10-11-2023, 07:07 PM
HI Doug
With regards to the 5.0/2.5 kHz option I did allude to this trap but did not expand on this.
I originally did not go too far into the over deviation issue as I was only trying to address the low deviation side of things. However someone responded to my other post so I added some more to that post on this.
Many, many years ago I organized the migration of all the fire services in WA to the 2.5kHz channeling into a continuous block of channels (by jumping in very early) with all fire agencies being fitted with them in common thus ensuring cross agency communications. However I failed in my goal to have them move away from mid band 70-85 Mhz. This is because one agency refused to change bands. The reasoning behind my band change proposal was limited availability of equipment on mid band due its limited use worldwide, susceptibility to electrical interference and, poor handheld performance. In the long run I have proven to have been on the right path. They have now have migrated away from mid band. But now, instead of a nice continuous block of channels, they now have channels all over the place (dependent on where free channels could be found for that area).
Years ago there was some work carried out on the use of SSB in place of FM for mobile communications as a way to generate more channels. As frequency stability, unlike now, was an issue then, it used a pilot carrier with the transmission. However this has since been overtaken by digital modulation developments and as result it did go progress much further than the testing phase.
In my early days 15 kHz deviation was used (commercial 60kHz channeling), but when the commercial service migrated to 5 kHz (30 kHz channeling) and lot of surplus commercial was available. In time old 5kHz surplus commercial equipment becoming available and amateur FM also eventually changing to 5 kHz. Commercial services have since moved to planned split frequency and simplex blocks on all bands using 2.5 kHz deviation and 12.5 kHz channeling. UHF used 25 kHz channeling but the frequency blocks splits were rearranged so as to fit in with more commonly used international ones.
2.5 kHz deviation is about as low as it is viable for FM. Digital modulation has now however enabled better use to be made of bandwidth and this is the area where progress is being made to achieve more efficient use of spectrum. It is unlikely that in the future FM will change much if at all from where it is now.
2.5 kHz systems are slightly more sensitive than the 5 kHz ones, but are more susceptible to interference (some lower band ones even incorporate noise blankers), produce a lower S/N ratio and the audio frequency band is less.
Fortunately I have more than one communications test set so I am able to measure deviation fairly easily. However this can only be achieved with direct connections or very strong signals. It is no good for weak signals. For this I use a modified receiver setup. This is as follows:
In my case I use an IC-2720 to receive signals. I adapted a EA K7222 Noise and Distortion meter with the option to switch to a rear socket connected directly to the data port of an IC-2720. It also has a BNC socket on the rear to permit viewing this on a CRO. The audio level is preset trimmed at the N&D Meter so that 30mV corresponds to 3.0 kHz deviation. A slight amount of frequency compensation is incorporated as the data was found to be close to, but not perfectly flat in frequency response. I was chasing perfection.
Now you can do the same with any other radio that has a data port. This should not require any frequency compensation unless you are chasing perfection. Any audio level meter will do.
A normal audio line or speaker output is not suitable as in includes de-emphasis and will produce incorrect results other than on the spot audio frequency that is used to calibrate the setup. Adding a emphasis compensation circuit to counteract the de-emphasis etc of the audio stages, while possible, would be extremely hard to actually achieve successfully.
To calibrate the above system all you need is a signal with a know level of deviation. 1 kHz is usually used. The level this is corresponds to the deviation reference level being received. Deviation can then be directly related to this level eg: 50% corresponds to half that level etc. It is a linear relationship. Varying the modulating deviating frequency will give you an idea how flat the frequency response of the data port measurement setup is.
73
Igor
With regards to the 5.0/2.5 kHz option I did allude to this trap but did not expand on this.
I originally did not go too far into the over deviation issue as I was only trying to address the low deviation side of things. However someone responded to my other post so I added some more to that post on this.
Many, many years ago I organized the migration of all the fire services in WA to the 2.5kHz channeling into a continuous block of channels (by jumping in very early) with all fire agencies being fitted with them in common thus ensuring cross agency communications. However I failed in my goal to have them move away from mid band 70-85 Mhz. This is because one agency refused to change bands. The reasoning behind my band change proposal was limited availability of equipment on mid band due its limited use worldwide, susceptibility to electrical interference and, poor handheld performance. In the long run I have proven to have been on the right path. They have now have migrated away from mid band. But now, instead of a nice continuous block of channels, they now have channels all over the place (dependent on where free channels could be found for that area).
Years ago there was some work carried out on the use of SSB in place of FM for mobile communications as a way to generate more channels. As frequency stability, unlike now, was an issue then, it used a pilot carrier with the transmission. However this has since been overtaken by digital modulation developments and as result it did go progress much further than the testing phase.
In my early days 15 kHz deviation was used (commercial 60kHz channeling), but when the commercial service migrated to 5 kHz (30 kHz channeling) and lot of surplus commercial was available. In time old 5kHz surplus commercial equipment becoming available and amateur FM also eventually changing to 5 kHz. Commercial services have since moved to planned split frequency and simplex blocks on all bands using 2.5 kHz deviation and 12.5 kHz channeling. UHF used 25 kHz channeling but the frequency blocks splits were rearranged so as to fit in with more commonly used international ones.
2.5 kHz deviation is about as low as it is viable for FM. Digital modulation has now however enabled better use to be made of bandwidth and this is the area where progress is being made to achieve more efficient use of spectrum. It is unlikely that in the future FM will change much if at all from where it is now.
2.5 kHz systems are slightly more sensitive than the 5 kHz ones, but are more susceptible to interference (some lower band ones even incorporate noise blankers), produce a lower S/N ratio and the audio frequency band is less.
Fortunately I have more than one communications test set so I am able to measure deviation fairly easily. However this can only be achieved with direct connections or very strong signals. It is no good for weak signals. For this I use a modified receiver setup. This is as follows:
In my case I use an IC-2720 to receive signals. I adapted a EA K7222 Noise and Distortion meter with the option to switch to a rear socket connected directly to the data port of an IC-2720. It also has a BNC socket on the rear to permit viewing this on a CRO. The audio level is preset trimmed at the N&D Meter so that 30mV corresponds to 3.0 kHz deviation. A slight amount of frequency compensation is incorporated as the data was found to be close to, but not perfectly flat in frequency response. I was chasing perfection.
Now you can do the same with any other radio that has a data port. This should not require any frequency compensation unless you are chasing perfection. Any audio level meter will do.
A normal audio line or speaker output is not suitable as in includes de-emphasis and will produce incorrect results other than on the spot audio frequency that is used to calibrate the setup. Adding a emphasis compensation circuit to counteract the de-emphasis etc of the audio stages, while possible, would be extremely hard to actually achieve successfully.
To calibrate the above system all you need is a signal with a know level of deviation. 1 kHz is usually used. The level this is corresponds to the deviation reference level being received. Deviation can then be directly related to this level eg: 50% corresponds to half that level etc. It is a linear relationship. Varying the modulating deviating frequency will give you an idea how flat the frequency response of the data port measurement setup is.
73
Igor