bias T for DC voltage measurement - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 07:07:41 pm
content: If you want to do precise DC voltage measurement on a RF line it might be troublesome, and you need to use a bias T.I see bias T that have up to two inductors in them, and a little bit of resistance and capacitance. But if you wanna measure something really precisely, I thought that maybe there is a better one that can be made with more isolation, i.e. by using a big series RF resistor. The DMM should not care because its input impedance is high, but this is not done because they expect you to introduce current through the bias T.Is this kind of thing produced?And it would not really be a bias T, because there is no need for AC coupling capacitor, since its more of a specific measurement tool for voltage, and if you want capacitor isolation you can add another block.I.e. you can get a 40+GHz RF resistor at 500 ohms to make the filter way more aggressive, instead of some puny <5 ohm resistor they put in these things (for current reasons) . Not sure if you can get a wide band resistor that has more ohms. I guess you can use two.Viking has resistors that are very high frequency up to 1Kohmhttps://www.viking.com.tw/en/category/Resistor-ARF.html

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 07:33:18 pm
content: 500 ohms? That's pretty high don't you think?My higher end VNAs include them as part of the test set. When the original NanoVNA was introduced, I included a couple of them along with a transfer relay in a single box using what ever I happened to have on hand. The VNA wasn't made for very high frequency use, so no big deal.I have a few from Mini-Circuits.https://www.minicircuits.com/pdfs/MBT-283+.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 07:41:19 pm
content: For a DMM with 10megaohm or 10 gigaohm impedance? 500 is lowThe amount of error you get from RF on the DMM would depend on the DMM and frequency, but the 'divider' error is known and small. Its something like 0.005 percent for 500 ohm in series on 10 megaohm multimeter. If its a 10GOhm multimeter the error is much smaller.. but regardless of what it is, its calculatable. Some random amount of small rectified RF 'signal' on the DMM is not something you can trace down easily for calculating. (and I don't know if you can even trust a figure that you determined for RF induced errors, it might not be a very stable number since the way the meter gets error is somewhat complicated compared to the usual error sources)And while I don't think its high enough to matter, it could help some with static charge or something developing on an antenna if you wanna measure it (RC filter), it might zap a meter. Raising the resistance might help a little bit with thatI thought the pasternack dual banana to coaxial adapter and a SMA connector on the can to hook up the DMM would be somewhat optimizedAnd then there is another block that can be made... to make a current shunt block for measuring DC current in RF system, if there is any.. that one is a bit more complicated then a voltage tap because its two signalsI might give it a shot to make a simple one for low frequencies

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 08:03:27 pm
content: Quote from: coppercone2 on May 25, 2025, 07:41:19 pmIf I am using a Bias-T it is to power some RF device I am wanting to measure. Not to connect a volt meter to the RF. For example, showing the original NanoVNA and home made Bias-T / transfer relay used to bias an RF transistor.https://youtu.be/GJNMnq8eD0E?t=2621

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 08:07:27 pm
content: that's what I mean, its only called bias-t because its the only RF part that I know is suitable for this. Its more like a measurement THere is something like a coaxial shunthttps://www.sciencedirect.com/science/article/abs/pii/S0263224121001548Much more interesting for pulsed DC instead of AChttps://www.ib-billmann.de/koax_e.phpBut for fast pulses you don't need the filter, and you won't be using a DMMI was also thinking about some kind of probe that has this filter built in, so you can probe DC on RF circuit boards preciselyA shunt might enjoy a twinaxial line, making it even more complicated

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 08:14:31 pm
content: Rather than a Bias-T you are looking for a RF tap or sampler. They are common.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 08:17:08 pm
content: https://www.microwaves101.com/encyclopedias/resistive-taps

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 08:17:40 pm
content: Can you give an example? I only see power taps and basically bias t's.I don't see anything optimized for like a multi meter DC measurementI think it would be considered a low frequency voltage tap (with minimal attenuation for the DC level).Kind of like the RLC filter I built for my multi meter for low frequency noise measurement.I think the power taps you show on the website have a load (divider) resistor? In this case the only 'divider' should be the DMM input impedance.

