1meg to 50ohm Impedance Matching Amplifier for general instrumentation use - Page 1
Forum section: electronics / projects

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 05:47:58 am
content: I've wanted such an amplifier for quite some time. This would allow testing of probes and other devices designed to drive a 1M scope input on a spectrum or network analyzer. It would also allow the use of a probe at a distance from the scope longer than it's normal cord distance. These trans-impedance amplifiers are available. One 400Mhz rated unit sells for $400. An older Tektronix version sells on Ebay for various amounts but requires an external supply and dealing with its output connector (GR type). I elected to try and build a simple one using a buffer op-amp on perf-board in a small aluminum enclosure. I was careful of the geometry and based the design on a example circuit in the datasheet. The unit has a negative gain of -6dB nominal due to the 50 ohm series output resistor. Tests on my spectrum analyzer showed the gain varies from about -7 to -5.8dB over the range of 0-100MHz. -3dB point is at 125MHz when using fresh 9V batteries. If you drop the voltage to 5V the -3dB point moves to around 110MHz.It should be possible to improve the performance with a PCB and better op-amp. The op-amp used for this first attempt was an OPA633KP. The present unit is more than good enough to use with my spectrum analyzer. Simply use the tracking generator and terminate into 50ohms at the tran-impedance amp input. Connect the amp output to the spectrum analyzer input. The response shown is the variance of the tracking generator and frequency response of the trans-impedance amplifier. Normalize the data and replace the 50ohm terminator at the trans-amp input with your device under test you are good to go...at least up to 100MHz and abit beyond

author: Someone
author_rank: Super Contributor
timestamp: February 20, 2018, 07:22:25 am
content: That term has a very specific meaning:https://en.wikipedia.org/wiki/Transimpedance_amplifierI think you are describing a more common impedance matching network.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 20, 2018, 07:29:33 am
content: A very important spec would be the distortion. You can tune out non-flat gain, but you can't tune out distortion. Any measurements on that?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 08:33:46 am
content: Quote from: Someone on February 20, 2018, 07:22:25 amInteresting. For years in the company I worked for over 30 years we used the term trans-impedance amplifier for any amplifier that was designed to match a higher (or lower) source impedance to some other impedance...with the most common being high to low version. It applied to photomultipliers like in the wiki you linked, but the term also applied to amplifiers like I described. It could be the term has changed meaning over time.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 08:36:16 am
content: Quote from: TheUnnamedNewbie on February 20, 2018, 07:29:33 amHmmm....I'm not sure how I would measure it. It does phase-shift a bit which is only visible at the higher frequencies.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 09:05:57 am
content: Quote from: TheUnnamedNewbie on February 20, 2018, 07:29:33 amI found this graph in the buffer op-amp datasheet I used. My load impedance is 100 ohms net, so this should apply for 1V rms amplitude signals for the lower frequencies shown.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 20, 2018, 09:38:03 am
content: In order to really measure distortion you need a basic understanding of what it is and is not. If phase is important for your application, AM-PM distortion could also be important but that is significantly harder to measure (since that requires coherent measurement).A very crude and simple way: put in a tone at fixed frequency. Measure the output spectrum. What you will see is that in said output, there is not only a component at your frequency but also at harmonics. As you increase the power of the input tone, you will notice that these harmonics grow faster than your fundamental (why follows from the mathematics involved).Do this at a few frequencies. As you approach the bandwidth of your amplifier you will find the distortion performance gets better since the limited bandwidth acts as a filter to get rid of these harmonics.Keep in mind that you need to be sure that what you are seeing is harmonics of your amplifier and not of your source. (there are various techniques to do this but I'm not going to go into them here as they would take too much space.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 09:47:13 am
content: Quote from: TheUnnamedNewbie on February 20, 2018, 07:29:33 amI decided to try a harmonics measurement at 10Mhz because that's where I can generate a fairly clean source using my old HP signal generator and a 10Mhz low-pass filter in series. The output was adjusted to 0dBm, and the results shown on the spectrum analyzer as "TA10Mhz Source". The Amplifier was then driven by the source (with 50ohm terminator) and the output of the amplifier attached to the spectrum analyzer as "TA10MHz Measured". As you can see the amplitude is down by a bit over 6dB and no harmonics can be seen...even the small ones in the source measurement...presumably because they are also down 6dB. Am I doing something wrong? I didn't expect it to be this good.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 20, 2018, 10:52:28 am
content: It is possible that your performance is good. Feedback lowers distortion and that is what you will also be seeing. I would suggest going up in power and observe how it changes. I don't know what power you can achieve?A similar property is the compression. As you increase power you will see that gain decreases. The point at which you see a decrease of 1 dB is called the 1dB compression point. You will find this spec expressed either as OP1dB or IP1dB, which simply refers to what port we are refering this power leven to (input or output: Ie, it compresses 1 dB when we input x dBm or when the output is delivering y dBm. The difference will be one dB less than the gain since ofcourse at this point the gain hass compressed by one dB).Also lowering the RBW of the spectrum to lower the noise floor if possible so you can see the harmonics.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 11:01:18 am
content: Quote from: TheUnnamedNewbie on February 20, 2018, 10:52:28 amThe max power I can generate from my 10MHz source is about 5dB higher than shown. When I do that I get the first harmonic spike just appearing. I will try increasing the RBW in future. I already employed averaging just to get the noise as low as you are seeing. Thanks for the insight and prodding me in a good direction. Today I ordered some filters so I can generate several "pure-ish" frequencies for future work.

