Factor 10 higher rise and fall times on new function generator - Page 1
Forum section: electronics / beginners

author: mcpt21
author_rank: Newbie
timestamp: June 11, 2024, 07:40:09 pm
content: Dear forum,I got myself a SDG 1062X function generator yesterday and was playing around with it today in conjunction with a R&S RTC1002 oscilloscope: I was generating square waves @ 100kHz with 4 Vpp and 50% duty cycle. Using the correct probe of the scope (no simple coax cable as already read in another post here) I get a nice square signal, however tr and tf are over 40ns. According to the spec (https://www.batronix.com/pdf/Siglent/SDG1000X/SDG1000X_Datasheet_EN.pdf) they should be in the order of 4ns. I tried both channels but they yield the same issue. Also Vpp does not seem to have an effect (which makes sense to me). Is there anything else I could be doing wrong or is the generator faulty?Any help is kindly appreciated.Regards,Martin.

author: Manul
author_rank: Super Contributor
timestamp: June 11, 2024, 08:18:49 pm
content: Sounds like your probe is set to 1X or you have bandwidth limiting somewhere else. I have same 1062X and yes, it does have 4ns rise and fall times while doing square wave output, a bit less than 4ns in fact.

author: Sensorcat
author_rank: Regular Contributor
timestamp: June 11, 2024, 08:20:56 pm
content: Have you got a 1x probe, or a 10x/1x switchable probe? Those have limited bandwidth in 1x, on the order of 10MHz.Have you got the BW limit set in the scope? Many scopes have a setable 20MHz limit for reduced noise.

author: mcpt21
author_rank: Newbie
timestamp: June 12, 2024, 06:24:36 am
content: Thanks for your replies!The scope is the 300MHz version so no worries there, I suppose.I have a 10x/1x switchable probe and just set it to 10x and adjusted the probe setting in the scope. Rise and fall times have now been reduced to around 6.5 – much better but still higher than specified.I just flipped through the menus and don't see a limiting factor anywhere. The scope does have a 20MHz bandwidth limit option but it's turned off.Since I switched the probe setting to 10x, the signal is now pretty much swinging in to the peak levels. Stable within 70ns but still?Thanks a lot for your support. I am fairly new to this domain...

author: Aldo22
author_rank: Super Contributor
timestamp: June 12, 2024, 07:36:55 am
content: Quote from: mcpt21 on June 12, 2024, 06:24:36 amThe small difference probably comes from the probes. What is the 10x bandwidth of these?You could test with a short coaxial cable.And up to 3% overshoot is stated in the data sheet.All in all, no reason to worry imo.

author: tautech
author_rank: Super Contributor
timestamp: June 12, 2024, 07:44:22 am
content: Quote from: mcpt21 on June 11, 2024, 07:40:09 pmSpecified risetime is not available if using a probe.Instead you need a direct coax connection.

author: radiolistener
author_rank: Super Contributor
timestamp: June 12, 2024, 05:18:15 pm
content: attempt to get nice rise time with probes is a stupid goal. The probe bandwidth is limited, it can't work good with wide bandwidth, this is impossible.If you want nice rise time the only way is to use direct coax cable with proper termination.

author: mcpt21
author_rank: Newbie
timestamp: June 12, 2024, 05:54:48 pm
content: OK, thank you very much for your input. I need to read more into this topic!With the current setup (direct connection by coax without termination) I get 4.56 Vpp instead of 4.0. That makes sense to me.By proper termination you are talking about a 50 Ohm resistance at both ends of the coax cable as suggested here:https://www.eevblog.com/forum/testgear/50-ohm-bnc-cable-recommendation-for-scopefreq-gen/?I need to get such a cable and/or 50 Ohm feedthrough terminators.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2024, 06:37:50 pm
content: The signal generator is (should be) terminated internally. Check the manual for details.You just need to provide one on the scope end. Often, scopes have this integrated as an input coupling option, otherwise a tee and terminator, or inline terminator, can be used.You can also read the wave at double amplitude with no scope-end termination (this is a "source terminated" configuration), but you get additional losses due to the cable, and the scope's slightly capacitive input, which tends to increase risetime readings.A 10x probe may be worse, but it depends on the probe type. Typically the probe tip has a 11M || 10pF or so characteristic, which has little loading on a 50Ω signal, but that's the DC to mid-frequency model, and the interesting stuff happens at high frequencies. At HF, the probe more closely resembles a "lo-Z probe", like a 450Ω resistor in series with a 50Ω transmission line and terminator. The probe's impedance changes gradually from DC to MF to HF extremes, exhibiting this R || (R + C) characteristic. The catch is, at HF, there isn't just R and C, there is also L, because the probe tip, and ground clip, have nonzero length. Typically the 10pF (and I use this loosely, because as said, it may have significant resistance, or have larger effective capacitance up here, or other perspectives) resonates with the ~0.2uH of this inductive path, giving a cutoff the probe is rated for (~100MHz).Characteristically, you'll find that higher-performance probes are physically smaller. The body/handle and cable might be typical, though the cable might be a bit shorter, but in particular, the tip and clip are made much shorter. A 500MHz probe is usually about half the overall size of a 100MHz probe, give or take the exact proportions of all its parts (which, presumably, the important parts are about 5 times smaller..!).Even a "100MHz" probe can still transmit sharp pulses -- I've viewed a <1ns avalanche generator with one before, for example -- you can still get something through it, it's far from a brick wall cutoff, but the amplitude and phase aren't as specified anymore andYou also tend to get some peaking, as 100MHz probes may be designed to pass a bit extra at the high end, compensating in part for a soft-rolling-off scope. Or the scope and probe system together may be rated for specified performance, but not any random combination. (This in general is kind of dubious, historically speaking; from the distributed amps of the 1950s and 60s, to ASICs and distributed CRTs through the 80s, to ADC acquisition up to present, frequency response has been all over the place, and there's no guarantee that any one scope will actually benefit from this peaking.)Probes sometimes come with a coaxial coupling; pull the tip off (it should simply snap on) and notice the coaxial shield and pin. This eliminates the ground clip inductance, but there may still be bandwidth limitations due to the internal structure of the probe.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: June 12, 2024, 06:49:51 pm
content: Quote from: mcpt21 on June 12, 2024, 06:24:36 amLooks like you are using the 15cm ground lead on the probe; that would cause the ringing. FFI seehttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/Probing a modern digital circuit is not trivial, since the signals have energy in the UHF and microwave bands. FFI on probes and using them safely and effectively, seehttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/