Rigol DG4162 Amplitude Inaccuracies  - Page 1
Forum section: electronics / testgear

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 17, 2013, 05:20:50 am
content: I recently purchased a DG4162 and overall I do like the instrument but here's some FYI:Here's what a 2Vpp sine wave looks like starting at 50MHz (where the signal displays just proud of the 2Vpp). At 65MHz, you'll notice the wave registers just under the 2Vpp then a bit lower at 66MHz (20mV lower). By 70MHz the error has become a bit gross, at 100MHz its displaying almost a 25% error and at 160MHz...I'm not finding much else to criticize but jeez.

author: Rerouter
author_rank: Super Contributor
timestamp: March 17, 2013, 05:28:29 am
content: are you sure your scope is not contributing some of that loss of amplitude? you failed to mention the model of scope

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 06:01:15 am
content: Are you using a 1:10 probe? Try direct BNC cable with 50 ohm feed thru terminator.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 17, 2013, 06:26:56 am
content: Quote from: Rerouter on March 17, 2013, 05:28:29 amScope is a Rigol DS2202 but I get the same results on my Owon - using the second channel on either scope doesn't change anything either.Quote from: EV on March 17, 2013, 06:01:15 amNo probe, just a direct BNC cable (I tried more than one). The results are the same with the cable terminated at 50 Ohms and with a direct cable (no termination) and the FG set to High-Z.

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 07:55:48 am
content: Here results with my DG4162 generator and DS2202 scope:In the first picture is sweep from 1 MHz to 160 MHz using Rigol 50 ohm feed tru terminator. DG4162 output is set to 50 ohm. The change between max and min level is under 1 dB ( 12 % ) and compared to 1 MHz level the change is under 0.5 dB ( 6 % ).In the second picture is the same sweep with T-Adapter and 50 ohm terminator. There is no change in the curve, if feed thru terminator is used.

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 09:07:05 am
content: From Rigol DC4162 specs:Amplitude Flatness (relative to 100 kHz, 1.25Vpp Sine wave, 50 ohm)Typical<10MHz: ±0.1dB<60MHz: ±0.2dB<100MHz: ±0.4dB<160MHz: ±0.8dBFor comparison in the picture is sweep with HP8642A generator from 5 MHz to 160 MHz.

author: kg4arn
author_rank: Unknown
timestamp: March 17, 2013, 11:03:22 am
content: I just measured my DG4162 with an Agilent DSOX3024A, 50 ohm input.I have previously measured my scope's frequency response as -1.2 dB at 160 MHz and -3dB at 220MHz (that's probably with an error of +/- 0.5 dB)The DG4162 sine wave output measures -0.9dB at 160MHz, which is within the error limits.I get the same results with my vintage HP 8654A signal generator.Not precision measurements, I admit, but they correlate and suggest that at least my DG4162 is performing well.

author: BravoV
author_rank: Super Contributor
timestamp: March 17, 2013, 11:34:05 am
content: Assuming the Rigol is not defective, crappy coax cable or bad terminator ? Try another set maybe ?

author: David_AVD
author_rank: Super Contributor
timestamp: March 17, 2013, 11:51:45 am
content: Quote from: StubbornGreek on March 17, 2013, 06:26:56 amIs the cable rated for the frequencies in question?

author: Unknown
author_rank: Unknown
timestamp: March 17, 2013, 12:12:22 pm
content: Don't expect accurate amplitude measurements from a scope. A scope is usually rated something like 2% at DC, and -3 dB (-30%) at its rated bandwidth. The accuracy between DC and its rated bandwidth can be anywhere in this range, it's only guaranteed to be less than -3 dB. I haven't seen flatness specs for the pass band, at least for low-end scopes.

author: Rufus
author_rank: Super Contributor
timestamp: March 17, 2013, 12:12:46 pm
content: The DG4162 user guide specifies a maximum output of 1v pk-pk into 50 ohm at 160MHz. What you will get with a bit of coax feeding a 1M/15pf (or whatever) scope input is anyone's guess.

