What do you mean by 'active transaction log'? There's always a portion of the log that's active.

If you want to check log behaviour:

Create one database switch it to simple recovery model, schedule backups (full) and make data changes

Create a second database, switch it to full recovery model, schedule backups (full) and make data changes.

You can't get log backup waits unless the DB has been in either full or bulk-logged recovery model and have just been switched to simple.