Batboard

I have read various reports about less radio coverage using narrowband. Also I have read Motorola spec sheets where the receiver spec for 12.5Khz narrow band is worse than for 20/25Khz spacing eg. 0.25uV for 12dB SINAD vs 0.28uV for 12dB SINAD.

However, in theory, a 12.5Khz narrowband receiver should be around 2-3dB more sensitive than a 20/25KHz radio - assuming that you are using narrower IF filters. In practice, I have found this is correct. Using narrowband results in much better reception of weak signals.

My understanding is that, generally :

25KHz spacing uses 15KHz IF filters
20KHz spacing uses 12.5KHz IF filters
12.5KHz spacing uses 7.5KHz IF filters

On some of my ham stuff, I can easily switch between 15KHz and 7.5KHz (sometimes actually 6KHz) IF filters and the narrow filter makes an enormous difference to improving the intelligibility of weak signals. Note however that a lot of ham stuff just uses the same wide IF filter for both wide and narrowband...but I can switch filters on my AOR 5000, Yaesu FT-817 and ICOM R1500 and the narrow IF filter makes a big difference to copying weak signals.

When I use my Motorola stuff, using narrowband always results in a hotter receiver. With my XTS3000, I can even switch in a Ultra narrow 5.76Mhz IF filter using 12.5KHz spacing...and there is a big improvement in sensitivity vs 25KHz spacing.

So why is it that Motorola (and other manufactures) always quote poorer sensitivity figures for narrowband....it should be the other way round!!!! Or is 12dB SINAD not a true measure of receiver sensitivity??

This subject has been discussed here before:
http://batboard.batlabs.com/viewtopic.p ... 40&start=0


The fundamental problem with your analysis is that it considers only the "static" case of sensitivity, i.e. a bench test type of measurement rather than considering the way signals propagate in a real world two-way radio environment. Those interested in a fully detailed analysis should study the EIA/TIA TSB-88 documents. A short analysis is contained in:

http://www.simulcastsolutions.com/userf ... _vs_wb.pdf

OK on the difference between static and faded sensitivity - interesting.

Best of both worlds - use wide 5KHz dev TX and narrow 2.5KHz RX!!! (for ham use only, of course)

My Motorola stuff will RX a 5KHz dev signal using a narrow filter no problem - no clipping or filter rattling!!

I also often increase the TX mic gain or switch in "whisper mode"(which just increases the mic gain) which helps to provide a better modulated TX signal.

I guess its a bit like shouting into the mic when you are working DX!!!! - same effect??

But I'm still wondering why narrowband 12dB SINAD sensitivity figures from the Moto spec sheets are worse than 25KHz measures..I presume a 12dB SINAD test is just a static bench measure...maybe not?

12db sinad is a hard number you can guarantee, reproduce, and publish. It is a bench test that is well understood and widely accepted.

I have now programmed my VHF XTS5000.

I'm very pleased with the analog sensitivity of the XTS5000 which compares very well to my most sensitive analog Motorola's...maybe the MCS2000 is very slightly more sensitive when trying to resolve "real audio" on a very very noisey signal...but there is not much in it.

However, the XTS5000 is definately more sensitive using 25KHz spacing vs 12.5kHz spacing...this is based on a real world modulated test signal (into a dummy load) and using the same external antenna.

The real world experience from my analog Motorola's and my XTS3000 (note 3000) have shown that as you narrow the IF filter you get better sensitivity...with the XTS3000, using the Ultra Narrow 5.76 IF filter and 12.5Khz spacing gives the best sensitivity...as you would expect. BUT with the XTS5000, 25KHz spacing is more sensitive (using the same test signal and external antenna) vs 12.5KHz spacing..why?

Maybe the XTS5000 only uses a single IF filter (for 25Khz) and the 12.5KHz IF filtering is done using DSP which involves additional signal loses with more layers of digital to analog conversion??

Any thoughts?