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 01:07:06 pm
content: I am curious about this as I've been looking at AWG and AFGs and trying to compare specs which is difficult because they all state them at different levels.One thing that I've noticed about the Rigol is that it avoids speccing its amplifier.I think what Rigol does is rely on the DAC directly for signals up to 1Vpp (into 50ohms) all its specs are at 0dBm or about 0.3V.It then has an amplifier with a fairly low frequency roll off giving x10 (or perhaps x8) up to 20MHz.The gain is set to x4 below 4V (if you look at the manual where it talks about limits on the output it is 11.25V above 4Vpp which is 1.125V at the input of the amp if it is x10,below 4V it is a limit of 4.5V which also equates to 1.125V at the input if the gain is x4).The x4 is able to go to 5Vpp if the DAC is slightly overdriven to 25mA (20mA is the typical from the spec sheet) and this is specced up to 60MHz and then it is set at x2 upto 120MHz and above this the amplifier is probably switched out or set to x1.In the tear down what were described as switched attenuators I think are just switched feedback resistors to set the gain on the output amp.It is interesting to note that gain flatness is specced for a 1.25V sine which may be the limit of the DAC so again the amplifier is not being specced at all.What all this means is that if you keep to low signal levels below 1Vpp you will get much better results than going to 2Vpp or higher where the low frequency amplifier is neededand there are no specs given.The original poster posted results for 2Vpp where the amplifier is needed. Most of the responses above have been for signal levels at 1V or below where the amplifier is not switched in (if I'm correct).It will be interesting to see peoples results for the higher amplitude settings. My suspicion is that Rigol is able to come in at a low cost on the 4000 series (even compared to its own 2000 series) because it is using a relatively cheap op-amp and setting different gain levels for the output. I would guess that much of the cost of more expensive units goes in the design of the wide bandwidth linear amplifier needed to change the 1Vpp DAC output up to 10Vpp that is specced for almost all such instruments.

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 01:37:25 pm
content: Quote from: jpb on March 17, 2013, 01:07:06 pm1 Vpp is max amplitude between 120 - 160 MHz for DG4162.

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 02:27:17 pm
content: Quote from: EV on March 17, 2013, 01:37:25 pmYes, but a comparison could be made between 1Vpp up to 120MHz and 2Vpp up to 120MHz.Amplitude makes quite a difference I think. I've also been looking at the TTi TG5011 which has separate specs for <1V and > 1V. Also at the Tabor WW5061 which in early specs used the level of 5V (it has been around a while and older spec sheets are available on the web) and then reduced it to 3Vpp and improved the high frequency distortion figures as a result.For example, the original Tabor WW5061 spec for Harmonic Distortion up to 25MHz was given at 5Vpp and the figure was -28dBc. The latest figure, for the same hardware, is -40dBc but this is given for 3Vpp. Then you are supposed to compare this with the Rigol which is -50dBc all the way up to 100MHz but it is given at 0dBm which is only 0.31Vpp or one tenth of the amplitude.I've found myself going round in circles trying to do a fair comparison between the Rigol DG4062, the TTi TG5011 and the Tabor WW5061 all of which I can get for about the same price (the Rigol new and the other two as ex-demo or old stock as their new list prices are much higher).

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 04:24:21 pm
content: Quote from: jpb on March 17, 2013, 02:27:17 pmHere are 1...120 MHz sweeps for 1 and 2 Vpp with DG4162 as you wanted.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 17, 2013, 04:41:01 pm
content: At 160MHz and 1Vpp, things don't look much better:

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 05:50:50 pm
content: Quote from: StubbornGreek on March 17, 2013, 04:41:01 pmYou have connected generator to scope with BNC cable and generator output is at HiRes mode? Change generator output to 50 ohm output mode and use 50 ohm terminator (with T-adapter) at scope input. You will get more reasonable results!

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 05:59:21 pm
content: Generators HiRes mode is for 1:10 probe input!

author: kg4arn
author_rank: Unknown
timestamp: March 17, 2013, 06:29:24 pm
content: Quote from: jpb on March 17, 2013, 02:27:17 pmSweeping from 1 MHz to 120 MHz at both 1Vpp and 2Vpp I can see no significant change in the output amplitude into a 50 ohm load.

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 06:52:31 pm
content: Quote from: EV on March 17, 2013, 04:24:21 pmThanks for that. I notice though that the scope is set for 500k samples per second, at least that is what it says at the top of the screen shot, this means it will be undersampling on frequencies above 250kHz so this may do odd things to the magnitude. I presume the sample rate is set by the time the sweep takes and the amount of memory available so it may be necessary to set different frequencies rather than doing a sweep.Anyway I don't want to waste your time with multiple experiments.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 17, 2013, 06:55:30 pm
content: Set the scope to peak detect acq mode and it'll run the ADC flat out all the time.

author: Rufus
author_rank: Super Contributor
timestamp: March 17, 2013, 06:58:24 pm
content: Quote from: jpb on March 17, 2013, 01:07:06 pmI haven't attempted to look at or understand the DG4162 circuitry but my initial assumption was it has DAC good for 160MHz sinewaves and an output stage that is only good for around 50MHz. It only does sinewave above 50MHz and they just let the output stage loose 10db at 160MHz. It will all be calibrated in software so it should be accurate, just the higher frequency you go the less amplitude you can have.I would also be surprised if the hardware for the 3 models isn't identical.

