Another radio amateur about to buy a DSO ... - Page 1
Forum section: electronics / testgear

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 11:34:09 am
content: After reading many threads on the subject here and suffering from the resulting analysis paralysis I havealmostdecided to go for a Siglent SDS1202X-E.One very helpful post from a couple of years ago showed that it has a 3dB bandwidth of around 300MHz which is fine for me as I work mostly below 50MHz.I also read about input noise, and how some scopes use relays to switch attenuators while others use tapped resistors. Does anyone know which way Siglent goes?Mike

author: Performa01
author_rank: Super Contributor
timestamp: November 11, 2023, 12:06:26 pm
content: Quote from: Mike99 on November 11, 2023, 11:34:09 amI don't know which post you're referring to, but actual measured -3 dB bandwidth of an SDS1202X-E is about 240 MHz. I have no reason to believe that the SDS1204X-E would be any different. Much more important, the amplitude drop for most Siglent DSOs usually won't eceed -1 dB at the rated bandwidth.Reliable (but partially a little outdated) information on all the properties of the SDS1000X-E series can be found in this thread:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1371771/#msg1371771Quote from: Mike99 on November 11, 2023, 11:34:09 amI'v never heard of "tapped resistors" in a DSO. All DSOs I'm aware of are using proper attenuators, yet there is a difference in implementation: many cheap instruments make do with just one attenuator for the entire vertical sensitivity range, whereas the SDS1000X-E has two of them (like the big boys), which enables a more optimal amplitude scheme and lower noise indeed in certain situations. On top of that, the SDS1000X-E is one of the very few DSOs with a true (full resolution) 500 µV/div setting. Others appear to have high sensitivities too, but these are mostly just softare magnifications which costs resolution and doesn't help to increase the SNR.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 12:36:15 pm
content: You'll have to set a max budget before talking what to buy.I would look for something with (in this order): 4 channels, 12bits ADC, 50 ohms input option, an internal generator channel (AWG) would be nice for bode plots. Siglent and Rigol are trustworthy brands while still affordable.In terms of bandwidth, in practice it's not much of a difference between 100 or 200 MHz. The logic analyzer option doesn't matter either, for the price difference usually there are better standalone USB LA.Both Rigol and Siglent can be unlocked, so for hobby use might worth buying the smallest model for a given series, then unlock it (for free) to its max performance and max software options. The hardware is the same for each given products line. This may add a lot of value for Rigol and Siglent, when compared with other not so hacking-friendly brands.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 12:54:48 pm
content: Points to watch out for, which might indicate a scope isn't the best tool for your requirements...Ignore headline figures, and understand how the "ENOB" affects distortion (time domain) and harmonics (frequency domain) analysis.Verify whether post-processing (e.g. FFT, protocols) operates on the entire captured buffer or just that portion which is visible on the screen. Understand the consequences of the buffer size w.r.t. frequency domain analysis.IMHO if you are interested in harmonics then consider a spectrum analyser for RF work, or a sound card for audio work. If you are interested in "higher level" radio signal analysis, consider an SDR.

author: Fungus
author_rank: Super Contributor
timestamp: November 11, 2023, 01:36:06 pm
content: Quote from: Mike99 on November 11, 2023, 11:34:09 amThe choice would be between that and the new kid on the block, the Rigol DHO802.Prices are similar, overall capabilities are similar, but the Rigol is more modern (eg. touch screen and windowing) and has 12-bits. Also HDMI out, VESA mount...For both of these you'd buy the base model and hack them to full bandwidth. Measured bandwidth would be about 200Mhz on the Rigol.Dave compares the FFT abilities here:

author: Fungus
author_rank: Super Contributor
timestamp: November 11, 2023, 01:38:55 pm
content: Quote from: tggzzz on November 11, 2023, 12:54:48 pmI don't think any low end model DSO does that. Rigol and Siglent are both limited to 1Mpts.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 02:12:00 pm
content: Quote from: Fungus on November 11, 2023, 01:38:55 pmI strongly suspect that is the case.I haven't investigated what high-end expensive scopes achieve.

