APX Automatic Frequency Control ( AFC )

[chrismol1]

AFC Disable​

This field causes the radio Automatic Frequency Control (AFC) algorithm to be disabled.
WARNING: The value in this field must only be modified by Qualified Service Personnel. Improper settings can cause unpredictable results in the radio.

AFC automatically corrects drifting of the radio's Reference Oscillator (RO) by resolving the frequency of the received RF signal and providing feedback on an error signal proportional to the degree to which the RO is mistuned, which in turn reduces the tuning error. However, it may be beneficial to disable AFC for noisy environments, where the error signal may track to an interference frequency instead of the allocated one.

\IMPORTANT: When AFC is Disabled, the Reference Oscillator must be kept accurately tuned. This selection applies for all Trunking Systems and therefore to all Trunking Personalities.
Accessed Only: When the radio is model/option capable.

Do you radio managers ever had any discussion about this?

 

[RFI-EMI-GUY]

Are those subscriber unit settings? I would leave the AFC alone, especially on 7/800 MHz systems.

My experience with AFC on the repeater side goes back to MICOR UHF community repeaters on the then Sears Tower. We had performance problems on a particular repeater (25 chan spacing back then) and when i looked into it ,

I discovered analog medical telemetry subcarriers on 12.5 KHz offset channels. These from some hospital in or near Chicago. Apparently HP made a bedside telemetry system with UHF transmitters at 50 milliwatts. This was plenty of signal to be heard on our receive antenna way up on a pylon. The solution was to disable AFC in the repeater receiver.

This could still be a problem today, for GMRS repeaters using MICOR due to FRS assignments.

 

[riveter]

In my experience, AFC has been a very powerful asset... mitigated only by how much of a crutch it can be in allowing users with seriously out-of-spec oscillators to stay in the field and think everything is fine (because it kind of IS fine for so long). Some things that finally come in for maintenance with "yeah it takes forever to connect to the system, it's been like that for months" are so far off frequency it's astonishing that AFC was even able to divine the adjacent presence of the control channel at all.

 

[prcguy]

AFC was fine in the days of 25KHz channel spacing, like maybe through the 1980s. It has no place with today's narrow spacing and much better frequency stability.

 

[Echo4Thirty]

AFC was fine in the days of 25KHz channel spacing, like maybe through the 1980s. It has no place with today's narrow spacing and much better frequency stability.

Unless your version 9.00 APX hasnt been aligned in a decade and has drifted. Some of these agencies just dont understand that radios need PMing too

 

[GTR8000]

3,000+ APX 6x00AN/7x00 mobiles and portables that were deployed on an ASTRO 25 TDMA LSM system in 2013-2014, most still on the original FW 6/7 which haven't been touched (except for some codeplug updates) in all that time, zero complaints/issues. Not only are they all still working perfectly fine on standard 12.5 kHz TDMA traffic channels, but somewhat unique to this LSM cell is the fact that there are two pair of immediately adjacent channels, and still no issues even when there is an incident where subscribers on scene are operating on all four TDMA slots within a 25 kHz slice of spectrum simultaneously.

If it ain't broke...

 

[hitechRadio]

There really is no reason to disable AFC. It just flat works, in the testing I have done in the past.

 

[RFI-EMI-GUY]

Unless your version 9.00 APX hasnt been aligned in a decade and has drifted. Some of these agencies just dont understand that radios need PMing too

I remember some NPSPAC 800 MHz portable radios back in the analog days which drifted outside their design tolerance in 18 months and needed to be PM'd. The TCXO's were green and it was just normal aging process that required them to be realigned at that point in life. The vendor claimed the radios had to be sent to depot , would not field align, which is nonsense if done in a climate controlled shop. There was no AFC back then to correct this. The base station/repeaters were locked to GPS/Rubidium and that included the RX LO also locked to the site standard. So subscribers had to be spot on or miss the target. If radios today are relying entirely on AFC, they still might need a PM for best initial acquisition time on the network.

 

[MTS2000des]

3,000+ APX 6x00AN/7x00 mobiles and portables that were deployed on an ASTRO 25 TDMA LSM system in 2013-2014, most still on the original FW 6/7 which haven't been touched (except for some codeplug updates) in all that time, zero complaints/issues. Not only are they all still working perfectly fine on standard 12.5 kHz TDMA traffic channels, but somewhat unique to this LSM cell is the fact that there are two pair of immediately adjacent channels, and still no issues even when there is an incident where subscribers on scene are operating on all four TDMA slots within a 25 kHz slice of spectrum simultaneously.

If it ain't broke...

I said the same thing until I installed two DiagnostX boxes on two of my sites and started finding dozens of radios off freq, collisions, excessive affiliations, etc. It certainly makes getting radios in that would never see the shop otherwise.

 

[KE4ZNR]

I said the same thing until I installed two DiagnostX boxes on two of my sites and started finding dozens of radios off freq, collisions, excessive affiliations, etc. It certainly makes getting radios in that would never see the shop otherwise.

DiagnostX has been a lifesaver for us. We have it on all 4 of our sites networked together and it helps us keep track of radios where the ref oscillator needs to be pulled back within spec. Being able to provide errant radio info to our agencies also keeps them engaged with radio maintenance.