Did you bench test it, or are you basing this solely on earball?

I decided against just using my "static" test gear (as per xmo's advice) for measureing sensitivity and instead used my real world "dynamic " gear..yes earball!!!

My XTS5000 is great and I am very pleased with it but it definately receives a real world "dynamic" test signal better using 25KHz vs 12.5Khz...but you need to be listening to a very weak signal to notice the difference. Also, if you listen to a weak signal (with the squelch open) you get the fake white noise on a really weak signal using 12.5kHz but still get a noisey real signal using 25KHz spacing.

So why is an XTS5000 more sensitive using 25KHz spacing vs 12.5KHz spacing...when its the reverse on all my other Motorola stuff including an XTS3000...all RXing the same weak test signal..same antenna etc?

In a perfect radio - a radio that adds no noise other than thermal noise - the noise power will increase as the bandwidth of the receiver (thermal noise being "white" and thus equal energy per unit frequency). Thus, for a given bandwidth of signal, the wider the perfect radio's receive path, the more noise power received, and the lower the signal to noise ratio will be (same signal power, more noise power := lower SNR).

Now, in the real world, radio's aren't perfect. They introduce noise beyond the thermal noise - in some cases, well beyond the thermal noise. If the radio introduces a fixed amount of non-thermal-noise power, then going to a wider bandwidth will spread that non-thermal noise power over a larger chunk of spectrum, and reduce the noise per Hertz. If you don't change the signal parameters, widening the noise can make the signal to noise ratio at any given frequency larger. In fact, this is the trick behind digital radio techniques like dithering - you deliberately spread the noise energy (in this case quantization noise), then later filter off part of it, improving the signal to noise ratio.

Thanks Wowbagger. So I guess its due to the DSP!! To be honest I'm glad that the 25KHz spacing works best as the XTS5000 does not like even slightly overdeviated signals when using 12.5 KHz spacing (2.5KHz dev). About 50% of the stations I listen to sounded great but the other 50% were distorting and clipping using 12.5Khz spacing....even though the band plan is theoretically 12.5khz spacing/2.5Khz dev!!..so many hams still use 5KHz dev as it sounds "louder". All my analog Motorola's will accept a 5KHz dev signal when using 12.5Khz spacing with no problems at all..audio still sounds great. I guess the XTS5000 DSP uses "cliff edge" filtering while the analog stuff is more "sloping"

One other thing I noticed is that the RX speaker frequency responce/audio characteristics can have an impact on resolving a very weak signal.

I was "real world" testing an MCS2000 and a GM350. On initially testing, they were both as good as each other..not much to choose. But I was listening to 2 very weak ham stations in QSO...it was very difficult to resolve the audio from either stations BUT I could resolve the audio better from "station A" on the MCS2000 and "station B" on the GM350. Both stations would have been running different equipment with different TX audio characteristics. All this testing was at the very margin of intelligibility...and using the same antenna.

When you perform 12 dB SINAD testing, the signal generator is adjusted to 60% system deviation (Per TIA/EIA 603D), So for a 25 KHz system running +/- 5 KHz deviation, you set the 1000 Hz tone at +/- 3.0 KHz. For a 12.5 KHz system, the deviation is +/- 2.5 KHz, so the tone is set at +/- 1.5 KHz. This reduction in the "transmitted energy", offsets the noise figure improvement you expect to see in the receiver.

So you have two things working against each other, a receiver having theoretically more sensitivity and a transmitter producing less "information" as Claude Shannon might say.

If you get into the faded environment (TSB-88) it is about a 3 dB penalty. http://www.leikhim.com/page13.php

g8tzl2004 said:

"I have read various reports about less radio coverage using narrowband. Also I have read Motorola spec sheets where the receiver spec for 12.5Khz narrow band is worse than for 20/25Khz spacing eg. 0.25uV for 12dB SINAD vs 0.28uV for 12dB SINAD.