author: G0HZU
author_rank: Super Contributor
timestamp: November 11, 2023, 02:19:45 pm
content: Quote from: Performa01 on November 11, 2023, 12:06:26 pmTake a look inside a Rigol 5000 series scope. There's a front end relay to select to the 50R path (and the 50R path looks to have a relay controlled attenuator) but the main 1Meg path is all done using a series of tapped resistors that feed to the first input IC via the various taps. There are no relay controlled attenuators on the 1Meg path. If you look at the resistor arrangement, you will be able to work out how it maintains a 1Meg input (for all the taps) but it inevitably does this in a lossy way. There's always a big fixed resistive divider at the input on every range. This fixed divider spoils the signal to noise ratio.Other scopes can deselect the attenuators using relays allowing much better noise performance on the sensitive ranges.So I'd expect the Rigol 5000 series scopes to have very high noise levels on the sensitive ranges on the 1Meg input. I've never used one but plenty of people complain that they are noisy.

author: G0HZU
author_rank: Super Contributor
timestamp: November 11, 2023, 02:26:54 pm
content: You can also look inside some of the budget Tek scopes to see a similar arrangement using fixed and tapped potential dividers. There are no relays involved in the selection of the attenuation ahead of the front end IC. These scopes will also be noisy on the sensitive ranges because of the fact that there is always some fixed attenuation involved even on the most sensitive range. Hence the signal to noise ratio is degraded.It's very similar to how you can observe how any scope gets noisy with a x10 scope probe fitted. The resistive divider in the probe degrades the signal to noise ratio seen on the display once the scope is set to x10 probe mode. The cheapo scopes that use the tapped dividers will suffer the same degradation in signal to noise ratio because there is always a fixed divider at the front end. You can easily spot it because it will be a large SMD resistor rated for high voltage and it will be somewhere in the region of 750k ohm. It will have some compensation capacitance in parallel with it. This can't be deselected so these scopes are designed to (inevitably) have a degraded signal to noise ratio on the most sensitive ranges.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 02:50:47 pm
content: Thanks everyone. I think you're telling me that the Siglent uses relays to switch the attenuators.I already have a quality SDR with spectrum analyser software so I'm only looking for oscilloscope functions. My current scope is a Tek 2430A - big, noisy, heavy.As for budget, ideally no more than £500 so I could afford the Rigol DHO812 (I don't need 4 channels for what I do) but it's only 100MHz bw and I don't like the front panel layout. At a push I could go for the Siglent SDS2202X-E.A 50 ohm input would be useful but I was planning to use a feedthrough terminator.Mike

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 03:11:34 pm
content: Quote from: Mike99 on November 11, 2023, 02:50:47 pmDHO900 goes up to 250MHz, but I don't have it so no idea how it is.https://www.rigolna.com/products/rigol-digital-oscilloscopes/dho900/

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 03:18:01 pm
content: Quote from: RoGeorge on November 11, 2023, 03:11:34 pmIt's too expensive for me (£740).Mike

author: Fungus
author_rank: Super Contributor
timestamp: November 11, 2023, 03:41:48 pm
content: Quote from: Mike99 on November 11, 2023, 02:50:47 pmThat's what it says on the front but real-life measurements show it's much higher.A lot of people here have bought the 70MHz model and unlocked it to 100Mhz with a license key generator. Measured bandwidth on the unlocked 'scopes is 200MHz (I've measured mine, several other people have measured theirs and got this result).eg. Here's Martin72 measuring (via rise time) 125Mhz bandwidth on a "70Mhz" model and 200MHz bandwidth on a "100MHz" model.https://www.eevblog.com/forum/testgear/split-from-rigols-new-dho800-oscilloscope-unbox-amp-teardown/msg5156580/#msg5156580Why is Rigol underselling the DHO800? We don't know. We only know it's a marketing decision.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 03:50:59 pm
content: Yes I know these can be hacked but I would be reluctant to try until it's out of warranty.Mike

author: ebastler
author_rank: Super Contributor
timestamp: November 11, 2023, 03:56:41 pm
content: Quote from: G0HZU on November 11, 2023, 02:19:45 pmI don't think the MSO5000 has an internal 50 Ohm input path at all. Are we thinking of the same scope here?

