Connecting Instek SFG-1003 to Rigol DS1052E - Page 1
Forum section: electronics / beginners

author: MikeK
author_rank: Super Contributor
timestamp: February 26, 2012, 02:07:58 am
content: I'm having a little bit of trouble getting my new Instek SFG-1003 function generator working with my Rigol scope. The scope doesn't always want to lock on the sine or triangle wave output, but will always lock on the square wave output.The scope will lock on the sine output if I keep the scale to 100mV or higher. Any less and it just jitters.I don't think I'm doing anything wrong, but would appreciate some help.The scope input is 1Mohm and the FG output is 50 ohm. Do I need to match impedence?Mike

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 26, 2012, 03:12:00 am
content: Are you sure you're triggering on the right channel and proper edge and voltage level? Sounds like a triggering issue to me.Also, when you say 'lock on' do you mean to say you're trying to use the "Auto" function? I don't know why that wouldn't work but you should probably just do it the way everyone else does and set all the triggering parameters manually.

author: robrenz
author_rank: Super Contributor
timestamp: February 26, 2012, 04:04:23 am
content: I think if you want the voltage setting of your FG to show up correctly at the scope you need to use a 50 Ohm pass thru terminator at the scopes 1Meg input.

author: MikeK
author_rank: Super Contributor
timestamp: February 26, 2012, 04:39:46 am
content: @TerminalJack505Yes, I'm using the Auto function. The trigger level is smack in the middle of the signal. It doesn't make sense to me that the display scale would affect the trigger lock.@robrenzWhat is this 50 Ohm pass thru terminator? I've tried using a BNC-to-alligators on the FG and the probe on the scope. And I've also tried using a 50 Ohm BNC-to-BNC cable between the two machines. Same results.

author: w2aew
author_rank: Super Contributor
timestamp: February 26, 2012, 04:52:31 am
content: Quote from: MikeK on February 26, 2012, 04:39:46 amNote - the AUTO Trigger function does NOT automatically setup the trigger parameters for you. All this function does is automatically initiate a trigger IF a trigger signal is not found within a short time period. You may want to review my video on this:This FG only goes to 3MHz - so really won't be bothered by reflections caused by the impedance mismatch. The only effect you'll see is that the amplitude will be 2x larger when loaded into a high impedance as you are doing vs. using a 50ohm termination.YES - the trigger IS affected by the vertical scale settings. The vertical scale settings adjust the attenuation seen by the signal before the vertical preamp and digitizer. The trigger pickoff is often after the attenuator. If your signal is small on the screen (<1 division), then triggering may be jittery. Do you have the same problem with unstable sine or triangle if you make the signal larger on screen (either by increasing the FG output or by reducing the vertical scale setting?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 26, 2012, 05:05:33 am
content: LOL. Sounds like the Auto function strikes again.I learned not to use this feature on scopes after getting a used Tek 2467 scope off Ebay. One of the first things I did was hook the probe up to the scope's calibration generator and hit "auto setup." The scope apparently locked on to some higher harmonic rather than the lower frequency square wave. Before I realized what was going on I thought my newly acquired scope was faulty.Just say "no" to the Auto feature, people.

author: robrenz
author_rank: Super Contributor
timestamp: February 26, 2012, 05:15:08 am
content: Quote from: MikeK on February 26, 2012, 04:39:46 amThis is my understanding of it which may have some flaws.A 50 Ohm output instrument assumes the recieving instruments input will be 50 Ohm forming a voltage divider. The FG set to 1Vpp will actually generate a 2Vpp signal before it passes through its internal 50 Ohm output resistor. The 50 Ohm of the FG combined with the 50 ohm input resistor of the recieving instrument form a voltage divider that halves the original 2Vpp to 1Vpp in the recieving insrument. When you just run your signal into that 1meg you have a 50 Ohm to 1 Meg Ohm voltage divider giving 1.99999 Vpp input into the scope instead of the 1Vpp that the FG is set to. You can use a BNC tee on your scope input and put a 50 Ohm terminator on one leg of the tee and input your signal into the other leg ot the tee. The 50 Ohm teminator is in parallel with the 1meg input resistor in the scope which yields 49.9975 Ohm. A pass thru terminator has a male and female BNC connector with a 50 Ohm resistor connected between the inner and outer conductor internally. Does the same thing as the tee with terminator.

author: MikeK
author_rank: Super Contributor
timestamp: February 26, 2012, 06:13:08 am
content: @w2aewAlan, the "Auto" function on the Rigol actually does setup the trigger parameters. It's not the same as the trigger's "auto" mode.The signal is only jittery when it's close to (and larger than) the full screen height. When it's about half-height or smaller there's no problem.I made a video of the problem:http://youtu.be/piO2V77z7uAIt starts off with a triggered and locked signal. Then I change the vertical scale and it loses lock. I move the signal into view and show that the trigger is in the middle. Then I change it to a square wave and it locks. Again I show the trigger location. Finally, I change it to a triangle wave and it loses lock again.

author: MikeK
author_rank: Super Contributor
timestamp: February 26, 2012, 06:24:46 am
content: @robrenzI found a BNC tee and 50 ohm terminator and tried it. It's fully stable at the levels I was using in my video, but it still goes jittery once I set the vertical so low that the signal doesn't fully fit the screen.