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 08:25:29 pm
content: I just thought it might be interesting to see whats going on there. I am thinking you just need to get it all eliminated down to 100KHz or so and then use a external switch "audio" filter or other attachments, so you can set the bandwidth. They are pretty good and characterizable .

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 08:33:03 pm
content: Here is a home made tap + load I put together when I was working on increasing the BW of my old 50MHz Tektronix current clamp. The magnet wire is providing the high frequency compensation.https://www.eevblog.com/forum/projects/home-made-pa-and-load/msg635851/#msg635851In that case, I needed the DC as the probe works down to DC. 100MHz was my goal for the Tektronix probe.

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 08:41:33 pm
content: Hmm, well I have one made for the tektronix clip on probe, I am thinking to make a pomona box with a vishay metal foil in there and a choke to experiment with (BNC)I like making the designs where you put a bunch of components floating in the boxes

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 08:43:38 pm
content: the way you stack those SMT resistors another thing is possible, to hollow out a LMR400 cable and put parts in there, and replace the braid with a brass or copper tube (needs some flaring work done on it), reuse the dielectric foam.Its big enough that you can get a coaxial component without a PCB and using cheap N connectors. It might need a bit of braid (soaked with something that resists solder ingress) to make a flexible part because its difficult to align it to put it together.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2025, 09:15:42 pm
content: I tried that technique to make some resistive probes. It was a big failure. I posted about them here if you want to search for them.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 25, 2025, 09:30:39 pm
content: This is one idea I've had in mind to monitor the lamp voltage or xtal voltage of an LPRO 101 based RFS or the 4.97877v of the temperature stabilised 7805 voltage regulator mounted on the attached fan cooled heat spreader held to within +/- 1mK of the 36.050 deg set temperature when the RFS is relocated into my garage/workshop for stability tests over a much wider range of ambient temperatures than I typically see in my hobby room (down to -4 deg in winter and as high as 32 degrees in summer) where I can utilise my 6.5 digit bench meter.The 7805 voltage provides a sanity check for the stability of the base plate temperature readings and the lamp voltage provides a clue to internal temperature variations (both show a small but measurable negative tempco in tens of micro-volts so it would be very useful to be able to observe these voltages over the much wider ambient temperature range in my workshop/garage - I'm not about to haul my 6.5 digit bench meter out to the garage since it would also be subject to the same temperature extremes well beyond its specified 23 +/- 5 degree working range.At this time of the year, I typically observe a diurnal variation spanning some 15 to 20 degrees C. Whilst it's not as extreme as the full -4 to +32 degrees, it's still a usefully wide range to verify the efficacy of any modifications to improve the RFS's temperature stability. Whilst the issue of stabilising the base plate to within a mill-Kelvin or three of the set point has been done and dusted, the remaining fly in the ointment is that of the top cover heat loss under cold extremes in spite of what I'd naively thought was an over-abundance of thermal insulation, which creates a thermal gradient varying with ambient temperature, sabotaging all my efforts at stabilising the base plate temperature to within mill Kelvins of the set point.I'm in the middle of building a MK II version of the current prototype using all the hard won lessons over the past four years of this ongoing project so making up a couple of "Bias Tees" to connect the voltage monitoring points over some 60 or 70 metres of coax to my hobby room bench meter would very useful when I have the the MK II RFS ready for extreme ambient testing.For this task, the bias T only needs to be a 5K1 resistor tap at each end with a series cap on the rf input and output ends (keeps the attenuation calculation nice and simple for a 10M meter input but I use the 10G input option anyway so can forget about any such calculation). Optionally you could add a 100pF silver mica or foil cap (low DA types) shunt to attenuate the 10MHz component out of the dc connections but I doubt they'll be needed in my case.Anyway this, as I said, is just an idea waiting for me to get a "round tuit" after I've finished building the MK II RFS ready for testing. The only other point worth considering is the state of the coax's insulation leakage. It shouldn't be a problem with new coax but worth checking if you're recycling older coax that's been exposed to the elements for the past three or four decades as one 20 or 30 metre length of RG213 of mine that I diverted from a mast on the back wall of the house into the garage (Note to self: check the insulation leakage of my coax link before making up any such bias Tees).

