Ivan,
High Q devices require a long settling time, often tens of milliseconds, before an accurate measurement can be taken. This is one reason we limit the step to 1 kHz–any finer resolution would take exponentially longer to measure.
In Spike, you can accomplish a high Q insertion loss measurement using a MAX HOLD trace and the TG CW utility, manually stepping across the frequencies of interest. Your “external” sweep with max hold is a good approach to this, and essentially automates this process.
