The primary purpose of the Model 24D is to provide two signals to the spectrum analyzer. The first is the rate of sampling necessary to allow the analyzer to display data in terms of orders, rather than frequency. The second is data for analysis that has been correctly filtered to protect against band limiting or aliasing of the input signal.. Given these two signals, the spectrum analyzer will perform correctly, unaware that its display will be an order normalized display, rather than the conventional frequency versus amplitude display. In addition, the 24D can supply the spectrum analyzer with a trigger signal which is synchronous with the running speed of a machine in order to allow the advantages of synchronous time averaging.