author: coppercone2
author_rank: Super Contributor
timestamp: May 25, 2025, 11:37:10 pm
content: There is something else I had in mind, but it used to be a pain (not anymore)I got lossy coaxial cable. The kind used for scope probes. The middle is like some resistor wire that is thin and bent.I think it would be the best thing to route a long run that is just measuring voltage.The problem is termination, you have a single fine wire that won't solder etc. But I believe using my silver solder pot idea (graphite crucible and 56% solder) it would be easy to 'silver' this wire (it works like tinning) to make it compatible with regular soldering and regular coaxial connectors.I imagine this might help on a 75 meter run, but I guess getting more shielding is important too. I assume it may be very hard, but of course you can slip on another braid over that. Just expensive as hell.with 10Gohm impedance it would hardly matter and I assume a distributed resistor might help things.Interestingly enough, for differential runs.. its rather easy to get a distributed resistor built into the twisted pair... just use resistor wire that is enameled ! Should be cheap too (other then the braid). IDK if that would help with something like long runs, or possibly hurt things some how. Manganin resistor wire with enamel.For alignment I came up with this idea: Use styrofoam balls and a thin vinyl tube that is slit open along its long way. Every 3-4 inches or so, put a styrofoam ball over the twisted pair of magnanin. Then press it into the vinyl or silicone or something tube through the cut. The tube is springy enough that it should hold the styrofoam ball tightly. Then you can run a bead of sealant across the tube to seal it, and the twisted pair should be in the middle. Then you can put a braid over that whole thing. In the end you should be left with a lossy lowish capacitance shielded twinaxial cable. All it would take is most of your sanity and a unknown period of time. I don't know how well it would handle bends (you would need to make where the balls are) and how stable the capacitance and stuff would be, and it would probobly wick in moisture because its hard to seal, but it might work if you live in a dry area for a while?200 feet = $100https://www.ebay.com/itm/302172775570?chn=ps&google_free_listing_action=view_item&gQT=0I should make a small prototypeI am curious if you can blow it through a empty coaxial cable with air, if you pull the center conductor out, like a tow missile

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 26, 2025, 03:03:30 am
content: @ coppercone2That sounds like the sort of madcap diy solution I'd sometimes consider to solve a problem but, thanks to my natural tendency to procrastination, I swiftly decide to take a more pragmatic approach, usually by not bothering to implement any solution at allIn my case, I was simply considering how to use the co-ax link between my workshop/garage and my hobby room to to connect the various voltage monitoring points (one at a time) to my bench meter rather than try running a dedicated circuit for this task.Describing the use of bias tees to hitch a 'free ride' to carry the test voltages to my hobby room bound bench meter from the workshop had reminded me that I'd overlooked the knotty issue of creating a ground loop with the setup as described.In short I'd have to dc isolate the coax shield from the DSO BNC grounds usingan isolation transformer or, 2nd best,a capacitor between the two. Even then, the bench meter might not be able to handle the magnitude of common mode noise despite both input terminals effectively floating to ensure that the zero volt connection would normally see only the DUT's local zero volt reference point over more typical test lead lengths in use on a test bench.So, that's two things for me to check out, insulation leakage in my co-ax link and whether noise induced on the outer braid of a long run of co-ax is going to prove insurmountable.A lithium coin cell connected to the isolated co-ax at the workshop end should give me some clue as to whether the bench meter will be able to cope with such a long run of test lead.I've just checked the leakage which is in excess of 100M with a total capacitance value of 4.9nF suggesting a minimum run of 50 metres which ties in with the 243ns phase shift I see whenever the RFS is transferred to the workshop using a 21v lipo battery pack to keep it powered during each transfer.I'll try the lithium coin cell test later today seeing that it is now silly o'clock in the morning.

