The FSVM is designed to measure the voltage level of various signals, usually in a telecommunications type environment. Later models will have more digital features and can often display readings in various formats. There needs to be a way to adjust the sensitivity of the meter, and this is done with a series of accurate attenuators. There also must be some way to set the frequency. Accuracy is crucial and even the older models have quite accurate dials, sometimes with crystal calibrators and the like.
Many models use a series of rotary switches to set each digit of the frequency.
Some models have a PLL system that lets you set the main tuning to the nearest megahertz or 100 kHz and then tune through the band segments, much like the Racal RA17 and other similar early PLL designs. Newer models with digital readouts often display the frequency to 10 Hz or even greater resolution. Later sets will have computer control, keypad entry, scanning, memories and even spectrum analysis capability and a price tag to match. The frequency coverage is obviously important. Most will measure down to a few kilohertz, but the upper end is more of a concern.