What is the difference? (TX Deviation)

[Zach489]

Hello, another question here,

I am wondering what the difference is between 4.0kHz, 5.0kHz, and 2.5kHz. How would I know what to use on a specific frequency? I am also wondering how I would figure out the difference between channel spacing (20.0, 25.0 and 12.5kHz). I have heard that these two selections would possibly make transmissions clearer or less noisy, however I'm having trouble wrapping my mind around the theory. Can anyone explain it to me? Thank you!

[Bill_G]

You have to look at the license to know what deviation is authorized. Channel spacing depends in the band.

[Will]

5khz deviation is used on 25 khz channel spacing.

4 khz deviation is used on 20 and 15 khz spacing.

2.5 khz deviation is used on 12.5 khz spacing.

[RFguy]

5khz deviation is used on 25 khz channel spacing.

4 khz deviation is used on 20 and 15 khz spacing.

2.5 khz deviation is used on 12.5 khz spacing.

Good info.

Also, (as said above) check your license for the Bandwidth/Emission Type . If it's 16K0, then it is a 25 kHz space channel and should be set for 5.0 Khz devation.

if it is 11K0, then it is a 12.5 kHz space channel and should be set for 2.5 Khz devation

[escomm]

5khz deviation is used on 25 khz channel spacing.

4 khz deviation is used on 20 and 15 khz spacing.

2.5 khz deviation is used on 12.5 khz spacing.

Good info.

Also, (as said above) check your license for the Bandwidth/Emission Type . If it's 16K0, then it is a 25 kHz space channel and should be set for 5.0 Khz devation.

if it is 11K0, then it is a 12.5 kHz space channel and should be set for 2.5 Khz devation

16 is 4KHz deviation. 20 is 5KHz. 11 is 12.5KHz and 7 is TRBO TDMA and 4 is NXDN FDMA. There are a bunch of others too

[RFguy]

16 is 4KHz deviation. 20 is 5KHz. 11 is 12.5KHz and 7 is TRBO TDMA and 4 is NXDN FDMA. There are a bunch of others too

Things might be different here, but In canada, 16K0 is +/- 5 KHz deviation and used for 25 KHz spacing for UHF and 30 KHz spacing in VHF.

[escomm]

Should have known Canada would have to do it different, eh

[Wowbagger]

I'll point out again the relationship between instantaneous deviation and bandwidth.

First, the emission designator gives you the bandwidth of the signal (e.g. 12k5 is 12.5 kHz occupied bandwidth).

Instantaneous deviation is how far the maximum signal would move the carrier from nominal center. 5kHz deviation means that the instantaneous maximum signal allows would move the carrier 5kHz from the nominal center.

When you modulate a signal, it gets wider - how much wider depends upon the modulation scheme. For FM, the mathematical expression that describes the signal is called a Bessel function. The rough rule of thumb is that the bandwidth of an FM signal is twice the sum of the bandwidth of the signal and the instantaneous deviation. Voice is assumed to be 3kHz wide, so in the case of 5kHz deviation, you get 2*(3kHz + 5kHz) = 16kHz occupied bandwidth. You then want to a guard band between channels, so you would go to a 25kHz channel spacing.

[Al]

IIRC Wow, the expression that you gave is also known as the Carson's Rule bandwidth for occupied bandwidth.

[RFguy]

I'll point out again the relationship between instantaneous deviation and bandwidth.

First, the emission designator gives you the bandwidth of the signal (e.g. 12k5 is 12.5 kHz occupied bandwidth).

Instantaneous deviation is how far the maximum signal would move the carrier from nominal center. 5kHz deviation means that the instantaneous maximum signal allows would move the carrier 5kHz from the nominal center.

When you modulate a signal, it gets wider - how much wider depends upon the modulation scheme. For FM, the mathematical expression that describes the signal is called a Bessel function. The rough rule of thumb is that the bandwidth of an FM signal is twice the sum of the bandwidth of the signal and the instantaneous deviation. Voice is assumed to be 3kHz wide, so in the case of 5kHz deviation, you get 2*(3kHz + 5kHz) = 16kHz occupied bandwidth. You then want to a guard band between channels, so you would go to a 25kHz channel spacing.

Good explanation and I would agree with most items, except one point. In your calculation, you say the 3 kHz is derived from the assumption that "Voice is assumed to be 3kHz wide". Actually the 3 kHz value that is used in that calculation is that is the highest frequency of modulated signal that id transmitted. A standard 2-way radio transmit audio bandwidth is 300hz to 3 kHz.

[Al]

Carson's rule uses the highest baseband frequency to approximate the occupied bandwidth, so 3 Khz is the accepted value for Ftop.

[ASTROMODAT]

Hi Wowbagger!

My understanding is that the FCC narrowband regulations (using analog FM) allow for a peak deviation of 2.5 kHz. If we assume the same 3 kHz for the top end of the audio pass band (e.g., 300 to 3kHz), then we have:

2*(3kHz + 2.5kHz) = 11kHz occupied bandwidth. Since the channel spacing for narrowband is 12.5kHz (vice 25kHz channel spacing for today's standard FM using 5kHz peak dev), how can a channel spacing of only 12.5kHz allow for a sufficient guard band between channels?

Like Bill O'Reilly would say, "Where am I going wrong here?"

Thanks for any help on this, Wowbagger!

[Wowbagger]

2*(3kHz + 2.5kHz) = 11kHz occupied bandwidth. Since the channel spacing for narrowband is 12.5kHz (vice 25kHz channel spacing for today's standard FM using 5kHz peak dev), how can a channel spacing of only 12.5kHz allow for a sufficient guard band between channels?

That allows for a 750Hz guardband on each side, which isn't too shabby. Also, remember FM capture effect: the radio will lock onto the strongest signal. So, if the edges of an adjacent signal are already attenuated, the receiver will lock onto the main signal in most cases (yes, if you had a strong signal next door AND the signal of interest was very weak, AND the signal next door was off frequency to the maximum allowable error, AND your receiver was off in the maximum allowable error in toward the neighbor, AND the signal you were listening to were off away from you... then you are having a really bad day, radio-wise.)

Of course, that does make keeping your radios in tune, on frequency, proper deviation, with proper filtering all the more important.

Also, if I remember correctly, on narrowband FM they attenuate the higher frequencies a bit, so the actual maximum frequency of the voice signal is a bit less than 3kHz.