Im playing with DSS FY-6900
and as many complain about jitter signal output , I'm trying to actually characterize it in numbers.
so gen source is 10Mhz OCXO now .. and I can run output up to 60M ; that frequency on pictures is actually 10.000 000.1
my guess I need to pick some most horrendous frequency but I don't know what is that ; like completely uneven , random one ?
I'm writing this based mostly on knowledge about the FY6600 I gathered from reading relevant threads of this forum about the FY6600. AIUI, the FY6900 has only minor modifications, so I assume that the following applies to the FY6900 too.
The FY6600 has an FPGA that is "clocked" from an external 10MHz crystal oscillator. The FPGA has an internal PLL (or has it two PLLs? – I would have to look at the schematics of the FY6600, find the FPGA type and then look into its datasheet, but I'm too lazy to do that now...) The PLL generates a 250MHz clock signal. This clock signal is used to "update" the DACs that generate the output signal.
This "DAC update frequency" is never changed and this means that the FY6600 can only generate "exact" signals with periods that are an integral multiple of 1/250e6 seconds, or 4 ns.
You tried a 10MHz signal, or a period of 100ns. so you have exactly 25 "DAC updates" for one period. Hence you won't see the jitter about which some people complained.
The "neighboring exact periods" are 24 and 26 "DAC updates" per period, or periods of 24*4ns -> 96ns and 26*4ns -> 104ns, i.e. the frequencies 10.416666...MHz and 9.651538...MHz, respectively. (Note that these frequencies cannot be exactly represented as decimal numbers, so it is impossible to set the generator output to these frequencies – or is it possible to set signal periods instead of signal frequencies with the FY6600/FY6900?)
So, if you set the output frequency for example to 10.2MHz, you will get an output signal where the period alternates between 24 and 25 "DAC updates" (96ns and 100ns) per period, so that the period, when averaged over a longer time, equals 1/10.2 µs. Similarly, when you set the output frequency to 9.8MHz, the output signal will have periods that alternate between 100ns and 104ns,
This should be easily visible on your DSO as two lines separated by 4ns when you look at the signal one period before and after the trigger time.
When you choose frequencies that are closer to 10MHz but not exactly 10MHz, the lines on the DSO screen showing the periods 96ns or 104ns should become darker.