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 07:06:23 pm
content: Quote from: kg4arn on March 17, 2013, 06:29:24 pmThat is a good result. But it may be that some examples are better than others given that others seem to have worse results. If Rigol can get consistently good results at higher amplitudes they should publicize them on the specifications. By only giving results at 0dBm (or 1.25V for amplitude flatness) it leaves the potential buyer with no idea as to performance at typical amplitudes (given that most of these instruments from Agilent, TTi, Tektronix, Tabor and so on have 10Vpp as a maximum I assume that typical waveforms would be in the range 3 to 5Vpp rather than 0.3Vpp).I don't want to knock Rigol who have given the user the option to trade off magnitude against frequency, and many users are probably happy with small signal levels if it allows them to go to much higher frequencies. It just seems to be a general problem with specifications is that each manufacturer chooses their own amplitudes, frequency ranges and so on to give figures and then it is very hard to compare. If you're a company you can get demo machines and do your own tests. If you're buying as an individual (like me) you have to rely on spec sheets and forums like this one.

author: Rufus
author_rank: Super Contributor
timestamp: March 17, 2013, 07:11:10 pm
content: Quote from: EV on March 17, 2013, 05:59:21 pmAt 160MHz a typical x10 scope probe has an impedance of around 70 ohms - hardly 'high' so it will be about 3db down fed from a 50 ohm source.I think the OP needs to get a bit of an rf clue.

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 07:19:33 pm
content: Quote from: Rufus on March 17, 2013, 06:58:24 pmLooking at the photos on this forum (in the review and also the video tear-down) there seems to be three relay switchers by the output amplifiers which I think actually switch gains (2x 4x and 10x) - this seems to fit in with the manual stating that there are different back voltages allowed on the ports depending on whether or not amplitude is above or below 4Vpp and frequency below 10kHz. The 2x gain is only needed well above 10kHz so is not mentioned in the manual.There is a fourth relay which I think switches in or out a final filter stage and I guess this is what determines the rise time of the square waves for instance.I think the hardware of the models is identical except perhaps for this last filter stage. The fact that they all have identical rise times for arbitrary waveforms indicates that this is the case - the arbitrary wave forms do not pass through this final filter (there is an earlier image suppression 160MHz low pass filter that they all have).The software could compensate for the gain dropping with frequency but could only be for typical values so some devices may be incorrectly compensated (I'm just surmising).

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 07:27:47 pm
content: Quote from: Rufus on March 17, 2013, 07:11:10 pmThe OP used a direct BNC connector, at least that is what he said. But you're correct, if a little harsh, in that if the scope wasn't set for 50ohms or didn't have a 50ohm pass through the 16pF of input capacitance would be looking like 60 ohms or so at 160MHz.

author: Rufus
author_rank: Super Contributor
timestamp: March 17, 2013, 07:35:41 pm
content: Quote from: jpb on March 17, 2013, 07:19:33 pmIt can be and probably is calibrated. The TG5011 for example has sine amplitude calibrations at 14 frequencies. 1 and 2 MHz then 5 to 40MHz in 5MHz steps then 43, 45, 47 and 50MHz.The close spacing at the high end indicating there is more to be calibrated there.The Rigol could have many more, the storage cost and time on an automated calibration set up is trivial.

author: kg4arn
author_rank: Unknown
timestamp: March 17, 2013, 08:21:06 pm
content: If anyone is interested...Here's a stepped sweep from the DG4162, 4 feet RG58, 30dB Minicircuits attenuator, Agilent N9320B 3 GHz Spectrum Analyzer.The results look very good for the sine wave amplitude response for 1Vpp (+4dBm) output.I guess that I have a good one (in the output amplitude regard).No argument from me that Agilent and Tek are better but I don't own any to compare. I have noticed that the DG4162 seems to have sweet spot frequencies where the the spurious responses seen on the SA are very few. Probably more expensive instruments do better in this regard.I think you get what you pay for and at least this unit is good enough for my needs.

author: EV
author_rank: Frequent Contributor
timestamp: March 17, 2013, 09:02:33 pm
content: Quote from: AndyC_772 on March 17, 2013, 06:55:30 pmThere is no change by using "peak detect" or "antialiasing"! Only the yellow color is more yellow ( not so light).

author: jpb
author_rank: Super Contributor
timestamp: March 17, 2013, 09:21:40 pm
content: Quote from: kg4arn on March 17, 2013, 08:21:06 pmThose results are well within the spec of 0.8dB and as you say are good. Agilent, Tek etc are much more expensive and don't go to such high frequencies (unless you pay a lot more money) but they do allow the full 10Vpp over the whole (reduced range). They also have more memory and more flexible memory with variable sample clock (in some cases) and more sophisticated filtering available but if you don't need the extra facilities it is not worth paying for them.My personal conundrum is deciding between the Rigol, an ex demo TTi TG5011 or an (very) old stock Tabor WW5061. They all have different pros and cons and I can't decide, I'm suffering paralysis by analysis.

author: opa627bm
author_rank: Regular Contributor
timestamp: March 18, 2013, 01:26:45 am
content: lol dat jetter ......

