Know Your FM bandwidths for CB radios

[rpcomms]

As we all probably know USA is going FM mode for the first time

Not a real issue in UK as we had this a while now,
but one technical issue that does keep cropping up is technical standards regarding FM mode on UK and EU bands (legal) radios operating on 27mhz.

so I've written up some notes for techs out there which may help them.

Old UK system MPT-1320 27/81-Now defuncted spec-NO LONGER USED

Max Deviation set at 1khz test tone typically 35-50mV RMS input to microphone socket (mic gain set to max)
20dB down on audio generator=60% modulation=1.50khz for ref
Set 2.20Khz with 15Khz LPF on FM Modulation meter or RF Comms test set.
Peak deviation with fist microphone inserted typically 2.2-2.50khz max no more
60% deviation=1.50khz on receive test--3 to 6dB change in deviation level (1.50khz to 2.50Khz)for maximum recovered audio
Typical filter 455E
bandwidth 11K0 F3E

Advanced Tech Users Only with proper test gear-WARNING
Engineer Notes-8K0 bandwidth modification (does not apply to new multi-norm sets)
To Convert to newer specs remove the 455E filter and replace with 455HT filter of same pin outs.
Set inject 1khz test tone @35-50mV into microphone socket
Set TX deviation to max 2.00Khz ,use 15khz LP Filter on FM Modulation meter.
CTCSS 300Hz deviation

RX changes
Make a note of audio value using a 4ohm resistive load connected to external speaker jack socket
-73dBm RF into set @1Khz test tone at 1.50khz deviation
Turm volume control on set for 775mV RMS (0dbM audio)measure on Audio Voltage meter or RF Comms Testset
Make a note of value

Change resistor value across 455khz resonator in FM RX demodulator chip to 3K8-4K2
ohms or discriminator type coil check schematics ,typically 47Kohms across coil
,change value to typical 68K ohms

set RF gen -73dBm 1Khz tone @1.25khz -new 60% level for 8K0 bandwidth system

check audio has increased in level ,peak discriminator coil as necessary.

ensure recovered RX same level as it would have been prior to modification at 0dB 0.775mV RMS
across speaker load

Re-check SINAD and sensitivity of radio once modified, note use 1.25Khz=60% as more audio is now recovered.
SINAD levels should improve due to the narrower filter.
Readjust squelch levels as needed max Carrier SQ typically 1mV max


Current Multi-Norm \FCC & ETSI EN Radios-Bandwidth 8K0 F3E (legal sets)
Covering EU and UK bands
Max deviation 2.00Khz Max @15khz LPF
60% deviation=1.25Khz -20dB down on microphone input level

CTCSS\DCS 300Hz deviation

RX 60%=1.25Khz deviation
Filter typically 455HT

MC3361 & 455Khz resonator-recommended resistor across 455Khz ceramic resonator was best found value 3K4-4K2 for maximum recovered audio -3dB @1.25Khz deviation at speaker



Export USA 10M Ham, Converted Ham HF gear on 27Mhz-A Note

These DO NOT Meet the new agreed, Ill call it Super Narrow FM mode (8K0 F3E)
in Most Cases

some sets meet FCC 10m USA specifications
Typical 4-5Khz max deviation
Lower RX recovered audio in discriminator due to a different resistor value for
11K5 to 16K0 FM wide\Mid bandwidths systems
CTCSS too high in deviation values

HF radios only support 12.5Khz channel spacing systems at 11K5 F3E bandwidths
in "Narrow FM mode"
Typical 2.50Khz max dev
350-380Hz CTCSS\DCS dev
RX discriminator set for 1.5khz (60%=3K2 ohms)FMN or 3.00Khz FM-Mid\Wide(60% typically 1K5ohms) deviation recovery of audio, value too low for SNFM mode at 10khz spacing system.

One newer SNFM 8K0 F3E system, this can lead to users thinking these new sets sound "quiet "on TX modulation and have to "Turned UP!"
when really its the resistor gain value in the fm discriminator coil or 455khz resonator that governs the recovered audio voltage fed to the AF amplifier chip that controls this.
Also wider FM bandwidth radios can sound too loud on the newer SNFM radios or distorts if it exceeds the built in 455HT filters bandwidth!

9 times out of 10,from practice most 10m export radios must be set up correctly to work on this new 8K0 F3E system for EU band
Most will get away using them on older MPt1320 radios with wider 11K0 bandwidths

Unfortunately there is a lot of mis-information on internet and confusion in the radio world regarding interoperability with these FM systems and different equipment types

Typical systems\Resistors values on descrimator 455Khz resonator
FM-Wide-25khz bandwidth-10m Ham FCC-1K5 ohms
FM-Mid-20Khz bandwidth-10m Ham FCC 1K5 ohms
Narrow FM-12.5khz bandwidth-Close enough for MPT1320 old 27\81 cb radios-3K2 Ohms
Super Narrow FM-10Khz bandwidth-CB\10m ham UK\EU land\New FCC-3K2 to 5K ohms typically(3K9-4K2 ohms seems about right)

there are others but above is most common

Hopefully this will help techs set up other radios types to the correct fm 8K0 system now currently the "default" standard on CB band and UK\EU 10M operations.
Help avoid pit-falls and misinterpretation's of actual what's going on.

I may post some schematic pics of fm chip decrim mods if figure out uploading pictures .lol

I have converted a few CB\Export USA Ranger sets sets to this newer SNFM 8K0 bandwidth system successfully with just
new filter
resistor
and deviation tweak\mod

All the new EU Multi-Norm CB sets should be compatible same as new FM FCC approved, hope this is of some help

Regards

Rob P 73's

[rpcomms]

Quick update

Regular new Multi-Norms for UK uses generally set for 1.6-1.90Khz deviation im finding which is fine for EU uses.

But as we still have a mix of old and new specs in UK band only my advise is just set
mic gain max
inject 1.0khz test tone into microphone sockets @50mV RMS
go into software service mode ,generally hidden.
FMOD or FOD (fm deviation) set it to 2.10khz max
CT DEV 300hz

max peak deviation 2.20khz (2.50khz with CTCSS tone) on voice.
and will give good results without infringing EU band use on Fm mode drastically

FCC-ETSI EU-DE-PL-IN-UK-I2 Band 8K0 F3E Multi-Norm Current
UK MPT-1320 HO spec 27/81 band 11K0 F3E Old Spec

Most CB dealers and Techs should be able to do this readjustment and comprimise fm deviation setting optimisation.

Some of the later 2023 models like Nanfone PNI do offer EU Dev=1.90khz and UK FM dev=2.20khz via software

Older and some newer HF or Multi-Band ham kit will not generally do 8K0 F3E 10khz spacing fm without modifications to both FM dev ,CTCSS Dev,and fm detector audio levels need to be increased.
Most at best do 12.5khz spacing 11k5 F3E 2.50khz devs and rx audio in some cases a little lower due to technical reasons.
While it may work to user its not optimised best for Super Narrow FM (S-NFM 8k) mode) ,while it will be ok when talking to the older type cb kit MPT1320 at 11K0 F3E being slighter wider filters more to Narrow FM (N_FM)mode.

Hope this is of some help.

Rob P 73's