Coax cable probe for oscilloscope - Page 1
Forum section: electronics / beginners

author: matgaw
author_rank: Newbie
timestamp: August 18, 2014, 06:53:54 pm
content: Hi,I wanted to build a poor man's x1 differential probe for my oscilloscope (as in Dave's video on measuring ripple & noise which I link below), because my oscilloscope only goes down to 20 mV/div at x10 and I wanted to measure power supply ripple/noise using differential technique.I already watched these videos:(EEVblog #594 - How To Measure Power Supply Ripple & Noise)(EEVblog #652 - Oscilloscope & Function Generator Measurement Trap)(EEVblog #453 - Mysteries of x1 Oscilloscope Probes Revealed)Unfortunately I'm completely clueless in the RF field and want to only do a simple thing - x1 coax probe for my oscilloscope, and I'm not sure about a few things.Here is what I came up with so far:If I understand correctly, the copper wire would be my measuring tip. It's AC coupled by capacitors C1/C2 with -3dB point @ 320 Hz which I believe is enough (I assume 50 ohms is the "load impedance" in this situation, not 100 ohms?). Then I have 50 ohms series resistor (as in Dave's video), then a 50 ohm coax cable (H1000 probably) - the shield is not connected to anything at tip end, then a 50 ohm termination resistor connected between the shield and copper wire on oscilloscope end, and a BNC connector for the oscilloscope.Yes, my oscilloscope doesn't have 50 ohm input impedance mode - don't blame it, I have to deal with this.I have a few questions:1. Where the hell to get the 50 ohm resistors? I never saw this value before. Is 49,9 or 51 ohms close enough? Do they have to be 1% or 0,1% (does it make any difference between these two tolerances)? Will standard resistor be OK or does it have to be some sort of special one? Can I solder two 100 ohm resistors in parallel or will it introduce some weird capacitive/inductive side-effect?2. In this situation, should the shield of the coax cable not be connected to anything at the measuring end?3. Does type of coupling capacitor matter? Which type should I use? After browsing some materials on the internet, I found that polyester or MKT would be ok. Can normal aluminium electrolytic be used?4. Do I understand it correctly, that this type of setup will draw a lot of current from the measured line and I should use it only for low peak voltage value signals? If I would measure a square wave of MOSFET driving signal - 12Vpp@500 kHz with this probe, it would draw 12V/100R=120mA from the source - right? So I can't use it for measuring the FET driving signal because it has high peak values, but I can measure a 12V DC output of switching converter with - let's say - 100 mVpp@500kHz ripple, right?5. If I use a 50cm cable with 40pF capacitance, and feed in a 1 MHz signal, the reactance would be 3980 ohms, which in parallel with 50 ohm resistor would be 49,38 ohms - so the signal would be attenuated by about 1,25% - right?Thanks in advance for helping me to solve this out!

author: matgaw
author_rank: Newbie
timestamp: August 21, 2014, 05:55:27 pm
content: Anyone has any knowledge about this topic?

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2014, 07:28:02 pm
content: What you are trying as shown in your schematic will certainly work to measure low impedance sources like power supply inputs and outputs.QuoteThe input impedance will be 100 ohms from the 50 ohm source termination in series with the cable impedance.QuoteTwo 100 ohm resistors in parallel will work but the termination resistances are not critical. 47 or 51 ohm 5% resistors or 49.9 ohm 1% resistors will work fine. Often two 100 ohm resistors are used in parallel because this allows for a better high frequency layout.Note that the resistor tolerances will affect the DC accuracy and therefor the common mode rejection when making differential measurements. Usually this may be trimmed using the variable gain feature of the oscilloscope.QuoteIf the oscilloscope and circuit share a common ground elsewhere, then the shields may be left disconnected (connected to each other but nothing else). Connecting the shields to the circuit ground which is shared with the oscilloscope may provide better performance.QuoteThe capacitor should have good high frequency performance. If the value does not need to be high, then a film capacitor is a good choice.QuoteYes, this is a problem. The input impedance of the probe is very low and it likely will not work correctly with all output stage drivers.QuoteThe cable capacitance is irrelevant when using it as a transmission line. At high frequencies it looks like 50 ohms. If the cable was unterminated *then* the capacitance would matter. This is why standard passive x1 probes have high input capacitance which limits their frequency response.Your 50 ohm cable design is going to have good sensitivity (although not as good as a x1 passive probe) and excellent frequency response however I suspect a pair of standard inexpensive x1 passive probes would work fine in your application. Their input capacitance will be higher but nothing a low impedance power supply measurement will care about and even a gate driver will not be affected by it much. Frequency response will be limited to about 20 MHz but higher bandwidth measurements are questionable at high sensitivity anyway. The oscilloscope's built in AC coupling will extend the low frequency response of x1 passive probes to below 60 Hz.Use x10 probes as a sanity check to make sure high frequency content is not being missed.

