Why do I calibrate FM frequency response?
and what is "first carrier null" ?
:-o Maybe YOU should stick to dmm's and scopes?
To go off topic slightly, I remember many, many years ago, I used to experimentally measure (just for the fun of it) the various Bessel Nulls. I calculated using an HP 8566B and it's associated noise floor, I could measure frequency modulation peak deviation in the cases I was measuring to within +/-0.25% of FM setting. I love Bessel Nulls. They bring out the nerd in me.