FM Deviation
VK6ZFG > 10-11-2023, 03:08 PM
Hi All
The other day I was listening to a QSO on 2M FM while mobile, One of the stations had the audio level so low that you could not tell they were talking. Only by turning up the volume to maximum did you stand any chance of making out some of what they were saying. Yet no one participating in the QSO ever said anything about it.
It was the last straw for me.
It resulted in me I posting the an article on the local VK6 user group to address the low deviation issue. Below is an updated version of that.
FM Deviation
With FM you should not ever need to adjust to volume level between different stations regardless of their signal level.
FM Modulation has the ability to provide good communications as long as sufficient signal level is present so as that the proper operation of the FM detector is achieved. While this condition is met a constant audio level is present regardless of signal strength.
A 12dB S/N ratio is used as the reference point for comparing receiver weak signal performance. In the past a 20 dB quieting level was used as this allowed the use of simpler test equipment.
The receiver bandwidth determines the FM deviation level it is intended to be used for. This is determined by the receive filter in the receiver. The deviation present determines the volume not the signal strength.
To obtain best audio S/N (signal to noise) ratio on FM the deviation needs to make full use of the receiver bandwidth available.
Deviation outside receiver bandwidth results in a distorted and/or loss of audio signal. On the other hand if full receiver bandwidth available is not made it results in an inferior S/N ratio than that possible. Wide deviation provides for a high S/N ratio and audio bandwidth but this comes at the cost of requiring a higher signal level for satisfactory operation. Narrow deviation on the other hand enables reception of weaker signals but at the expense of a lower audio S/N radio. You can’t have it both ways.
The overall effect of over deviation varies depending on a number of factors such as the deviation level, audio frequencies involved, signal strength and mute setting. No simple answer is possible.
Some quick tests on my IC-2720 produced the following results:
RF Level/AF Freq Audio Lost* Audio Lost* Mute Status
-120dBm/1000Hz 7.8kHz ~17kHz muted
0dBm/1000Hz 8.4kHz ~200kHz** unmuted
-120dBm/2700Hz 6.2kHz ~8.7kHz muted
0dBm/2700Hz 7.1kHz ~200kHz** unmuted
* level lost occurred was with audio with some loss in quality
** max deviation possible from my communications test set
With strong signals a higher level of deviation may be usable. This is probably due the receiver filter appearing wider with its skirt selectivity coming into play.
FM transmitters normally include some form of deviation limiting circuitry and it is relatively easy to set the maximum deviation level. But you cannot take this for granted. I found that the Ranger RCI-2950/2970 series does not include any form of deviation limiting. Deviation on this is entirely dependent on the audio level going in. It is a somewhat poor design.
Broadcast FM stations use wide band FM (+/- 75kHz) so as to be able to provide a wide audio level and wide frequency range at a low noise level for music programs. They compensate for the higher signal level requirement by using high power levels.
Two way radio communications use narrow band and can achieve the similar coverage as FM Broadcast transmissions but lower power levels. They only provide voice audio bandwidth. These normally use (+/- 5kHz) in amateur service (commercial services now use +/- 2.5kHz as it provides for more usable channels). Unfortunately a down side to low deviation levels is that while the sensitivity is increased the S/N ratio achievable is reduced and it becomes more susceptible to impulse interference.
The transmitter deviation needs to be controlled so that the maximum deviation does not exceed that required for the system to avoid distortion. At the same time the deviation should be kept close to the maximum allowable so as to maintain received audio S/N ratio as high as possible. Maximum deviation is usually limited by using a clipper with audio filtering after it. Sometimes this is achieved using fast acting compression. A slight amount of audio clipping and/or additional compression can be used to keep the average audio level up and increase talk power (speech processing as often used on SSB). The amount of "speech processing" you can achieve will depend on the limiter used.
My observation of amateur FM radio communications is that the deviation of many stations is rather low resulting in poor communications quality. My base radio data port feeds a meter calibrated to indicate the deviation level present along with an oscilloscope so it easy to compare transmissions.
With a 5kHz deviation system, the goal should be to maintain a deviation around the 3kHz on speech so as to achieve maximum talk power. Unfortunately the deviation on many stations is around 0.5kHz or lower. This corresponds to the drop in audio level of some 15 dB on what it should be. Would many SSB operators deliberately only operate their 100W radio so that is effectively only operates at the 5W level? I don’t think so.
The low deviation could be due to the radio not been set up correctly. Having a new radio from the factory however does not necessarily mean it is set up correctly.
In my case, many years ago, I bought a new Icom IC- 2720, loaded channels into it and as no one ever commented on the audio continued to use it for many, many years until one day Will 6UU commented that I had low audio. I found this interesting and eventually checked the deviation. To my surprise I found that the maximum deviation was only 1.5kHz not 5kHz.
Mmm, for a radio straight from the factory???
After much searching on the internet I found clues as to what it could be. I, of course, like all other Amateurs had read the user instruction manual – well the parts that were relevant to me at the time, or at least I thought so. I however had missed the one on page VIII of the Icom IC-2720 Instruction Manual.
"Although you have purchased a brand new transceiver, some settings may be changed from the factory defaults because of the QC process. Resetting the CPU is necessary to start from factory default.”
Now I had always assumed that the QC process is supposed result in a better product going out from the factory door. Obviously I was mistaken. I checked a later Icom radio manual and found a similar statement present using slightly different wording. I don’t know if this still is the also case with other Icom radios or with to other brands but it is something to consider when setting up the radio.
= Don't Assume anything -
-
I saved the IC-2720 memory channels, reset the radio and reloaded the memory channels. And, guess what, the max deviation was now where it should have been.
Now getting back to the low deviation levels on many amateur FM stations this may be due to:
1 Factory reset requirement.
2 Deviation level not set correctly (beware of 5/2.5 kHz option trap).
3 Deviation level set correctly but insufficient mike gain.
I suspect that it is more likely that most of the amateur stations probably have the deviation level set correctly but are suffering from low mike gain. Many are probably using mobiles as base radios. Mobiles are set up to expect a reasonably high audio voice level as they are used in a noisy car environment. However in the quieter base station environment one naturally talks at a much lower level. If this is the case, the mike gain needs to be increased (if possible) to compensate for this, or failing that remember to always talk at a higher level.
Handhelds are another issue. They are rather difficult to set up as they are used in a very variable environment.
So, if you have a QSO with someone that has low audio.
Let them know
It will give them the opportunity to do something about it.
With FM you should never need to adjust the volume level between stations.
73
Igor
VK6ZFG