Output from TXCO - Page 1
Forum section: electronics / projects

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 06:56:40 am
content: Hi,I'm trying to replicate the Microchip Ethernet Starter Kit II on my own board. Their design has the use of a TXCO (temperature controlled XTAL oscillator). I'm using ECS-TXO-2520 - the 50MHz variety.So I have a board and a new scope (Siglent SDS2204X Plus) and was poking around looking at things and familiarizing myself a bit more with the scope when I looked at the output of this TXCO.According to the scope the output swings from about 4.07V down to -0.5V or so !! The Vdd is 3V3.There are two 'impedance matching resistors' (Microchip term) where the TXCO output is split to two different chips, a LAN8740A and a PIC chip both running 3V3 Vdd.Why would the output of the TXCO be much more than 3V3 ?I'm wondering if I'm seeing things ...

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 16, 2024, 07:09:05 am
content: Those spikes in the edges are normally caused by the scope probe itself, basically acting as an unloaded transmission line.Probe in 10x mode mostly gets rid of this.

author: fourfathom
author_rank: Super Contributor
timestamp: January 16, 2024, 07:26:32 am
content: And make your probe ground connection as short as you can. There are small ground clips that attach to the probe, eliminating the ground lead.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 16, 2024, 07:28:37 am
content: Yep, I've also noticed a great difference when attaching those "springs"

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 09:57:35 am
content: Thanks guys for the response.The signal is not a spike at all. In fact, it is almost sinusoidal. The Vpeak is around 4.07V and the lowest is -0.5V or so.

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 10:03:29 am
content: My concern is that it is up to 0.8V above the chip voltages of 3V3.

author: tggzzz
author_rank: Super Contributor
timestamp: January 16, 2024, 10:31:08 am
content: Quote from: swmcl on January 16, 2024, 06:56:40 amHave you checked and adjusted the probe's HF compensation?Are you using a short (1cm) or long (15cm) probe shield connection? The inductance inherent in long probe shield connections resonates with the tip capacitance at around 100MHz.

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 10:50:11 am
content: Standard 200MHz probe. Calibrated on the machine.I'll have a go at making a short gnd lead for it tomorrow.Thanks for the reply tggzzz.

author: tggzzz
author_rank: Super Contributor
timestamp: January 16, 2024, 11:33:57 am
content: Quote from: swmcl on January 16, 2024, 10:50:11 amYou're welcome.FFIhttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/andhttps://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/

author: fourfathom
author_rank: Super Contributor
timestamp: January 16, 2024, 04:23:46 pm
content: Quote from: DavidAlfa on January 16, 2024, 07:28:37 amYes.I always lose mine, and should order a good assortment. In the meantime, I sometimes make my own using a tiny paper clip, or just bare copper wire. Not really "springy" enough, but it works in a pinch (and free!)As for the OP's situation, the only way to get an output signal that exceeds the power supply rails is with an inductance (or transmission line) somewhere. This could be due to probe characteristics, circuit board layout, parasitics within the TCXO (unlikely), or some combination of these things. Can you temporarily disconnect the TCXO output (cut a trace?) and see what the floating output looks like? This would eliminate many of these factors. While you're at it, probe the TCXO power and ground pads, as inductance here can also cause this.But I am a bit puzzled by the trace you show, since the factors I mention usually result in actual ringing, not a semi-sinusoidal waveform. Perhaps with the 50 MHz signal the ringing is slow enough to create this waveform?Where are you probing this signal? Probing midpoint or at a discontinuity on a transmission line (trace) can show "interesting" signals where reflections meet with the incident signal. But, the signal at the end of the trace will still look reasonable (and that's what matters.)

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 09:58:35 pm
content: I found a spring earth fitting as part of another scope probe (new).The results are seen in the attachment.This is a win for me as I've not been up at these speeds before and my panic over the high voltage is subsided !Thank you to all.

author: swmcl
author_rank: Contributor
timestamp: January 16, 2024, 10:01:08 pm
content: I wonder at what speed I should consider using a springy earth fitting ... 10MHz ?

