External clocking on HSDIO devices allows for operating at frequencies that are not integer divisors of the onboard clock. For example, supposed you are using the PXI 6562, which has can use an onboard clock from 48 Hz to 200 MHz, configurable from 200 MHz/N; 1 ≤ N ≤ 4,194,304. If you would like to operate at 120 MHz, the closest onboard clock rate you could obtain is 100 MHz. In this situation, you would need to provide an external clock signal. 

 

There are many different options of providing an external clock signal, but here are the two most common and useful methods of externally clocking your HSDIO devices. 

 

The 5404 is a 100 MHz frequency generator, which can specifically generate sine waves from 9 kHz to 105 MHz, with a frequency resolution of 1.07 µHz. With an SMB output, you can simply use SMB to SMB cables to wire from the output of your 5404 to the SMB CLK IN terminal of your HSDIO device. This device is an excellent low-cost solution for providing accurate clock signals to your HSDIO cards. 

 

In the case that it is necessary to operate in the 100+ MHz frequency range, it is also possible to use National Instrument’s RF Signal Generator product line. Specifically, the 5650 has a frequency range from 500 kHz to 1.3 GHz, and is the lowest frequency range out of the entire 565x series and thus the cheapest. This device can also be used to provide a low amplitude sine wave (with < 1 Hz resolution) which can be used as a external clock reference for the HSDIO devices. Since the HSDIO clocking circuitry uses zero-crossing, a TTL logic level is not necessary to drive the clock. 

 

This hardware requires the use of the RFSG driver software. The simplest way to generate a clock example is to use the “Single Tone Generation” RFSG shipping example. Simply specify the frequency that you’d like to generate at, and a power level of -10 dBm to 10 dBm should be enough power to drive the HSDIO’s clocking circuitry. However, it might be safer to generate at the maximum power level for this frequency range, at 10 dBm. In terms of hardware connections, you will need to use an SMA to SMB cable to send the signal from the 5650 signal generator to your HSDIO device.