author: tautech
author_rank: Super Contributor
timestamp: November 11, 2023, 04:06:46 pm
content: Quote from: Mike99 on November 11, 2023, 02:50:47 pmYou do get more features with 2kX-E models.1M/50 Ohm input path.WebserverBode plotWiFiAnd it's a 350 MHz design......

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 04:07:39 pm
content: Quote from: Mike99 on November 11, 2023, 03:50:59 pmThe unlock is a number. Can be manually introduced, or removed at any time. Won't void the warranty.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 04:24:08 pm
content: Quote from: tautech on November 11, 2023, 04:06:46 pmNow that's interesting. If I bought the SDS2202X-E (200MHz) could I open it out to 350MHz?Mike

author: G0HZU
author_rank: Super Contributor
timestamp: November 11, 2023, 04:26:04 pm
content: Quote from: ebastler on November 11, 2023, 03:56:41 pmWhoops, sorry for any confusion caused. It turns out that I was describing the Rigol 7000 scope. I was watching Dave's Rigol 5000 teardown and he had overlaid a screenshot of the 7000 front end. I got the two mixed up and it's only the 7000 that appears to have the 50 ohm path.Both of them appear to have the tapped resistive network on the 1Meg path though. They look very similar in this respect so I'd expect both of them to be quite noisy because of this.

author: 2N3055
author_rank: Super Contributor
timestamp: November 11, 2023, 04:28:30 pm
content: Quote from: ebastler on November 11, 2023, 03:56:41 pmIt does not. You need to step up to DS/MSO7000 for 50 Ω Path. Also only DHO4000 has it..

author: 2N3055
author_rank: Super Contributor
timestamp: November 11, 2023, 04:29:55 pm
content: Quote from: G0HZU on November 11, 2023, 04:26:04 pmYou are correct. They are noisy..

author: tautech
author_rank: Super Contributor
timestamp: November 11, 2023, 04:30:42 pm
content: Quote from: Mike99 on November 11, 2023, 04:24:08 pmYup. Not difficult at all.IIRC -3dB BW =~400 MHzMember xrunner posts about his here:https://www.eevblog.com/forum/testgear/oldish-vs-newish-test-equipment/msg4685039/#msg4685039

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 05:42:42 pm
content: Quote from: tautech on November 11, 2023, 04:06:46 pmAre they 50ohm inputs or 1Mohm//50ohm//15pF inputs?What's the VSWR, which is relevant at RF frequencies?

author: tautech
author_rank: Super Contributor
timestamp: November 11, 2023, 05:44:58 pm
content: Quote from: tggzzz on November 11, 2023, 05:42:42 pmhttps://int.siglent.com/u_file/document/SDS2000X-E_DataSheet_DS0102E-E02A.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 05:49:27 pm
content: Quote from: tautech on November 11, 2023, 05:44:58 pmVSWR isn't specified, which often happens where the manufacturer just puts a 50ohm resistor in parallel with the15pF18pF.Any chance you could put a VNA on one of yours, in order to settle the issue?

