Measuring probe bandwidth with NanoVNA - Page 1
Forum section: electronics / testgear

author: ozkarah
author_rank: Regular Contributor
timestamp: April 11, 2022, 01:43:37 am
content: Hi guys?Is there an easy way to measure an oscilloscope probe (1x and 10x mode) bandwidth using a NanoVNA.I tried several ways but had no success yet. I am open to suggestions..

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: April 11, 2022, 02:10:27 am
content: Found this one using a oscilloscope, maybe that can help you in any way..https://www.signalintegrityjournal.com/articles/1092-quick-simple-way-to-measure-the-system-bandwidth-of-a-scope-probe-system"There are a number of ways to measure the transfer function of a measurement system. Unfortunately, using a VNA is not one of the. We really want to be able to include the scope’s amplifier and whatever DSP equalization is built into the scope’s electronics, in addition to the passive cables and probes. Other than in a calibration lab, we can’t pull these pieces out and connect a VNA from the input to the output."https://www.rs-online.com/designspark/a-simple-method-to-verify-the-bandwidth-of-your-probe

author: joeqsmith
author_rank: Super Contributor
timestamp: April 11, 2022, 02:38:49 am
content: The 10X scope probe is going to want a 1M input. The VNA is 50 ohms. Imagine a 10M in series with a 50 ohm...Shown with LiteVNA's Port2 attached to a battery powered, 50 ohm, DC coupled buffer which has a 1M input impedance. The scope probe tip is attached to the Lites Port1 and the it's output attached to the buffer's input. Step attenuator was a just in case....With a voltage ratio of 10X we expect 20dB. System was not calibrated but it appears we are in the ballpark. This is one of my two for $20 Hantek PP-150 probes. I still use these frequently and not had any problems with them.I have some resistive probes which would use a 50 ohm input. These could be directly coupled to the LiteVNA.

author: bob91343
author_rank: Super Contributor
timestamp: April 11, 2022, 02:45:28 am
content: The general rule is that the probe bandwidth is that frequency at which its input capacitance equals 50 Ohms. This does not allow for minor things such as series inductance. However the resistance being high means that the probe is almost entirely capacitive (again, ignoring the tiny inductance of the tip and connection to the resistor).Measuring this with the nanoVNA is simple enough. Use a formula or a nomograph to compute the frequency.Basically this implies that the capacitive reactance will load the source impedance by 3 dB at the corner frequency. Or 1 dB at half the corner frequency.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 11, 2022, 12:36:41 pm
content: Terminated the input to the buffer amplifier using a 50 ohm thru. Set the step attenuator to 10dB. Attach Port1 to the terminated input and then normalize the data. Trace now displays 0dB.Next, remove the 50 ohm thru on the input of the buffer. With Port1 now driving a very small capacitance, we would expect the voltage to roughly double. We now measure 6dB.My Hantek probe was switched to 1X and inserted between the buffer and Port1. We would expect to see 6dB again at very low frequencies which we do. It drops about 3dB at 5MHz and 6dB at 8.5MHz.From the following paper:https://brsbrs.medium.com/how-do-i-probe-b2a186994c13QuoteI made no attempt to compensate the probe.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 11, 2022, 01:35:11 pm
content: For a quick sanity check, I used an RF generator to swept from 50kHz to 10MHz in 1 second. Hantek probe set to 1X was attached to my scope and probe attached to my RF generator. Scope was set to 100ms/div and triggers off start of the sweep. So basically its 1MHz/division. At 5 divisions out (5MHz), we are a little under 1.5 divisions vertical or about 3dB.

author: nctnico
author_rank: Super Contributor
timestamp: April 11, 2022, 01:58:56 pm
content: Quote from: bob91343 on April 11, 2022, 02:45:28 amI never heard of that one. AFAIK the generally accepted method is to connected the probe tip to a 25 Ohm signal source and measure the other end. When using a probe for a 1M Ohm input, the instrument for the measurement should also have a high impedance input to mimic an oscilloscope input. A buffer amplifier is needed in case of the NanoVNA.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 12, 2022, 12:09:19 am
content: Comparing my Hantek with my LeCroy 10X probe. Data was again normalized. The 50ohm terminator remained attached between the scope probe tip and Port1. We don't have the 6dB gain and the normalization would have accounted for it. Once the probes were inserted into the buffer, they were adjusted for flatness.

author: ozkarah
author_rank: Regular Contributor
timestamp: July 19, 2022, 01:44:57 pm
content: Thank you very much Joe,Meanwhile, I had time to look at your VNA application and learned a lot.Could you give me more information about the voltage buffer you use?I want to build my own. I looked into many op amp circuits, but it is not something I have experience.Do you have a recommendation on a kit, circuit or opamp for that?Best regards...