Singular or plural form of property, when describing an object with N-count for the property [duplicate]

Solution 1:

John Lawler coined the Eleven-year-old boy rule which is also mentioned in another answer.

The general rule is that one-word modifiers precede the noun; modifiers of more than one word follow the noun.

Here, you are using a hyphen to make a one-word modifier ("two-mode") so it goes before the noun, as you have it. It's an adjective, describing your analogue filter.

Adjectives are never inflected for number in English.

Note that foreign adjectives in English are often inflected for number, like beaux arts — but that's not English. English adjectives (red apple/red apples) don't change. They are always expressed in a singular form.

Because adjectives are never inflected for number, the correct form is two-mode.

_{This question is a good, generalised question on the subject which we haven't had before. It can serve as a destination for duplicates, and we need an explicitly explanatory answer.}

Solution 2:

You would say "two-mode analog filter".

Other examples could be "five-speed gearbox", "four-horse sleigh" or "two-party system".