author: tautech
author_rank: Super Contributor
timestamp: November 11, 2023, 06:05:15 pm
content: Quote from: tggzzz on November 11, 2023, 05:49:27 pmNo chance, no 2kX-E currently in stock.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 06:13:02 pm
content: Quote from: tautech on November 11, 2023, 06:05:15 pmShame.Are there any others with a 1Mohm//*pF and a nominal 50ohm input, but no VSWR spec?Ditto 50ohm with a VSWR spec?Either would provide a useful clue.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 07:14:03 pm
content: Quote from: tautech on November 11, 2023, 06:05:15 pmWell now I'm leaning towards the 2202X-E! I'm going to the dealer's on Tuesday for a demo of various models and planning to walk out with one, so if I take my NanoVNA with me I'm sure they'll let me look at the VSWRMike

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 07:40:44 pm
content: Quote from: Mike99 on November 11, 2023, 07:14:03 pmExcellentwe look forward to hearing your conclusions.In that position I would want to try some form of quick and dirty experiment at home, to debug my experimental technique without embarrassing myself in public. Me a pessimist? Shurely shome mishtake

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 08:10:32 pm
content: Quote from: tggzzz on November 11, 2023, 07:40:44 pmIt shouldn't be a problem measuring VSWR. I will try it on my Tek 2430A first.Mike

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2023, 08:34:28 pm
content: 18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$. No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 09:01:51 pm
content: Quote from: RoGeorge on November 11, 2023, 08:34:28 pmPossibly, but many scopes - including from reputable manufacturers like HP and Tek - don't bother. Hence the desire to have it measured.One scope that gets it right has two attenuated (one 50ohm, one 1Mohm), and an RF relay to switch between them. That has the advantage that an overload pops the relay back to 1Mohm before the 50ohm circuits are damaged.

author: bdunham7
author_rank: Super Contributor
timestamp: November 11, 2023, 09:09:16 pm
content: Quote from: Mike99 on November 11, 2023, 07:14:03 pmQuote from: RoGeorge on November 11, 2023, 08:34:28 pmThere were two previous discussions on this:https://www.eevblog.com/forum/beginners/whats-the-input-capacitance-of-an-oscilloscopes-50-ohm-input/https://www.eevblog.com/forum/testgear/need-50-ohm-input-on-an-(old)-oscilloscope-that-has-only-1m-ohm-inputs-bnc-tee/msg4484371/#msg4484371They're not long, and you'll find that I tested a Tek 485, Tek 2465B and a Siglent SDS2354X+ with a NanoVNA. The 485 is best of all, but the Siglent is essentially perfect to 350MHz, as good as the 2465B and more than likely at least as good as the 2430A since the attenuator design is similar to the 2465B. The Siglent had a VSWR of 1.43 @ 500MHz, but it only curved up significantly over the 400-500MHz span. Below that it was low enough to not worry about, IMHO.Also in that discussion I measured the Siglent with an LCR meter and it seemed to indicate that the input capacitance of 18pF had some 10k ESR. Some other theories and diagrams were presented, but what seems clear is that the input capacitance specified for the 1M input is not simply the equivalent of an 18pF directly across the input. Thus statements observing that an 18pF capacitor would have a very low impedance at the upper end of the scopes BW aren't really relevant because the input has additional complexity. An 18pF capacitor with a 10k resistor in series still looks a lot like 18pF if the rest of the circuit is 1M. But even you just slap a 50R resistor across all that, the 18pF now looks like about 10k, not 25R, and the phase angle will be nearly zero.Ultimately I don't have accurate enough stuff to try and model these inputs with any real precision, but it seems pretty clear to me that it isn't just 50R with 18pF across it and then they simply fail to mention the capacitance. The results from that would be really bad, people would notice.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 09:17:30 pm
content: Quote from: tggzzz on November 11, 2023, 09:01:51 pmThat's what the 2430A does. It has a 50 ohm termination resistor and thermal sensor. The manual states 1.3:1 VSWR to 150MHz. I'll measure it shortly.Mike

author: tggzzz
author_rank: Super Contributor
timestamp: November 11, 2023, 09:30:27 pm
content: Quote from: Mike99 on November 11, 2023, 09:17:30 pmSounds like the earlier HP1740, except that doesn't have the relay. You can guess a common cause of failure!