author: w2aew
author_rank: Super Contributor
timestamp: February 26, 2012, 02:07:50 pm
content: Quote from: MikeK on February 26, 2012, 06:13:08 amOK, I was referring to the AUTO Trigger mode, not the Auto-Setup, which is what you were referring to. I understand that the auto-setup does it's best to setup the vertical, horizontal and trigger.I think I now know what it happening with your setup. The Instek SFG-1003 is a DDS type function generator. DDS stands for Direct Digital Synthesis. This means that the waveforms are created using a DAC (digital to analog converter) from data samples produced by an FPGA. It is possible (and likely) that the waveform produced by the generator has small "wiggles" on it due to the finite number of voltage levels/steps that the DAC can produce. The DDS usually with dither the samples a bit to smooth these out over the long term. The waveform frequency is adjusted in most DDS generators using a phase accumulator - which has the effect of decimating or replicating samples. This generation process creates waveforms that have a bunch of small "micro-steps".However, your new scope has enough bandwidth to actually SEE and trigger on these small little steps. So, for the waveforms with "slower" edges like the sine and square wave, these microsteps will be more apparent, and the scope is triggering on them which is what is causing your instability.You can verify this a couple of ways. Try engaging the vertical bandwidth limit (20MHz?) on your scope if it has it. That may filter the micro-steps enough to smooth them out so that it triggers cleanly. The other way you can verify this is to zoom way in on the waveform and do a single acquisition - you will likely see these microsteps. I have a Leader function generator that is also a DDS, and the wider bandwidth scopes "see" the same thing that your new Rigol does.

author: MikeK
author_rank: Super Contributor
timestamp: February 26, 2012, 04:01:56 pm
content: Well, I turned on the bandwidth limit. Same results. I also turned on the digital low-pass filter set to 1kHz (same as the signal) and also got the same results.It might be time to downgrade to 2.04 firmware?

author: w2aew
author_rank: Super Contributor
timestamp: February 26, 2012, 04:15:05 pm
content: Quote from: MikeK on February 26, 2012, 04:01:56 pmTry zooming in vertically and horizontally a lot, and I'll bet that you see little wiggles on the rising (or falling) edges of the waveform.On the BW limiting - probably not having an effect due to two things - the wiggles are too low in frequency (are in the passband), and possibly the Rigol picks off the triggering before the bandwidth limiting. The fact that it is digital BW limiting as opposed to analog, then it is very likely that the triggering is picked off before the filtering is applied, so "no change" in the triggering is what you'd expect.Have you tried running the FG up higher in frequency rather than the 1kHz you show in the video?I really think this is due to the FG, and not a fault of the scope.

author: w2aew
author_rank: Super Contributor
timestamp: February 27, 2012, 01:10:22 am
content: MikeK - here is a video illustrating what I believe is happening with your Rigol scope and Instek function generator...

author: Rufus
author_rank: Super Contributor
timestamp: February 27, 2012, 02:03:00 am
content: Quote from: w2aew on February 27, 2012, 01:10:22 amI have an SFG-2004 and it shows a similar problem. The output is a bit noisy with 3-4mV spikes. There is a pattern to them, looks like some noise from the digital circuitry is coupled to the output. The generator 20db attenuator attenuates the spikes as well so best to use it for small signal output. HF reject trigger coupling obviously helps too.Edit: Looked more closely at the noise, bit bigger than I thought. Some traces attached.

author: MikeK
author_rank: Super Contributor
timestamp: February 27, 2012, 02:53:32 am
content: Yeah, but do you have trouble getting a trigger lock on the signal?

author: w2aew
author_rank: Super Contributor
timestamp: February 27, 2012, 03:47:54 am
content: Quote from: MikeK on February 27, 2012, 02:53:32 amNo trouble at all - the trigger IS locked on the signal - it just happens that the signal has imperfections on in, and the trigger sees them - so it's doing exactly what it should be doing. If you want to trigger only on the "ideal" waveform, then you'll have to see if your Trigger controls such as HF Reject, etc., or other trigger features that might be part of your scope are able to not trigger on the anomalies. Or, maybe bring the TTL output into Ch2 (if it operates simultaneously with the analog output at the same frequency) on the scope and trigger on that instead, and then sine/triangle on CH1 will be stable.

author: Rufus
author_rank: Super Contributor
timestamp: February 27, 2012, 03:58:41 am
content: Quote from: MikeK on February 27, 2012, 02:53:32 amYes, I said it shows a similar problem. If you are triggering on rising edge and one of those spikes occurs as the falling edge of the signal passes through the trigger level the rising edge of the spike will be a valid trigger and cause a sweep where the actual waveform appears 180 degrees out of phase.The scope is doing what it is supposed to and it isn't an unusual problem which is why most scopes have an HF reject trigger coupling mode to help trigger on slow waveforms with high frequency noise.The generator being that noisy isn't great, but, they are about as cheap as generators in a proper package come. Did you try the 40db attenuator on your generator?Edit: If you want to investigate the presumed noise on your generator set it to 0.1Hz sine, minimum output level and AC couple the scope input. The scope won't see much if anything of the 0.1Hz leaving just the noise to look at - that is how I got the traces I posted.

author: MikeK
author_rank: Super Contributor
timestamp: May 03, 2012, 02:59:13 pm
content: Thanks guys. I'm checking this out again so that I understand what's going on.And thanks for the video, Alan. I somehow didn't see it back in Feb. I'm watching your "debug and repair" video now.

author: MikeK
author_rank: Super Contributor
timestamp: May 03, 2012, 05:23:54 pm
content: Well, I zoomed in on the signal and it just looks like noise. Not a nice rider like Alan had.And with a 3.5V Peak-to-Peak signal from the SG it has about 5mV of noise, which is not as good as the claimed distortion of better than -55dB. Maybe I'll contact Instek to see what they have to say, but there's no warranty on this model.

author: saturation
author_rank: Super Contributor
timestamp: May 07, 2012, 02:38:31 pm
content: I see no noise above the noise floor of the Rigol 1052e I used to test it several years ago.Quote from: MikeK on May 03, 2012, 05:23:54 pm