Noise

Noise from a Hitachi FT-5500 MkII tuner, in stereo mode and IEC A weighted. At -79dB it is extremely quiet.

WHAT IT MEANS

Whilst VHF/FM tuners have improved in most areas noise is not one of them. Modern tuners border on having audible hiss, measuring around -65dB, little different to cassette tape with Dolby B noise reduction. Because there are few silences in radio broadcasts this largely passes unnoticed. However, Radio 3 aficionados may well detect faint hiss in its not infrequent silences. A hiss level of 68dB or better is needed if near perfect silence is wanted.

VHF/FM is noisy with weak aerial signal.  Most tuners need around 1mV  from the aerial – quite a lot – for hiss to fall to a minimum in a condition known as ‘full quieting’. Our noise measurement is in this condition, so it is the best the tuner can do in effect.

HOW WE MEASURE IT

We set our Leader 3216 Standard Signal Generator to 4mV output and 100% modulation at 1kHz to set the 0dB reference, in stereo with the pilot on. Modulation is then switched off. Noise is measured using an IEC A weighting filter. This is insufficient to eliminate the influence of residual pilot tone where mpx filtering is weak or absent. In such circumstances an external 19kHz notch filter is switched in (it has no affect upon response below 16kHz) or an IEC Tuner filter selected in our Rohde & Schwarz analyser. The results of using either method are identical. Spectrum analysis shows when pilot tone suppression is satisfactory.

Linked to noise measurement is the full quieting threshold we always quote. This is the RF input level needed for the tuner to achieve full quieting, -1dB. So if a tuner has a -69dB noise floor, its full quieting threshold is the signal needed for -68dB noise. It hovers around 0.8mV for good tuners.