Distortion

Distortion at 50% modulation, stereo, in a Hitachi FT-5500 MkII (wide IF & RF). This is as good as it gets.

WHAT IT MEANS

Distortion in VHF/FM tuners is relatively benign. There a few ‘nasty distortion’ mechanisms and in most designs a small amount of second and third harmonic exists that is aurally tolerable up to 0.5% or so.  VHF/FM tuners can  be very linear and distortion free, although there is a trade off with selectivity and most modern tuners are very selective (i.e. they reject interference from nearby stations) they have higher distortion. FM demodulators are nowadays are much more linear than in the heyday of VHF/FM and so sound quality is usually little affected by distortion. This assumes a reasonably good VHF/FM tuner assembly has been bought in. Cheap ones give poor results however, high distortion introducing muddle. Our measurements can pick out the good from the bad.

Ultra low distortion VHF/FM generator, the Leader 3216.

HOW WE MEASURE IT

We use the low distortion internal generator of our Leader 3216 Standard Signal Generator to modulate in mono (L+R), L, R and L-R with pilot on (i.e. transmitting in stereo) at 100% and 50% modulation and measure distortion on our Rohde & Schwarz UPL analyser.