Wideband current probe (70 kHz-350 MHz) - Page 1
Forum section: electronics / projects

author: suj
author_rank: Regular Contributor
timestamp: February 16, 2023, 03:44:57 pm
content: I am currently working on a low phase noise quartz oscillator project and there was a need to select a drive level of quartz crystal and measure its effect on the phase noise of the oscillator. I decided that I needed a current probe. A quick scan of ebay showed that a DIY approach instead of a Tektronix CT1 or CT6 would be many dB cheaper. I do not need the characteristic to be flat, I'll calibrate the probe precisely at the discrete frequencies I'm interested in (mainly 5 and 10 MHz). I have looked through the posts on this forum, read the chapter on current probes from "Oscilloscope Probe Circuits" by Joe Weber (Tektronix) and also watched two videos on YT on the FesZ Electronics channel treating the DIY approach. Then looked through the ferrite core box looking for something that would be suitable. My choice was the Laird 35T0135 toroidal core (the bandwidth from the bottom should start quite low). I was planning to use this core for BALUN as well, but it had a bit too small internal diameter to fit a coaxial cable I used. So for BALUN I used 4 pcs. of Fair Rite 5943000101 cores. I don't need a lot of windings in the secondary winding, I use a spectrum analyzer as an RF voltmeter and the sensitivity is sufficient. Schematic below. Please note, I wound 6 turns and not 5 turns as I planned and wrote on the schematic.I assembled it on a small piece of universal PCB trying to make everything as small as possible. I immobilized the wires of the secondary winding of the transformer with a drop of neutral silicone sealant and sealed everything in heat shrink sleeve. This is how it looks likeI then prepared a "fixture" and set up a measuring bench to do the calibration. Central of two SMA female connectors linked by wire. Before measuring, feed this wire through the ferrite core of the transformer and then solder.To make the output level of the generator independent of cable attenuation etc I used a power splitter just before the measured circuit and the ALC of the generator was controlled by the power meter probe. Even though ALC in the first pass I measured the actual voltage by connecting a spectrum analyzer instead of fixture. In the second pass I connected a fixture loaded with 50 ohms and with a spectrum analyzer and measured the voltage at the output of the probe. I collected the data in a spreadsheet and calculated the mV/mA conversion factor. I also marked the operating bandwidth (probe output voltage >= 0.707 of the maximum voltage) on the spreadsheet. Not bad.OK, but what about the effect of the probe on the tested system? How will the insertion into the circuit affect the DUT? I measured this with my beloved HP 4396B Swiss Army Knife. I prepared a piece of wire that I used to compensate the 43961A/16092A fixture of the impedance analyzer (previously calibrated). I then threaded this wire through the transformer core, loaded the transformer output with 50 ohms and measured the impedance from 100 kHz to 1 GHz.Before measuring each time, consider whether an additional 9 nH will ruin the operation of our system. Due to the fact that I built this circuit for a specific application this should not be a problem for me. I haven't checked the operation with an oscilloscope, but I don't anticipate any problems. Perhaps I will also check the cooperation with the VNA - here the 4396B is perfectly suitable because its bandwidth starts at 100 kHz and the S-parameters test-set is external rather than integrated so we have direct access to the generator output and receiver inputs. This allows me to create various configurations of fancy test-sets. I am very satisfied with the result especially since I spent one evening to create.[EDIT 25.11.2023: I corrected a total disaster with the display of images in my post]

author: moffy
author_rank: Super Contributor
timestamp: February 16, 2023, 09:48:05 pm
content: Thank you for sharing. I'll save this post if I should ever need a similar current probe.

author: RoV
author_rank: Regular Contributor
timestamp: February 16, 2023, 11:01:57 pm
content: I am a bit surprised, because the ferrite kind 35 that you are using is suitable for transformers up to 10-100 kHz, then it starts to become lossy and the permeability drops to very low values. It shouldn't contribute to the flux linkage between primary and secondary at the high frequencies that you measured. Are perhaps the two windings linked regardless of the toroid, just because they are close?With a secondary of 6 turns and 25 ohm loading (50 ohm direct + 50 ohm on the analyzer) you should obtain 4.16 mV/mA, that is quite close to your measurement and this confuses me.The impedance seen across the primary turn should be ideally real, at 25/(6**2)=0.69 ohm (secondary load seen by the primary, transformed by the turns ratio). This seems to hold from 300 kHz to 3 MHz; at 100 kHz the turn inductance wins and transforms the impedance magnitude; over 3 MHz the core loss increases the real part of the impedance, causing the higher readings.

