Frequency response

Frequency response of a Hitachi FT-5500 MkII, used as a benchmark. This shows what VHF/FM can do.

WHAT IT MEANS

VHF/FM tuners can have flat frequency response to 16kHz when fitted with an 19kHz pilot tone filter – see the Hitachi FT-5500 MkII trace above. Some omit this and are flat to 20kHz. Both will have a basically even tonal balance. If there are peaks or dips, or an early roll off, usually due to pilot tone filter behaviour, influenced by termination accuracy, then the tuner will sound bright and sharp, or dull and warm.

A good VHF/FM tuner has a very similar bandwidth to CD, offering an accurate and high quality source of music, especially from live broadcasts.

Leader 3216 VHF/FM Standard Signal Generator.

HOW WE MEASURE IT

We send random noise from our Rohde & Schwarz UPL analyser to our Leader 3216 Standard Signal Generator, modulating in mono at around 30% mod. depth. Since the Leader has no equalisation for audio, a 50µS (UK) post equalisation network is used, connected to the output of the tuner under test. This gives a net flat response. The noise is then sent back to the UPL and its amplitude measured in FFT mode to provide a publishable response curve, like those seen here.

The tuner’s response in stereo is also checked to ensure the difference channel is identical to the sum channel and their responses do not differ (rare but possible). The tuner is not measured in stereo to avoid wide band random noise triggering the stereo decoder.

Post equalisation network, and mpx filter.