author: coppercone2
author_rank: Super Contributor
timestamp: May 26, 2025, 03:12:28 am
content: oh i think it will work fine for that I was thinking about like what might happen with alot of digitsand I really wanna find a use for the scope probe wire I found lolbut the silver thing is real, like for tektronix they have some weird ass ceramic insualtor crimped connection for that lossy wire on some of their old accessory probes (active, hv, etc) . good luck replacing the wire. if you can silver it, its possible to maybe drill out part of the crimp and solder it in therelike if you shorten the lead because its frayed near the adapter end

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 10, 2025, 06:20:43 pm
content: @coppercone2(BTW, is that a shaped explosive charge reference by any chance?)Apologies for the belated response. I did actually run the coin cell test later that morning and the results matched your prediction. However, when I was disconnecting the 1 metre bnc to croc clips adapter from the 50 or so metre length of feeder, I saw an immediate rise to around 3 volts which carried on rising over the next two or three hours before reaching peak just north of the 9 volt mark by which time the light drizzle had become more of a scattered shower, ending my trips to the workshop.I had intended to do more testing but got distracted by other activities whist I pondered the surprisingly high voltage charge on a mere 100pF's worth of test cable capacitance. Presumably the cable manufacturer had pre-charged it during the dielectric stress test required to assure it met its specified 300v rating and dielectric absorption had held the charge ever since, effectively turning it into an electet.All this would probably not show with a standard 10M DMM but it's amazing what a bench meter with a 10G ohm option can revealAnyway, that was a fortnight ago and with the sunny weather that we're experiencing this afternoon, I thought I'd take another look at the setup.At the time of the original test, I'd gotten as far as shunting the workshop end of the cable with a 150K ohm resistor which completely flattened the DA induced residual 2 or 3 volts I could see with the other bnc to croc clip test lead connected to the end of the feeder co-ax and left open circuit. At this point I gave up any further testing leaving the 150k shunt attached until this afternoon's test session.I was curious as to how much charge voltage I would see on this second test lead after removing the resistor and hauling it back into my hobby room. Surprisingly, I saw an initial negative charge (wrt the shield) of just over 2 volts which faded down to zero after a couple of minutes before it started climbing to positive vales where it reached a value of circa 2v. It's now hovering around the 2.3v mark.With a little more fiddling around with the test setup, it's now dawned upon me that most of that phenomena is due to the bench meter test leads being charged up by the electric field gradient within my hobby room environment. There is some (small) dielectric absorption effect in the bnc to croc clip test leads but I'd have to acquire a banana to bnc socket adapter before I can do a proper testRe-testing the workshop feeder showed only half a volt after a ten minute rest which after shorting it out to 0.000000v almost immediately recovered to quarter of a volt before starting a slow climb back to half a volt.Damn! Ireally needa "BNC Female to Dual female 4mm Banana" adapter for doing this test (I should have a pack of the four permutations of bnc to 4mm banana adapters in my hands by the end of the week - only one UK ebay seller that could provide what I needed amongst a four varieties pack) And, whilst I was checking Banggood to see if they happened to be selling a "BNC Female to Dual male 4mm Banana" adapter (they only showed the ass backwards type), I was reminded to check out the price of nano V3s and placed an order for a 5 piece pack).Anyhow, getting back on topic and phantom voltages aside, my own conclusion is that there shouldn't be any problem using the co-ax feeder to send the lamp / xtal voltages back to my hobby room despite their 20K impedance (+ another 5.1K in each "Bias Tee"). Once I have the "BNC Female to Dual male 4mm Banana" adapter in my possession, I'll repeat my tests and report back on the results.

author: coppercone2
author_rank: Super Contributor
timestamp: June 10, 2025, 08:37:04 pm
content: no its a antenna type called a biconical antenna that I made a few of, you need to be proficent in making sheet metal cones to make themthey have all sorts of adapters for dual banana connectors including ones you can strap parallel loads on or ones that you can put a series load on (isolated dual banana connector) which are useful for low frequency work of all kindsthey make it alot more practical then using coaxial project boxes (pomona) if you need quick work, that is solder posts, just need to be careful soldering them because you can damage it from heat,i.,e.https://www.tequipment.net/Pomona1837.aspdual isolated available tooyou can use those types for rapid prototyping then make a final more understood prototype in a shield box for instance. all those adapters are useful for being practical with your work

