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Agilent PN 8791-5 Tips on External Clock Operation with FASS Product Note Abstract This product note provides helpful tips and examples of external clock operation with the Agilent Technologies 8791/10 Frequency Agile Signal Simulator. Examples of successful external clock operation are considered in the context of two real applications: secure communication receiver test and radar target simulation. Figure 1. External clock operation with FASS Introduction However, there are some applications System Clock Fundamentals Standard operation of Agilent where exact timing is critical, partic- Let's begin by understanding how FASS Technologies FASS uses an internal ularly in many radar and secure com- clocks are distributed. Figure 2 shows clock of 134.217728 MHz (227 Hz) to munication simulations where trigger a block diagram of the system clocks provide synthesized frequency resolu- jitter or timing skew between the sim- where CLK represents the system tion of 0.125 Hz nominal. This power- ulator and the D.U.T. may be intolera- clock, whether internal or external. of-two timebase was selected to pro- ble. In such cases, the FASS can be vide "convenient" frequency resolu- successfully synchronized to the sys- When you are using the standard tion when used with the direct digital tem under test by using a suitable INTERNAL system clock, connect the synthesizer inside FASS. Because the external clock source. Of course, 134.217728 MHz signal, CLK OUT direct digital synthesizer uses a bina- there are certain ramifications of (134 MHz), from the Agile Upconverter ry phase accumulator, any timebase external clock operation that must be (AUC) to the EXT CLK INPUT of the other than a power-of-two would understood before one can proceed Agile Carrier Synthesizer (ACS). result in an awkward frequency reso- without consequence. Some of these lution. issues concern the range of valid To use an EXTERNAL user-supplied clock operation, triggering and syn- clock, you must input the clock into The tradeoff here is that the power- chronization, effect of external clocks the EXT CLOCK INPUT of ACS. For of-two timebase--while providing a on spectral purity, and possible resid- specified operation, the clock must convenient frequency increment--has ual Doppler drift. We will examine not exceed these conditions: a non-power-of-ten reciprocal timing these effects along with two practical interval. Specif