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 18, 2013, 05:04:23 pm
content: Using the same RG58 cable and changing the generator to 50Ohm resistance (from High-Z) and including an in-line 50 Ohm terminator, gives me results well within the instrument's specifications.Some clarity would be nice on this - I'm trying to "get an RF clue." I was under the impression that the High-Z function emulates a 50Ohm termination... Would anyone care to educate me on what my misunderstanding is?Thanks to all those that have participated in the thread.

author: jpb
author_rank: Super Contributor
timestamp: March 18, 2013, 06:41:55 pm
content: Quote from: StubbornGreek on March 18, 2013, 05:04:23 pmAs far as I understand it all the "High-Z mode" does in the DG4062 is adjust the amplitude value, it doesn't change the 50 ohm impedance of the generator. If you set High-Z mode and then ask for 2Vpp you'll get 2Vpp at the inside point of the 50ohm source impedance as the assumption is there is no voltage drop across the 50 ohms as it is connected to a High-Z that passes no current. If the mode is set to 50 ohms and you set 2Vpp then the voltage at the inside of the 50 ohm internal impedance is set to twice this (4Vpp) to allow for the fact that there is a 50 ohm load so the voltage at the output will be one half (half the voltage across the internal 50 ohms and half across the load 50 ohms).When you set high Z mode the source impedance was still 50 ohms. You then connected it to the 1M/16pF input of your oscilloscope. The problem was the 16pF capacitance, at 1MHz the impedance has a magnitude of around 1k much higher than the internal 50 ohms so the measured voltage is close to the value you set of 2Vpp. At 160MHz the impedance has dropped to only about 60 ohms which is similar to the internal 50 ohms (they are different phases of course, one being real and the other capacitive) so the voltage will have almost halved.With the 50 ohm pass through load the load starts at 50 ohms (the 1M + 16pF being high impedance) and at 160MHz it is down to about 1/sqrt(50^-2+62^-2) or about 39 ohms which is not that far off the original 50 ohms (I've somewhat simplified the fact that you have different phases, though I've allowed for it a bit in calculating the 39 ohms).The most accurate results will be obtained with a scope with a proper 50ohm input impedance.I've not touched on the effect of the cable. With the 50 ohm pass through the 50 ohm cable is terminated in something close to 50 ohms so is well matched so still looks like 50 ohms at the generator. Without the pass through the 1M/16pF impedance will be transformed by the cable but as the wavelength even at 160MHz is still quite long (1.875m in air, maybe one third of that in the cable depending on the dielectric constant) for most of the frequencies the unmatched cable will look like extra capacitance at the generator and will look inductive between the generator and the oscilloscope. It is complicated to work out the effect accurately, you could do it on a Smith chart. The point is that where the cable is not well matched it will have an effect, where it is well matched (50 ohm impedance each end) it should just introduce a slight time delay.I hope all the above makes sense.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 18, 2013, 07:15:42 pm
content: Quote from: jpb on March 18, 2013, 06:41:55 pmVery much so, thank you - I really appreciate the clarification.

author: EV
author_rank: Frequent Contributor
timestamp: March 18, 2013, 07:44:54 pm
content: Quote from: jpb on March 18, 2013, 06:41:55 pmHere is the same sweep (1...160 MHz) when DG4162 is connected directly with BNC cable to Tektronix TDS3032 scope 50 ohm input. DG4162 is in 50 ohm output mode.

author: Rufus
author_rank: Super Contributor
timestamp: March 20, 2013, 07:55:38 pm
content: Quote from: jpb on March 18, 2013, 06:41:55 pmThis is the impedance at the end of of a 1.1m RG58 BNC cable connected to a DSO-X 3000 scope input. 200 ohm/div, 0 ohms at the bottom, 1 to 200MHz.Then the same with the scope input at 50 ohms. 2 ohm/div, 50 ohms in the middle.

author: jpb
author_rank: Super Contributor
timestamp: March 20, 2013, 10:53:33 pm
content: Rufus, that illustrates the point very well!My background is in microwave engineering (monolithic circuits 10GHz and above), we tended to view any frequency below 1GHz as dc so I tend to forget that there is quite a difference between 1MHz and 200MHz!

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: March 21, 2013, 03:26:52 pm
content: Thank you for the visual representation - it lends further clarity. BTW, nice scope!Quote from: Rufus on March 20, 2013, 07:55:38 pm