However, in theory, a 12.5Khz narrowband receiver should be around 2-3dB more sensitive than a 20/25KHz radio - assuming that you are using narrower IF filters. In practice, I have found this is correct. Using narrowband results in much better reception of weak signals."

It's been over 30 years since I was in radio engineering classes, but if I recall, if you decrease your FM modulation index by holding you max modulating frequency and decrease your deviation, doesn't that have an adverse effect on SNR? True, your bandwidth decreases, but so does the relative amount of energy in your sidebands?

As you decrease your modulation index much below 1, you begin to lose a great degree of the noise immunity that is a benefit of FM. So, if my old brain remembers correctly, the FM modulation index is deviation divided by the max information frequency? If we keep our max audio frequency at 3 kHz on our comm channel, and have 5 kHz deviation, our modulation index is 1.67. Dropping our deviation to 2.5 kHz gives us a mod index of 0.83.

So yes, decreasing bandwidth decreases noise, but in FM systems, decreasing bandwidth by decreasing deviation also decreases the information energy that we place in our sidebands.

Kind of over simplified, but your SNR will not improve by 3db if you cut your deviation in half, because you are losing the signal intensity too.

Have I forgot too much FM theory, or is this about right? This would explain why SNR is not better by going to narrowband FM.

Waiting for my loss of memory on this to be ridiculed... Any fresh-out-of school engineers to set me straight on this?

AL7OC wrote:As you decrease your modulation index much below 1, you begin to lose a great degree of the noise immunity that is a benefit of FM. So, if my old brain remembers correctly, the FM modulation index is deviation divided by the max information frequency? If we keep our max audio frequency at 3 kHz on our comm channel, and have 5 kHz deviation, our modulation index is 1.67. Dropping our deviation to 2.5 kHz gives us a mod index of 0.83.

I think you are right on with all of this. I am almost 50 years out of engineering school but I don't think the laws of physics have been repealed since then LOL.

We were taught that the advantages of FM over AM are a function of the modulation index. As the modulation index is decresed to around 1 or less, the FM advantage disappears.

I started in 2-way work when 40F3, 15 kHz deviation and 5 kHz audio, was standard and the difference between AM and FM was spectacular with the quieting and crystal clear audio. The original "narrow banding" to 16F3 was a noticable degradation but still fine for communications use and better quality than AM.

The present narrow band FM has really noticable degradation in SNR and I hear complaints from users that capture effect has also been impacted. The comments are that on a system with co-channel interference, distant signals that used to be overcome by local users now result in just an unreadble mess. I beleive theory also says capture effect is a function of modulation index.

Wowbagger's point about the noise added by the receiver circuits is also very relevant to the real world situation.

The bottom line seems to be that FM is so degraded at low modulation index that Digital in the better alternative in most cases.

True; the capture ratio is degraded as well. Yes digital starts looking attractive when you are forced to narrowband. You will have improved range, though arguably the voice quality is not the same. Motorola would call this result of "Fait Accompli", or "a done deal - money in the bank" if you are an insider!

there is no argument about voice quality, sorry

digital is inferior to analog and that is all there is to it and nothing will ever change that. you cannot overcome the inherent losses of a conversion to 1s and 0s and back again, especially on such low throughput with dsp and other software tricks

One thing to keep in mind with SNR. Going from wide to narrow didn't change the amount of noise on a frequency only the deviation of the radios communicating on the frequency changed. Since you are listening twice as hard for hanf as much deviation, the ratio changed. Filters did get narrowed but not narrow to the point it cuts off at 3 KC or anything that drastic. A current production Motorola radio will stop receiving at about 4.5KC in narrow band mode Where before it was around 10KC in wideband. Point is that since the radio changed, the SNR measurement changed. And remember that although SNR is an industry standard test, published numbers are meant more to compare radio X with radio Y. Try looking at SNR (12 db sinad) vs BER some time... the numbers are different, but measure the same basic thing