmitter. However, it's low enough to not cause problems as soon as you move it just a few feet away - harmonics are always filtered out, but some filtering is better than others - and in older kit, better filters are more expensive and worse still, bigger! You mustn;t discount harmonics - because very often intermod problems are when harmonics from one piece of kit interact with another. All sorts of nasty maths - sums and differences to calculate. However - interference is thankfully quite rare in the harmonically induced manner UNLESS the VHF kit has been tweaked. It was very common, working in a seaside in the summer, for the DJ to pull his Shure or Trantec (same thing) VHF transmitter apart and have a tweak - the nominal 50mW or so could be perked up quite a bit, and ranges of 400-500m are easy to generate - at the expense of filtering - sproggies springing up all over the place, in band and out! We'd frequently find club DJs outside our venue, talking to punters and their voice coming out of a sea front pub seconds later! Joe mentions 173 and 863 being a long way apart, but the first harmonic of 173 comes in at 346, and double that and we're up to 692 - add 173, and we're quite close - so our harmonics can easily couple to produce at least something at our working frequency - and although the strong UHF one usually wins, if they walk into one of those dead spots, the VHF nasty one is perfectly capable of lifting the squelch. Link to comment Share on other sites More sharing options...
ramdram Posted April 7, 2011 Share Posted April 7, 2011 If fading memory serves correctly stuff could sometimes get in on the IF, so for those interested: http://en.wikipedia.org/wiki/Intermediate_frequency Link to comment Share on other sites More sharing options...
pjb304 Posted April 7, 2011 Share Posted April 7, 2011 I've had a similar experience but the other way around. It was with a VHF radio mic and a PC with a CPU speed of around 1.6GHz - I can't remember the exact figures. If the PC was turned off the radio mic behaved perfectly, turn the PC on and you got interference. I'd forgotten all about it until reading this topic, but it sounds like it was exactly the same problem. I think the solution which we used in the end was just move the PC further away, unfortunately I wasn't that involved in the maintenance of the church sound system at that time so I don't know the full details. It's nice to now understand the details of what was going on a bit better though :) Link to comment Share on other sites More sharing options...
paulears Posted April 7, 2011 Share Posted April 7, 2011 http://earsmedia.co.uk/radioharm.jpg Here's the proof. I dug out one of my old Trantec 3500s on 216.100MHz, and the harmonic is on 864.4MHz - a Sennheiser G1 tuned to 864.3MHz has a 6 bar signal close in, and at 12 feet away it had dropped to 3 bars, vanishing from the display at around 20 feet or so. Link to comment Share on other sites More sharing options...
Joe Mitchell Posted April 7, 2011 Share Posted April 7, 2011 Yes you are absolutely right Brian. I didn't make it very clear my answer. I guess I was trying to politely say I've yet to come across a problem like this with half decent radio mic systems in the last twenty five years. That isn't to say it can't and never will happen. I've had the same thing occur on old CB Radio Bands back in the 80's so I have actually experienced this problem just not with decent kit... So far :-)175MHz to 863MHz is as far apart as the UK and Australia :-)As I pointed out above it's only a factor of 5 which isn't much in harmonic terms. Many vhf radio mic start with a local oscillator based around a crystal, often running around 6-7Mhz. This then gets fed into a series of three triplers to end up with something around 175Mhz. A tripler is nothing more than something that distorts the incoming signal and then filtering the result to leave the desired third harmonic. However, as well as the wanted third harmonic you'll get odd-order harmonic all the way from DC to light. If your filtering is not up to the job then you'll be generating all sorts of frequencies. I stand corrected by those far more technically qualified than I... The picture of the VHF Trantec transmitter and Sennheiser EW G1 receiver picking up the signal even within a few feet is something I have never ever come across though I do understand harmonics and knew they got very close but I have genuinely never had that happen and I use those same UHF and VHF frequencies together virtually every week. I have one of those old Trantec systems (sold to me as a Beyerdynamic product) and I also have a G1 receiver and will have a play before it gets traded in just for fun. I'll try the same experiment with all Sennheiser kit and see what happens for my own curiosity. Thanks very much for the lesson Brian/Paul. I knew in the back of my mind it possibly could happen but with the power output of the gear we use (not that power output is a real factor especially just a few feet away) I've just never had this issue happen to me ever. JoeBrian is quite right - and Joe missed it - VHF equipment has the capability to interfere with UHF, and in nearly every case I've come across, proximity and quality of filtering is the usual problem. When VHF was still popular, many people used the extended frequencies, up to about 216MHz - and there is significant output from a typical lav pack at 4 times the nominal frequency of the transmitter. However, it's low enough to not cause problems as soon as you move it just a few feet away - harmonics are always filtered out, but some filtering is better than others - and in older kit, better filters are more expensive and worse still, bigger! You mustn;t discount harmonics - because very often intermod problems are when harmonics from one piece of kit interact with another. All sorts of nasty maths - sums and differences to calculate. However - interference is thankfully quite rare in the harmonically induced manner UNLESS the VHF kit has been tweaked. It was very common, working in a seaside in the summer, for the DJ to pull his Shure or Trantec (same thing) VHF transmitter apart and have a tweak - the nominal 50mW or so could be perked up quite a bit, and ranges of 400-500m are easy to generate - at the expense of filtering - sproggies springing up all over the place, in band and out! We'd frequently find club DJs outside our venue, talking to punters and their voice coming out of a sea front pub seconds later! Joe mentions 173 and 863 being a long way apart, but the first harmonic of 173 comes in at 346, and double that and we're up to 692 - add 173, and we're quite close - so our harmonics can easily couple to produce at least something at our working frequency - and although the strong UHF one usually wins, if they walk into one of those dead spots, the VHF nasty one is perfectly capable of lifting the squelch. Link to comment Share on other sites More sharing options...
cedd Posted April 7, 2011 Share Posted April 7, 2011 A useful tool in these instances is an intermod calculator. I've used PRF intermod calculator before with good results. You'll see very quickly that as others have suggested, it's the odd harmonics you're looking for. One of the reasons why harmonic-rich sources are such bad causes of interference - electrical arcing is a very good example. If you're picking up interference in a venue, I'd always go looking for the dodgy emergency light first - happened on more than one occasion. It's dead simple software, but does the job. Ah yes triplers - varactor diodes if I remember rightly? They run very hot on large transmitters, good place to go looking for a fault if you ever need to repair an instrument landing system...... And intermod's don't only happen within your equipment. They just need a stage of non-linear gain to get going. We've seen them in antennae, antenna connectors, rusty mountings, the works. You can go looking for them if you've got another receiver that has a signal strength meter on it. Don't be tempted to go looking for the strongest signal, instead using a dipole aerial, use the null that occurs at the end of the antenna to look for where the interference vanishes. In other words, point the aerial directly at different things and when your level drops, you're probably pointing at the source. Dirty fix, but I've succesfully found sources on more than one occasion, and Ofcom still use this method occasionally when looking for interference (it was an ofcom inspector that taught me it!). Link to comment Share on other sites More sharing options...
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