author: suj
author_rank: Regular Contributor
timestamp: February 16, 2023, 11:20:32 pm
content: Quote from: RoV on February 16, 2023, 11:01:57 pmI think I can check it. I will apply the DC current via primary winding and will measure the inductance. If coupling pri/sec is no thru the core then the saturation effect schould not be visible.Edit. Not today. The bias port in my impedance analyzer is rather suited for the voltage biasing (varicap, transistor junctions etc). The true test will be checking the pri/sec ratio with DC current. I have to prepare a different setup with the signal generator, bias-tee and the spectrum analyzer.BTW. The core material I did not choose from the lotteryI am posting a graph of the initial permeability for the material used by Tektronix (from the book mentioned in the first post) and also the Laird 35. The 35 ends at 1 MHz and the Tek material goes further. But...I am a bit surprised too.

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2023, 01:39:27 am
content: quartz crystal current drive is in the ma,range.Many XO ckts,have one side,of,the xtal at or,near return.Easiest solution insert a small current sense resistance in series with the xtal. Voltage can be differentially sensed.Depending on the Q and xtal resistance a 1.100 Ohm series R may workJon

author: suj
author_rank: Regular Contributor
timestamp: February 19, 2023, 02:12:17 am
content: Quote from: jonpaul on February 17, 2023, 01:39:27 amOf course you are right but I chose this approach for several reasons. One of the oscillators I'm going to test uses serial resonance and is the quartz crystal is placed in an amplifier coupling loop with no ground connection - in which case it's harder to use this approach. I agree that the easiest way is to place a small resistor on the ground side of the resonator. Most of my equipment is matched to 50- this probe is suitable for that. Well, and the project turned out to be surprisingly simple and cheap and the results exceeded my expectations many times over.Today I checked the effect of DC current flowing through the primary winding on the pri/sec coupling ratio. I put together the test configuration as shown in the photo. I inserted a bias-tee between the generator and the fixture with the probe connected to spectrum analyzer. This limited my frequency from below due to the capacitance of the coupling capacitor in the bias-tee. I conducted tests at 2 MHz and 250 MHz. At the output of the primary winding I gave a more powerful 50load.I checked the conversion factor without flowing current and with a DC of 500 mA. At a frequency of 2 MHz, the drop in the coupling factor with DC current flowing through the primary winding was -4.5%. At a frequency of 250 MHz with 500 mA DC, the conversion factor decreased by 15.7 % compared to no DC in the primary winding. So the coupling is clearly through the core.The results are so surprisingly good that I think I will decide to build such a probe with a slightly more robust mechanical design. Maybe some 3D printer design? Or glued onto a small PCB.

author: suj
author_rank: Regular Contributor
timestamp: February 20, 2023, 10:54:04 pm
content: Today I tested the probe in cooperation with an oscilloscope. I have a Rigol MSO2000A series oscilloscope, 300 MHz,two 1 Mohm/50 ohms switched inputs. I used an LF modulating generator from an SMF100A microwave generator as the signal source. First a 10 MHz sine wave. Everything is absolutely correct, the ratio is as I measured with my spectrum analyzer. OK, the sine is nothing particularly difficult. How about a rectangular wave? Well, it's a 500 kHz rectangular wave. The shape is not perfect, but the main thing is that the ratio still agrees. The blue color is the voltage across the 50 Ohm resistor (the input impedance of the oscilloscope shorting the primary winding to ground) and the yellow signal is the output voltage of the probe (the secondary winding of the transformer) loaded in parallel with a 47 ohm resistor and the input impedance of the oscilloscope (50 Ohm). Now I'm sure that I can also measure quite accurately also currents deviating from sine. I consider this a success.BTW. The secondary winding has 6 turns and not 5 turns as I wrote in the schematic. When winding it under the microscope, I didn't particularly concern myself with counting the turns, just tried to wind the coils uniformly. This was noticed by RoV (I think he counted the turns in the photo).