author: G0HZU
author_rank: Super Contributor
timestamp: November 11, 2023, 09:48:30 pm
content: Quote from: bdunham7 on November 11, 2023, 09:09:16 pmAs I mentioned in that other thread, by deliberately adding some inductance in series with the 50R termination the circuit will mimic approx 50R in parallel with a negative capacitance and it will do this over a wide bandwidth. The negative parallel capacitance will cancel some of the existing 18pF over a fairly wide bandwidth.So that simple trick alone will mask some of the input capacitance when the 50R terminator is selected.The other way to do it is to arrange the 50R termination as a potential divider made up of a series 20R and then switch in a shunt 30R with a relay when in 50R mode. This will mask the 18pF input capacitance yet further and when combined with the inductance trick, the scope could achieve a decent VSWR over a large bandwidth. The penalty will be a higher noise floor due to the attenuation introduced by the 20R and 30R potential divider.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 11, 2023, 10:04:20 pm
content: The 2430A service manual doesn't show the input arrangement in enough detail but I've just measured mine.Sorry about the layout I can't get the hang of the attachments.Also ignore the cal description, it is calibrated over the full measurement range.R+jX for the 1M input.VSWR for the 50 ohm inputMike

author: Fungus
author_rank: Super Contributor
timestamp: November 11, 2023, 11:57:59 pm
content: Quote from: Mike99 on November 11, 2023, 03:50:59 pmAll you're doing is entering a license key as if you'd bought it from Rigol. Inputting a few numbers can't void the warranty.nb. It can easily be removed, too.

author: Performa01
author_rank: Super Contributor
timestamp: November 12, 2023, 11:26:03 am
content: I don't have an SDS2000X-E, only SDS2000X Plus. I honestly cannot tell whether the little 2000X-E has a true 50 ohms path like the SDS2000X Plus (and higher models) have, but the following demonstration should still be universal to show how an external through termination is only an option at low frequencies. For higher frequencies, a proper internal 50 ohms path is mandatory to get a reasonable VSWR , hence reasonably accurate measurements.Look at reply #237 here:https://www.eevblog.com/forum/testgear/rigol-dho804-test-and-compare-thread/msg5114334/#msg5114334

author: Mike99
author_rank: Frequent Contributor
timestamp: November 12, 2023, 01:13:22 pm
content: Quote from: Performa01 on November 12, 2023, 11:26:03 amThat's very interesting, thank you. It's nudging me towards the 2202X-E which has a selectable 50 ohm input.Mike

author: Performa01
author_rank: Super Contributor
timestamp: November 12, 2023, 03:38:20 pm
content: Quote from: Mike99 on November 12, 2023, 01:13:22 pmIt would be a sensible choice especially for a radio amateur. While the SDS1202X-E really is the bottom of the barrel instrument in the modern Siglent lineup (and has less features than the SDS1004X-E), the SDS2000X-E series gives you not only higher sample rate and bandwidth together with the 50 ohm inputs required for that, but also more software features like Search and Bode Plot.The latter should be interesting especialy for a ham, since it works with any Siglent AWG and offersup to three channelsup to 120 MHz at a high dynamic range >100 dB. So you cannot onlycharacterizeyour newly designed three-way crossover network for your loudspeaker in one go, but alsoall sorts of (IF) filters. Yes a VNA is faster, but the Bode Plot can be used down to 10 Hz at high accuracy.EDIT: Yeah, for three channels we need a four channel scope, so the SDS2000X-E is limited to just one channel...