author: matgaw
author_rank: Newbie
timestamp: August 21, 2014, 08:25:28 pm
content: Thanks, your answers are very helpful! Unfortunately I can't find any passive x1 probe (which is not switched x1/x10) in local retailers so I wanted to build my own cable using Dave's suggestions.Can I use resistors which are a multiple of coax impedance to lower the current? E.g. 100/200/500 ohm resistor on both ends instead of 50 ohm? Will it still has the same good frequency response or is this a bad idea?

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2014, 10:16:07 pm
content: Quote from: matgaw on August 21, 2014, 08:25:28 pmWhat is wrong with switchable x1/x10 probes? The cheap Chinese 100 MHz ones I have work great.QuoteYou can use a higher value resistor in series to raise the attenuation and lower the loading. A 450 ohm series resistor for example would yield a x10 probe with 500 ohms of input resistance. The parallel termination resistor at the oscilloscope end needs to remain the same for good frequency response.A x1 passive probe is just a low capacitance lossy coaxial line. You might get away with just putting a 1k resistor in series with the coax and no termination. DC input resistance would be about 1 Mohm and bandwidth would be about 1 MHz. Use a lower value series resistor to raise the bandwidth.

author: matgaw
author_rank: Newbie
timestamp: August 23, 2014, 01:16:34 am
content: The reason why I don't want to use cheap chinese probe set to x1 is here:Both channels are measuring the same signal - about 2 MHz square wave. Channel 1 (red) is full bandwidth (100 MHz scope) x10 100MHz probe. You can see some ringing and overshoot coming from the ground clip + some high frequency noise.Channel 2 (yellow) is also full bandwidth (!) but this time using x1 probe - as you can see, all the noise and high frequency stuff disappeared and the frequency response is very bad - and it's only 2 MHz...Thanks for your advice. I will set up this coax-based probe and report back how good it works and how it compares to this cheap x1/x10 probe

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2014, 02:06:44 am
content: The x1 probe rise time shown indicates a bandwidth of about 10 MHz which is typical. Some x1 probes are faster but not by much.If you are making measurements on switching regulators, then you may be get better results using a x10 probe and average mode on your DSO to reduce noise allowing higher sensitivity.

author: w2aew
author_rank: Super Contributor
timestamp: August 23, 2014, 03:54:21 am
content: Quote from: David Hess on August 23, 2014, 02:06:44 amDavid is right. What you are seeing with the 1x probe is absolutely normal - mainly due to the fact that the input impedance is the scope's 1Mohm in parallel with 100-120pF or more. The capacitance comes from the the coax capacitance and scope input capacitance. Nature of the beast of any 1x probe on a 1Mohm input. Note that the Tektronix P2220 10x/1x switchable probe is rated for 200MHz in 10x mode, and 6MHz in 1x mode. Have you watched my video on the basics of 1x and 10x passive probes?

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2014, 04:51:55 am
content: Probe bandwidth also depends on source impedance. All of probes I am familiar with are specified using the 25 ohm impedance from a terminated 50 ohm transmission line which usually does not reflect real world usage.