Some AFSK sources such as TNCs, APRS trackers etc do not have facilities for sending single modem tones for the purpose of level adjustment which must be done using data packets which are a complex arrangement of high and low tones.
Whilst there is an upper limit to the deviation that a receiver can accommodate, the link limit with narrow band FM radios intended for voice communication is usually set by a peak limiter in the transmitter.
In that case, adjusting the tx audio drive so that it is just below the limiter onset provides the maximum deviation that is possible (without distortion) with that transmitter and receiver combination
The peak limiter is usually located after the pre-emphasis circuit, so its effect is firstly on the highest frequency tones.
This article describes a method of using a PC or soundcard oscilloscope on one transceiver’s rx audio output to find the threshold of another transceiver’s tx limiter.
Data packets are then send and the tx level adjusted up until the peaks as observed on the PC oscilloscope reach a maximum and will not go higher.
The tx level is then reduced so that peaks are around 80-90% of the maximum to provide a little headroom for variation over time.
The following video demonstrates the technique.
The software used for the demonstration was Soundcard Oscilloscope, and there are other packages which may suit.
This is not the only way to adjust these transmitters, but it is a valid technique that works well if the transmitter has a properly adjusted limiter (eg as they leave the factory). The method does not use expensive equipment, and works even on devices that offer no test tone facilities and must be adjusted on data packets.