Wireless - What is Maximum deviation range +-75kHz?

[felixm]

Ok this was listed as a specification of a really cheep VHF wireless, here and I don't ever remember seeing it anywhere else and if someone could tell me what it is, if it is really a spec. after all.

I have copied the rest of the specs below

• Features:
  • Dual signal VHF high band
  • Frequency Range 109-120MHz
  • Maximum deviation range ±75kHz
  • Frequency Response 100Hz-10kHz
  • Signal to noise ratio >80 dB
  • Modulation mode FM
  • THD <1%
  • Audio dynamic range >80dB
  • Range 50 feet

[Derrick]

Deviation is a term used to describe the amount which a signal moves about a center frequency in an FM transmitter. Maximun Deviation +/- 75Kc simply means that is it's limits of usable MODULATION. You can, in most cases, drive it harder, but that usually results in noise being generated outside the desirable limits of the FM system you are using. And like anything overdriven it will become distorted.

Example: let's say you have a transmitter on 174.000 MHz (megahertz) if you properly modulate (drive) it, the signal would actually deviate 75Kc ABOVE 174.000 and 75Kc BELOW 174.000 (Kc = Kilocycles commonly refered today as kilohertz, same thing)

So we now have a signal that occupies a portion of radio spectrum from 173.925 to 174.075 MHz. This is considered wideband FM. This is the same deviation that commercial FM broadcast stations are suppose to limit themselves to. But that's another story...

Now, What can we do with this wideband FM signal? Well if you have a matching FM wideband receiver, you will be able to send some pretty decent audio wirelessly from point A to point B. However, try and listen to it on a narrowband receiver, (scanner etc...) it will sound like trash. Why? The narrowband receiver is typically limited by filters to a maximum ability to receive +/- 5Kc. +/- 5kc 0r +/- 2.5Kc which is normally used in Fm two-way radio. This is what is deemed needed for proper transfer of voice grade analog two-way radio communications.

But wait!, we want a HIGHER quality signal from our wireless microphone or instrument than just two-way radio quality right? How do we get it? Well like anything else, to get more takes more. Takes more what? BANDWIDTH. We have gone from +/- 5Kc of radio spectrum bandwidth to +/- 75Kc. Now we can transfer a much higher quality signal. Remember though, you gotta have a matching wideband receiver.


I just went back and looked again at the specs you posted. This particular unit seems to operate from 109-120 MHz. If this is correct, DO NOT USE this unit in the US (or other areas, depending) I will double check and correct if needed. The radio spectrum this unit covers is in the aeronautical band. I'm pretty sure this is NOT FCC type accepted for us in the US on the frequencies listed and likewise depending on the governing authority in your area if outside the US.

OK, I clicked on your HERE link and I do not suggest spending your money on this unit if you are in the US looking to purchase this unit, without getting further information. Call Papa and ask'em if this unit is FCC type accepted for use in the US. If they say yes, ask them for the FCC acceptance ID number (It'll be on the FCC sticker). Then post that number here for me, please.


Hope this helps. I'll be watching for any questions and further information.

[felixm]

I had no plans to buy or use this thing. It was something that someone I know found on Craigs list and wondered what I thought of it and I told him not to buy it. But I was wondering about the spec, I have never seen in a manual for Shure, Sennheiser or any other mid-line UHF wirelesses.

Thanks for the quick response.

[Derrick]

You're welcome. And tell your buddy not to pull the trigger on this one.

[mbenonis]

Very nice explanation, Derrick. And yeah, the 109-120 Mc frequency spec raised my eyebrows a bit too...

PS - Want to add Deviation to the glossary for us?

[Derrick]

Thank you for the suggestion, I would have not thought to do that. I've added it.

[Blah067]

Originally Posted by Derrick

I just went back and looked again at the specs you posted. This particular unit seems to operate from 109-120 MHz. If this is correct, DO NOT USE this unit in the US (or other areas, depending) I will double check and correct if needed. The radio spectrum this unit covers is in the aeronautical band. I'm pretty sure this is NOT FCC type accepted for us in the US on the frequencies listed and likewise depending on the governing authority in your area if outside the US.

OK, I clicked on your HERE link and I do not suggest spending your money on this unit if you are in the US looking to purchase this unit, without getting further information. Call Papa and ask'em if this unit is FCC type accepted for use in the US. If they say yes, ask them for the FCC acceptance ID number (It'll be on the FCC sticker). Then post that number here for me, please.

You are correct, if this device actually operates in the 109-120 MHz range, it's operating in a range (in the US) reserved for "AERONAUTICAL
RADIONAVIGATION". So yeah...don't use this in the US....

P.S.-Even though I already knew what deviation was, I have to say that was a very good explanation! Thanks!

[Chris15]

108 - 117.975 MHz is allocated in all ITU regions for Aeronautical Radiolocation and 117.975 - 137 MHz is allocated in all regions for Aeronautical Mobile Use.

Ergo one of 2 things is true, they either meant to type 209 - 220 MHz or they have a product that has the potential to play havoc with air trafiic globally...

[Blah067]

Originally Posted by felixm

But I was wondering about the spec, I have never seen in a manual for Shure, Sennheiser or any other mid-line UHF wirelesses.

Thanks for the quick response.

While I won't speak on behalf of any Sennheiser gear, I did find this in two manuals for Shure gear I have off hand...

ULX Series...
Modulation
±38 kHz deviation compressor-expander system with pre- and de-emphasis.

UHF-R Series...
Modulation
FM (45 kHz max. deviation), compander system with pre- and
de-emphasis

[FMEng]

When the transmitter and receiver are designed as a system, there is no reason for the end user to care what the deviation is. Companding plays a huge part in wireless mics, which means that audio performance is not as directly related to the amount of deviation or frequency modulation used. In other words, you could not draw any conclusions from the deviation specs of two models with different companding schemes.

All the user needs to know is whether the system is operating within proper parameters, which is why they put audio level meters on them.