DMR Vs. NXDN

[Ubbe]

You must always use different multiples of TX frequency differencies on a site or you will have a 100% risk of getting trouble with the receivers at the same site and sometimes on other nearby sites.

/Ubbe

 

[mmckenna]

I have heard the same. If using data, 12.5KHz is the way to go, if only using voice, 6.25KHz is fine.

12.5KHz NXDN (Narrow) gives you 9.6kb/s, before you take away for the headers and the like.
6.25KHz NXDN (Very Narrow) is 4.8kb/s before headers, etc.

So, Yeah narrow for the data capacity.

 

[mmckenna]

I believe it requires a very sable time source to keep the transmitters and receivers rock stable.

The Kenwood repeaters have an option for a high stability OCXO to do this. For trunking systems, you only need one, and can loop the signal through the other repeaters. Gets to be more important with 6.25KHz channels.
Or, you can run an external 10KHz GPS based source.

 

[AD0TY]

I was implying that I heard it was possible, not that it was practical.

 

[speedway_navigator]

an external 10KHz GPS based source.

I thought it was 10MHz.

A guy I worked with was called in to help out on a simulcast paging system that had problems. He did a facetime chat with the local tech while looking at the spec an and thought the xmtr timing was off. The tech said it was plugged into a GPS reference, so they switched to the GPS being used by the public safety trunk system and that cleared up the problem.

Never assume nothing.

 

[mmckenna]

I thought it was 10MHz.

It is 10MHz, typo on my part.

 

[speedway_navigator]

Why do GPS receivers have a 1 PPS output?

Interesting post on OCXO ( which I never heard of, I'm from TCXO days) and GPS timing.
Even after 40+ years in 2 way I learn stuff on this site every week.

 

[slicerwizard]

His advice was to keep using narrow rather than very narrow on any system using data. I believe that includes transmission of over the air alias.

It does not.

Just to note though, if you stack two 6.25 kHz channels right next to one another in 12.5 kHz of spectrum, you will have a 3rd order intermod product 6.25 kHz away from your high channel. So it seems you could can't practically space three right next to one another.

Doesn't seem to be an issue for the Toronto site: https://www.radioreference.com/apps/db/?sid=8070

 

[Ubbe]

Oh-oh! They also have 4 channels evenly spaced at 100KHz from each other. Put that towers frequencies in an intermod calculator and all goes red.

/Ubbe

 

[domes]

Licensing evenly spaced trunking channels is standard FCC practice. The FCC started that practice when they first opened 800MHz trunking in the1980's. 800MHz Licenses were issued in 5 ch blocks, evenly spaced by 1MHz with 45MHz between TX & RX. That allowed the manufacturers to make standardized 5ch combiners with a standard receiver filter, coupler & bottom end pre amp. They also made a standard duplexer with 5 MHz windows so you could connect the 5 transmitters & 5 receivers onto one antenna. The upper PS trunked systems are licensed in 5ch evenly spaced blocks. The UHF T-bands that are available in some parts of the country are issued in evenly spaced 5 ch blocks. You don't see evenly spaced in the 450-470 band because of the legacy of haphazard coordination of single channel repeaters before trunking was permitted on 450. I would think that most UHF trunking operators, like myself, would prefer standardized, evenly spaced channels because it makes it easier to combine and control intermod and other RF noise prblems.

Reading thru some of the above comments, there seems to be some major misunderstandings about intermod, and licensing channels. Any 2 (or more) transmitters can cause IM. That's just the nature of radio. You can calculate it on paper but that doesn't mean it actually happens in the real world. The most common and most offensive IM product comes from 2 X A - B = C. Double tx freq A and subtract tx freq B = IM freq C. The C product on paper doesn't mean the world comes to an end. IM doesn't just happen in thin air. A & B have to mix in some device, typically in C's receiver or in transmitter A or B or their individual or shared antenna systems. If you don't happen to have a repeater that receives on C then it isn't a problem, and if you do, good duplexers & antennas with quality spec repeaters, instead of cheap mobiles butted together, may be enough to prevent the problem. At sites with multiple repeaters it is good practice to put all the receivers on one antenna with filtering that blocks out the transmits. Transmitter combiners do a much better job than duplexers at attenuating IM and harmonic products, however the 2A-B product may still be a problem if A & B are on same combiner so it is usually necessary to separate the A & B transmitters on different combiners/antennas.

Combining repeaters is not really as big a deal as some would like you to believe. Because of the wide separation of TX to RX freqs in the upper bands, IM seldom falls in the RX band. It can be more of a potential problem in the UHF bands with closer TX/RX separation and the various splits that are interwoven. If you do the math you will see that close spaced TX freqs are not likely to produce IM products that fall in the RX range. TX freqs that are randomly spaced far apart are more likely to produce hits in the RX range. Wide transmitter modulation causes stronger IM products than narrower. Wide receivers are more susceptible to receiving and mixing offending transmitter frequencies..

 

[domes]

DMR, NXDN/6.25 and NXDN/12.5 all have the same "voice compression" and all use the same device, AMBE+2 Vocoder that compresses voice to 2400 BPS. All 3 also use the same technique of 4-level FSK-frequency shift keying to transmit ones & zeros in binary code to send the 2400 voice bits plus other bits necessary for radio functions & features. All 3 use data compression. DMR also compresses data by time, NXDN does not. Analog comparisons of bandwidth, modulation & over the air "fidelity" do not apply to digital systems.

Even though all 3 transmit data with 4FSK they use entirely different technologies to "modulate" and "demodulate" the data. DMR is a past century, wideband technology that was developed in the 1980's. DMR technology is bandwidth dependent. NXDN/6.25 is not bandwidth dependent.