DG1022Z frequency error? - Page 1
Forum section: electronics / testgear

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2022, 08:48:46 am
content: If I set my function generator to 25,000,000 Hz and I connect a frequency meter to it then it reads exactly that.If I reduce the frequency to 24,999,980 it still reads 25,000,000.If I take it a bit further to 24,999,970 it reads 24,999,979Could someone that has a DG1022Z check and see what there’s puts out when set to 24,999,980? I know the firmware is not perfect in these things and maybe this is something new, or maybe it’s just my frequency counter is a bit dodgy. If someone else sees the same problem then we’re onto something.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 04, 2022, 10:03:16 am
content: Most probably not a bug, just that the DDS can't resolve such small time differences. The time difference between the periods of the frequencies you were testing is very, very small, in the range of tens of femtoseconds (here about 16 fs):Frequency PeriodHz ns ps fs25 000 000 40.000 00024 999 980 40.000 03224 999 970 40.000 048

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2022, 11:30:24 am
content: If that is the case, it's a pity the sig gen displays so many decimal places, giving you the false idea that you can adjust the frequency with such a fine resolution. it would not have been hard to reduce the number of displayed digits at higher frequencies so it wouldn't be misleading.

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2022, 11:40:31 am
content: The specs in the attached page say the frequency resolution is 1uHz. At low frequencies, sure. But not at higher frequencies?

author: Someone
author_rank: Super Contributor
timestamp: April 04, 2022, 11:54:13 am
content: Quote from: Circlotron on April 04, 2022, 11:40:31 amWhat was the gate time on your frequency measurements? What was the accuracy of the frequency counter?The data sheet/specifications gloss over it, but the programming manual reports only 7 "effective" digits.

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2022, 12:11:29 pm
content: Gate time was 10 seconds.Don't know the accuracy of the counter, but after running all day, when measuring 10MHz the counter showed approx 0.3Hz low.

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2022, 12:57:02 pm
content: Quote from: RoGeorge on April 04, 2022, 10:03:16 amI put a 24 meg sinewave from each channel of the sig gen into each channel of the scope and adjusted the phase so both waveforms overlapped perfectly. Then I adjusted the frequency of channel 2 first 0.0001Hz above 24MHz then the same amount below. It took a minute or two but you could see the sinewave creeping ever so slowly out of phase, the direction depending on whether channel 2 was higher or lower frequency than channel 1. So it certainly seems to have good frequency resolution. I should have timed how long it took to slip one complete cycle so I could calculate the actual frequency difference but the rate it was going it would have probably taken about 15 minutes. (edit - would take 10,000 seconds, nearly 3 hours) Maybe I’ll do it tomorrow. It almost 11pm here.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 04, 2022, 01:11:50 pm
content: Just tested it on a DG4000 at 25 MHz and a 53131A. It does at least 0.1Hz steps, I can't test the smaller steps yet because both are still warming up.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 04, 2022, 01:20:14 pm
content: Just out of curiosity, I've just checked on my Rigol DG4102 DDS (upgraded to DG4202 - max 200MHz sin), with its own counter set to count for 10 second (the counter's gate set to 10 seconds). The counter shows 12 digits.The generator shows max 14 digits for setting the frequency, but the last 4 of them apparently have no effect.Generator set (MHz) Counter display (gate 10s)25.000 000 000 000 25.000 000 000 024.999 999 999 999 25.000 000 000 024.999 999 999 998 25.000 000 000 0...24.999 999 990 000 24.999 999 988 324.999 999 980 000 24.999 999 976 724.999 999 970 000 24.999 999 965 0 sometimes 24.999 999 976 724.999 999 960 000 24.999 999 953 4 sometimes 24.999 999 965 024.999 999 950 000 24.999 999 941 7 sometimes 24.999 999 953 424.999 999 940 000 24.999 999 930 1 sometimes 24.999 999 941 724.999 999 930 000 24.999 999 930 124.999 999 920 000 24.999 999 918 524.999 999 910 000 24.999 999 906 824.999 999 900 000 24.999 999 895 2 sometimes 24.999 999 906 8kHz Hz mHz uHz kHz Hz mHzI think it is expected for a DDS to measure slightly different alternating frequencies (but not from an RF generator).

author: RoGeorge
author_rank: Super Contributor
timestamp: April 04, 2022, 03:35:44 pm
content: Quote from: Circlotron on April 04, 2022, 12:57:02 pmThat's a very good idea to check if something changes, though I'm not sure that means the true time resolution is really in the femtoseconds range. From time to time, the lower frequency channel will repeat the same amplitude value in the next DAC clock (relative to the other channel), so with time the phase seen on the oscilloscope will skew.A DDS has a table of a given length for the DAC output values. Then it calculates how many addresses in that table it has to jump at each clock, so to get in the end the requested frequency. However, the real clock of the DAC is much lower than the time resolution required by a 1uHz change in 24MHz. DG1000 has 200MSa/s, so a 200MHz DAC (DG4000 has a 500MHz DAC). It can update the DAC value at best once at each 5ns, but because the lookup table for the ADC can be much longer, it appears as if the frequency resolution would be much finer than it really is.In less words said, the resulting frequency it's like the stroboscopic effect while looking at a rotating wheel, except the DDS "wheel" also moves in steps (given by the length of the lookup table).To achieve such fine resolution, the wheel must skip or repeat a step here and there at each turn. On average it will match the fine resolution, but when looking at each "stroboscopic turn" it will have same jitter (1 "stroboscopic turn" = 1period in the set frequency). Since usually there is some analog filter after the DAC, the jitter from the ideal period is reduced even more.