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2024, 10:20:23 pm
content: Quote from: swmcl on January 16, 2024, 10:01:08 pmMore to the point is edge rate aka rise/fall time (which also translate to frequency, but only indirectly).

author: rhb
author_rank: Super Contributor
timestamp: January 16, 2024, 10:21:44 pm
content: Quote from: swmcl on January 16, 2024, 10:01:08 pmEvery speed!Dig up the articles on this by Bob Pease and Jim Williams. Ground loops will eat your lunch at any speed. And fast edge digital logic will probably eatyouas an after dinner snack clocked at 100 kHz.Have Fun!RegBTW to make your own ground springs wind copper wire around a drill bit smaller than the probe ground sleeve and never worry about losing them again.

author: ataradov
author_rank: Super Contributor
timestamp: January 16, 2024, 10:30:07 pm
content: Quote from: rhb on January 16, 2024, 10:21:44 pmAlternatively, you can just keep in mind the type of artifacts caused by the ground lead and just ignore it. If you only need to estimate the frequency of the cock, then captures shown above are sufficient for that.Using ground springs is not going to be practical in a lot of cases. What if I want to observe SPI and need 4 channels? Are you going to hold 4 probes?Use the spring to make sure your signal integrity is ok, then use ground leads and work on the logic, ignoring artifacts.Quote from: swmcl on January 16, 2024, 09:57:35 amIt had the "spike". The spike just happened to last about the same time as half the period.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 16, 2024, 10:44:30 pm
content: Beyond the influence of the ground lead on the frequency response, ground leads very easily couple noise inductively (as they make a nice loop), so you can get a lot of noise completely unrelated to the signal of interest, which can lead you to the wrong conclusions. As long as you're aware of it, you'll deal with it appropriately, and not every situation requires probe earth clips. Just be aware of it. Another unrelated, but that can similarly lead to the wrong conclusions in terms of artificial artefacts is not being careful with the current sample rate of your scope. Depending on your horizontal and acquisition settings, the sample rate can be just too low for the bandwidth of the signal and lead to aliasing which will make you see something that just doesn't exist. That can particularly happen when dealing with "square wave" signals as those contain a lot of harmonics and thus potentially very high frequency content.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 16, 2024, 11:32:15 pm
content: It'sTCXO.

author: tggzzz
author_rank: Super Contributor
timestamp: January 16, 2024, 11:39:45 pm
content: Quote from: ataradov on January 16, 2024, 10:30:07 pmOnce a scope has verified signal integrity, for logic signals flip to the digital domain and Use something like a logic analyser or protocol analyser.Use techniques like this; I used to have a picture of a paperclip doing something similar, but it has vanished.

author: tggzzz
author_rank: Super Contributor
timestamp: January 16, 2024, 11:44:02 pm
content: Quote from: rhb on January 16, 2024, 10:21:44 pmYup.Signal integrity requires verifying setup and hold times. Hold times are more problematic since they are short and cannot be resolved by reducing a clock frequency; they screw at 1Hz just as effectively as at 100MHz.

author: Smokey
author_rank: Super Contributor
timestamp: January 16, 2024, 11:48:05 pm
content: Quote from: ataradov on January 16, 2024, 10:30:07 pmI had this thought to put uFL headers on circuits I know I'm going to be probing that need decent low inductance grounding. Then I got a uFL to BNC cable, and one of those BNC->Scope Probe adapters.I haven't finished the board yet, so I can't say how well that will work, but that's the idea. In theory, you should be able to run this setup on all 4 scope channels at once without needing to balance any spinning plates.

author: fourfathom
author_rank: Super Contributor
timestamp: January 17, 2024, 12:40:14 am
content: Quote from: Smokey on January 16, 2024, 11:48:05 pm

author: Smokey
author_rank: Super Contributor
timestamp: January 17, 2024, 03:07:43 am
content: Ya, we will see. My idea wasn't for a high cycle count end use, but more for development. You can always swap out the uFL when it wears out. But if I needed a ton of repeated connection, there are other options, like SMA.Or if you wanted to go crazy:https://www.digikey.com/en/products/detail/teledyne-lecroy/PK106-4/3587113

author: fourfathom
author_rank: Super Contributor
timestamp: January 17, 2024, 07:01:58 am
content: I actually like U.FL connectors and use them for 50 Ohm interconnects between modules, or for the occasional 50 Ohm test connection where I don't want to have a permanent SMA jack. The U.FL jacks are extremely inexpensive and take up very little board space. It's just going to be a problem using it to connect to a high-Z testpoint.

author: rhb
author_rank: Super Contributor
timestamp: January 17, 2024, 01:17:28 pm
content: u.FL to SMA cables are very cheap which is important given the very few mating cycles for the u.FL. Designing test points at key nodes is an important aspect of good design. Use 0 ohm jumpers to enable them as needed.For hi-Z, some pads and a short twisted pair of 26 AWG wound into springs at one end will let you tack solder in test leads if you need to make multiple connections in a small space.It's the area of the loop that matters. The most important part is understanding what's going on (i.e read Bob & Jim on the topic). After that it's down to human ingenuity, which has no bounds in the long term.Have Fun!Reg