Intermodulation

[Michael P Clark]

I am hoping someone might be able to clarify the intermodulation specs that Lectrosonics gives with it's wireless systems. Is it only an issue when blocks are next to eachother(ie. block 21 and 22, block 26 and 27)? or would it encompass ALL the blocks in a bag(ie 21, 26, 27)? I have one in block 21, two in block 26, and one in block 27. When coordinating frequencies among these blocks, am I only limited to the actually presets that Lectro has in their Frequency Chart? This seems to narrow the frequencies available down to only 8 usable frequencies in a block. Only 32 available according to this concept, negating the other 224 available frequencies. I am sure I am not understanding this concept fully, so please enlighten me! How can I figure out which frequencies play nice with eachother, if I am in an area where there is alot of RF pushing my choices outside of Lectro's Frequency Chart?

Mike

[Philip Perkins]

I am hoping someone might be able to clarify the intermodulation specs that Lectrosonics gives with it's wireless systems. Is it only an issue when blocks are next to eachother(ie. block 21 and 22, block 26 and 27)? or would it encompass ALL the blocks in a bag(ie 21, 26, 27)? I have one in block 21, two in block 26, and one in block 27. When coordinating frequencies among these blocks, am I only limited to the actually presets that Lectro has in their Frequency Chart? This seems to narrow the frequencies available down to only 8 usable frequencies in a block. Only 32 available according to this concept, negating the other 224 available frequencies. I am sure I am not understanding this concept fully, so please enlighten me! How can I figure out which frequencies play nice with eachother, if I am in an area where there is alot of RF pushing my choices outside of Lectro's Frequency Chart?

Mike

My policy with this is to always try it out and see what really works in that place at that time.  Where I live a lot of times frequencies that are supposedly occupied will be useable enough in certain areas, and others that are supposedly open will have something on them.

Philip Perkins

[David Terry]

Mike,

“Intermod” interference occurs when the fundamental frequency or the harmonics one RF system beats against another system and causing the rf waves to interfere with or cancel each other.  As the number of RF mics in a system increases, so does the probability of inter-modulation interference. Symptomatically, one would notice decreased range of the radio mic, but you would most likely not hear the other ”interfering” RF channel.

Since your mics cover several blocks, I suggest that you phone Lectro and ask for two or three sets of compatible frequencies.  I have three sets of frequencies for my block 27 mics.

Lectro does us a great service by publishing the intermod tables, but the tables list a limited number of frequencies.  I wish that Lectro would publish the formula that they use for calculation of compatible frequencies. An even more elegant solution would be to embed their formula in a web page that would contain a compatible frequency calculator.  The user could enter a frequency, and the calculator could generate a list of compatible freqs.

Everyone should coordinate the frequencies in his own bag (or cart).  When I had only two 205s their range was excellent, but when I added the third and fourth unit to the rack, the four units barely functioned.  A single call to Lectro for compatible frequencies made all of the radios work extremely well.  Recently I replaced the rack of 205 receivers with Venue VRTs.  The spectrum analyzer can give the user a hint about intermod problems as well as other frequencies in the two blocks that Venue sees.

If you need a more elegant and accurate answer, ask Larry Fisher at Lectrosonics after NAB.

David Terry

[Michael P Clark]

Thanks guys. I'll be giving them a call before NAB, I hope! It's good to know that there are more combinations than what they show on there cards. An expanded version for all combinations, for anywhere in the US would be nice. 

Thanks

Mike

[SoundHound]

RF Intermod and Frequency compatability is an art not a science.  The number of combinations that make Intermod goes up geometrically (very FAST) as more transmitters are added.  Strange things happen!  Not the least of which is that there are, typically, other sources of, environmental, RFI to spice up the soup.  Nothing works better than actual testing with some alternates for a back-up. Remember the RFI Intermod intensity varies with the square of the distance between transmitters.  So a little extra room between transmitters can do you a lot of good.