author: ebastler
author_rank: Super Contributor
timestamp: November 12, 2023, 04:07:33 pm
content: Quote from: Performa01 on November 12, 2023, 03:38:20 pmBut the three-channel plots would only work with a four-channel scope, right? The SDS2000X-E series only comprises dual-channel models, of which one would be used for the input and one for the (single) output of the DUT, in my understanding.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 14, 2023, 02:54:43 pm
content: The deed is done. I bought the Siglent SDS2202X-E and they gave me a discountHere is the input VSWR measured on an FA-VA5 VNA (because my NanoVNA has a couple of inband spurious signals).Mike

author: bdunham7
author_rank: Super Contributor
timestamp: November 14, 2023, 03:04:28 pm
content: Quote from: Mike99 on November 14, 2023, 02:54:43 pmAre you using the basic calibration elements that came with the FA-VA5 or the better ones that they sell separately?

author: Mike99
author_rank: Frequent Contributor
timestamp: November 14, 2023, 08:44:05 pm
content: Quote from: bdunham7 on November 14, 2023, 03:04:28 pmShortly after I put that picture up I realised it was nonsense, and yes I do only have the basic cal kit for that VNA.I fixed the problem with my NanoVNA and tried again, using the SMA cal kit that came with it. Not ideal at higher frequencies but needs must. The result may not be spot on but it's the shape I expected.Mike

author: Performa01
author_rank: Super Contributor
timestamp: November 15, 2023, 11:11:27 am
content: Well, this is a 200 MHz scope, right?From the plot I can see a VSWR <1.5 up to 340 MHz, so there is nothing to complain about.Most likely there is a difference between the direct path up to 100 mV/div and above that, when the first internal attenuator gets switched in, e.g. 200 mV/div. With attenuator, the VSWR usually gets better.

author: 2N3055
author_rank: Super Contributor
timestamp: November 15, 2023, 01:02:43 pm
content: Quote from: Performa01 on November 15, 2023, 11:11:27 amAnyways, it shows 50Ω path isnotmade by simply putting 50Ω terminator in parallel with 1MΩ/20pF input...

author: ebastler
author_rank: Super Contributor
timestamp: November 15, 2023, 01:13:17 pm
content: Quote from: 2N3055 on November 15, 2023, 01:02:43 pmThis would be a nice opportunity to demonstrate the difference.@Mike99, any chance you could repeat the measurement with the scope's input set to 1 MOhm and an external 50 Ohm pass-through terminator? Thanks!

author: Performa01
author_rank: Super Contributor
timestamp: November 15, 2023, 03:39:17 pm
content: Quote from: ebastler on November 15, 2023, 01:13:17 pmWas this not demonstration enough?https://www.eevblog.com/forum/testgear/another-radio-amateur-about-to-buy-a-dso/msg5164407/#msg5164407

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2023, 04:22:43 pm
content: Quote from: Mike99 on November 14, 2023, 08:44:05 pmWe all have the occasional brain failure!Looks like I was right to question the statements in the earlier post shown belowA cursory inspection of the document referred to by Tautech would lead you to think that it has a 50ohm input.Channel Flatness (Inner 50 Ω)DC - 60% (BW): ± 1 dB60% - 100% (BW): + 1 dB/-3 dBMax. Input voltage1 MΩ: ≤ 400 Vpk (DC + Peak AC <= 10 kHz)50 Ω: ≤ 5V rmsA closer inspection reveals what I regard as specmanship...ImpedanceDC 1 MΩ: (1 MΩ ± 2%) || (18 pF ± 2 pF)DC 50 Ω: 50 Ω ± 2%Of course only an owner can say whether that is significant in their application.Quote from: tggzzz on November 11, 2023, 05:49:27 pm

author: ebastler
author_rank: Super Contributor
timestamp: November 15, 2023, 04:35:00 pm
content: Quote from: Performa01 on November 15, 2023, 03:39:17 pmOh, right. For some reason I only remembered the pulse responses from your tests, not the return loss plots. Thanks for pointing them out again. Yes, those will do!So the 2000X+ performs significantly better still than the 2000X-E -- which makes sense, given that it is specified up to 500 MHz. I am curious what it takes to improve the high-frequency matching. Presumably some of the "ingredients" cost money, and were hence not justifiable for the entry-level scope? So, besides properly chosen R and C as well as careful routing, what is required to get this right?