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 20, 2025, 01:51:54 pm
content: @coppercone2That pack of four bnc to 4mm banana adapters turned up about a week ago and, to cut to the chase, allowed me to properly test out the concept of using bias tee adapters to connect the lamp / xtal and calibration pot reference voltages over the 10MHz co-ax link to my hobby room bound SDM3065X 6.5 digit bench meter. The results were that this would do the job, just as you'd as suggested (and I had hoped for).The reason for not responding immediately was due to my looking to find a way to eliminate the 70ms processing time required to update an ssd1306 oled every 1000ms to display plenum and base plate temperatures and the absolute pressure which had been interrupting the 140 sps temperature data stream used to control the base plate temperature to within a mK of a setpoint target of 37.050 deg C.I'd decided that the best way to solve this issue would be to add a second nano3 into the BoM to offload this timehog code and shoot the three float values across once a second, effectively cutting the 70ms .delay down to just less than a millisecond. I had spent the previous weeks trying to track down examples of a one way SPI master to slave method of transferring float variables without seeing any useful examples, hence (out of desperation/exasperation) my plea for help that I'd posted here:https://www.eevblog.com/forum/microcontrollers/using-another-nano-3-via-spi-to-drive-an-i2c-only-oled-display/msg5943310/#msg5943310As per usual with my seemingly unique programming problems, whilst awaiting a response beyond merely noting some interest in my posted attachments, I landed up tracking down a solution by pumping DDG's flavour of chat gpt (rather vigorously). It had taken me six attempts before it finally gave me a useful enough example for me to write the slave code and shift all of the display related code off the primary nano3 onto the slaved nano3.If you're interested you can read the whole thread in little more than five minutes (7 posts in all). If not, suffice for me to state that this solution was a resounding success that had matched all my expectations.We're in the middle of a heatwave right now so I've just moved the RFS into the workshop where the temperature readings have just dropped back a degree or two from the recent 33 degree high due to the appearance of scattered cloud cover (Sod's Law in action).I'm not expecting to see the frequency stability I'm hoping to observe with the totally revamped version that I'm still in the middle of assembling, just wishing to satisfy curiosity regarding base plate temperature stability under these more extreme ambient temperature conditions. Holding the base plate to a constant temperature is not sufficient by itself without ensuring a constant fixed temperature gradient within the LPRO over the entire specified range of ambient temperature limits.An hour after moving the RFS into the workshop and the initial 143ns phase delay has only now started touching the 144ns point suggesting a frequency shift of ~2.5μHz, about what I'd been seeing over the 24 to 25.5 degree variations in my hobby room over the past few days, so to my mind, surprisingly better than I'd been expecting.However, in view of the GPS's 1 to 2ns jumps back and forth every hour or three (I suspect the existence of two or three rogue gps satellites being the source of this annoyance), it's really too soon to tell how accurate this impression might be.Anyway, enough of that. I've yet to make up the bias Tees to monitor any of these voltages at the hobby room end. Not a priority at this time since I'm only interested in how well my temperature control algorithm is performing with regard to maintaining the base plate to its set point in the face of this heatwave induced temperature extreme.[EDIT some 2 hours and 40 minutes later]I've attached three scope screenshots showing the before and after results of the RFS transfer to the workshop where the temperature has remained close to the 32 degree C mark. Ch1 is the ZED9FT GPSDO (doing its own version of the Rocky Horror Picture Show "Time Warp" dance step - a 1 or 2 ns 'jump' backwards and forwards every 1 to 3 hours or so). CH2 is the RFS under observation with CH3 showing the diurnal wanderings typical of a single frequency Timing GNSS receiver module (M8T in this case) based GPSDO.Thus far, the RFS appears to be performing far better than I would have had any right to expect out of my bodged version of the revamped cooling airflow setup I'll be using in my mark II RFS.