author: bdunham7
author_rank: Super Contributor
timestamp: November 15, 2023, 04:45:04 pm
content: Quote from: 2N3055 on November 15, 2023, 01:02:43 pmFrom the earlier discussions that I linked above, I'm not sure that is an entirely accurate explanation. The external 50R across the existing 1M/20pF input also does not behave as you'd expect from a simple model using the stated values and depending on the exact terminator and setup, can perform much better than predicted--even if it isn't as good as the internal termination.My theory was that the 20pF isn't just 20pF directly across the input but rather that it has some significant series resistance, not enough to matter in a 1M circuit but enough to take it out of the picture in the 50R case.

author: G0HZU
author_rank: Super Contributor
timestamp: November 15, 2023, 10:46:04 pm
content: Quote from: Mike99 on November 14, 2023, 02:54:43 pmSounds good! If you plan to use it for ham stuff (including transmitter testing) make sure you don't exceed the AC voltage limits (vs frequency) for a typical digital scope.For the internal 50R load this is usually about 5Vrms across all frequencies within the bandwidth of the scope.For the 1Meg input, I'd recommend you keep all RF signals below about 10Vrms, especially at mid to upper HF and into VHF. The scope will only be rated for high AC voltages at low frequencies. So you can't expect it to cope with (say) 400Vpeak above a few kHz. There may be a derating curve for AC voltage versus frequency somewhere in the manual.Even a 12W PEP 27MHz CB radio can pose a risk of damage to some digital scopes unless you use some form of inline attenuation.

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 12:26:55 am
content: Quote from: G0HZU on November 15, 2023, 10:46:04 pmThanks for the advice! I'm having fun playing with low level stuff at the moment. Love the FFT function!Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 12:29:32 am
content: Quote from: ebastler on November 15, 2023, 01:13:17 pmI don't have a pass through terminator, only a 6dB BNC attenuator.Mike

author: tggzzz
author_rank: Super Contributor
timestamp: November 16, 2023, 09:18:44 am
content: Quote from: Mike99 on November 16, 2023, 12:29:32 amA 6dB passthrough attenuator followed by the imperfect internal attenuation will bebetterat reducing VSWR than an external terminator. That technique is frequently usedinsideequipment where components/subsystems have poor impedance matching.Basic principle: gain is easy and cheap, wideband matching is difficult and expensive. But don't stick an attenuator in front of a low-noise front end, of course

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 11:53:43 am
content: I have made some more measurements.First was the 50 ohm input bandwidth, measured using an RF millivoltmeter:Next was input VSWR. The graph compares the 50 ohm input with the 1Mohm input, the latter using a T piece with a 50 ohm load.Finally, the impedance of the 1Mohm input.Mike

author: Orange
author_rank: Frequent Contributor
timestamp: November 16, 2023, 12:52:39 pm
content: Hi Mike,An impedance plot for 50 Ohms would make this information complete :-)

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 01:07:59 pm
content: Quote from: Orange on November 16, 2023, 12:52:39 pmOK I'll do that later today.Mike

author: dshorthill
author_rank: Contributor
timestamp: November 16, 2023, 03:34:14 pm
content: I use this device to monitor the RF output of a 1 kw ham amplfier on my 1204xe Siglent scope. I have a bnc t with a 50 ohm termination on the scope end. Works great.https://www.ebay.com/itm/193686855709Davek1xd

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 05:19:34 pm
content: Quote from: dshorthill on November 16, 2023, 03:34:14 pmHi DaveYes I have a similar device, not as high power but it gives -50dB coupling up to 500MHz. Very useful.Mike G8GYW

author: Mike99
author_rank: Frequent Contributor
timestamp: November 16, 2023, 05:21:22 pm
content: Quote from: Orange on November 16, 2023, 12:52:39 pmHere it is ...Mike