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2018, 11:15:40 am
content: Quote from: Insatman on February 20, 2018, 11:01:18 amThe "first harmonic"is10MHz, so I would hope you could see it!Perhaps you mean 2nd harmonic or third harmonic?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 20, 2018, 11:37:21 pm
content: Quote from: tggzzz on February 20, 2018, 11:15:40 amYes...I meant the 2nd harmonic.

author: MasterT
author_rank: Frequent Contributor
timestamp: February 21, 2018, 02:22:44 am
content: I recently built similar device, using VCA821. It has better freq. performance, low THD and variable gain. I see the price is lower on mouser, almost half compare to OPA633.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 21, 2018, 03:37:40 am
content: Quote from: MasterT on February 21, 2018, 02:22:44 amI checked on Digikey and the VCA821 is about a $1 cheaper than the OPA633. It's in a SOIC14 package whereas the OPA633 is DIP. I will buy a couple of VCA821 for experiments.

author: Insatman
author_rank: Frequent Contributor
timestamp: March 31, 2018, 10:20:25 am
content: I recently designed and had fabricated a PCB for the amplifier I built on perf board earlier. A few circuit tweaks were implemented. The results are a bit better but generally similar. Response is within +/-1dB from DC to ~100Mhz. The nominal gain is -6.2dB. The -3dB (plus nominal -6.2dB) is ~125MHz. Crude checks for Harmonic distortion were made at 10MHz and 100MHz. No harmonics were observed at 10MHz. At 100MHz the 2nd harmonic is about 40dB down and the 3rd harmonic is over 50dB down. The PCB uses all through hole components except for some optional SMD capacitors on the back side. They were installed for these tests. The PCB can accommodate a variety of coaxial IN/OUT connectors. BNC straight or right-angle or SMA types. Various screen shots from my SA, photos and the schematic are shown. The circuit is intended to be powered by a pair of 9V batteries, but any decent supply could be used.I intend to experiment with other op-amps suggested on this thread and those efforts are in progress.

author: kony
author_rank: Regular Contributor
timestamp: March 31, 2018, 10:54:46 am
content: Why do you call this trandimpedance amp? It is not.

author: tggzzz
author_rank: Super Contributor
timestamp: March 31, 2018, 11:37:45 am
content: Quote from: kony on March 31, 2018, 10:54:46 amCorrect.That has already been pointed out to the OP:https://www.eevblog.com/forum/projects/1meg-to-50ohm-trans-impedance-amplifier-for-general-instrumentation-use/msg1431533/#msg1431533While it isn't unreasonable to make a mistake and then correct it (e.g. see comments about harmonics), it really doesn't look good for someone to repeat a mistake after it has been pointed out to them.As I taught my daughter, "it is OK to makenewmistakes"!Hint for the OP: it is a transimpedance amplifier if and only if the input is a current (I) and the output is a voltage (V). Then the gain is V/I, which has units of impedance - and hence the name transimpedance amplifier.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 05:40:01 pm
content: Quote from: Insatman on February 20, 2018, 08:33:46 amTerms do change. For instance in the 1960s Tektronix referred to differential circuits as push-pull which leads to some confusion now if reading a circuit description without a schematic. Modern confusion with the terms difference amplifier, differential amplifier, and instrumentation amplifier all sometimes referring to the same thing (in my opinion incorrectly thanks to Texas Instruments) so this is hardly new.I have only heard the term transimpedance applied to this function mistakenly for the reason others identify; a transimpedance amplifier is a current to voltage converter. The oldest oscilloscope documentation I regularly use from the 1960s just calls these circuits cathode or source followers which is not very descriptive of the actual function. Tektronix started using the term buffer amplifier in the 1980s but this is still not very descriptive.Steve Roach who worked for HP in the 1990s on oscilloscope front ends and wrote a chapter in Jim Williams' "The Art and Science of Analog Circuit Design" about signal conditioning in oscilloscopes (1) called this function an impedance converter but to me that implies a passive device so I tend to use the term impedance buffer. Of the terms you could use though, transimpedance amplifier is just going to lead to confusion.(1) You might want to look this up for some ideas. Also consider the design of the Tektronix P6202A active probe which provides for a gain of 2 to drive a double terminated transmission line. I would keep the discrete impedance buffer but replace the transconductance amplifier with a gain of 2 current feedback operational amplifier for simplicity.

author: tggzzz
author_rank: Super Contributor
timestamp: March 31, 2018, 06:42:30 pm
content: Terms do indeed sometimes change, but all too often individual companies:screw up, e.g. Tek measuring time in units of conductance (S)deliberately obfuscate, to try to gain a marketing advantageThe latter is particularly frequent in the software arena, and isn't unknown in the hardware areaAs is frequently the case, Lewis Carroll characterised the problem...Quote

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 08:00:27 pm
content: Quote from: tggzzz on March 31, 2018, 06:42:30 pmWhere was this? I give them some slack for using upper case S for seconds in CRT readouts for increased legibility. There have been complaints about the SI system increasing confusion by replacing the upside down omega with upper case S.My general solution is to write out Sec or Seconds and use the archaic mhos instead of Siemens. I would rather be thought archaic but understood rather than modern and misunderstood. I also put slashes through my zeros and dashes through my 7s and Zs.

author: Insatman
author_rank: Frequent Contributor
timestamp: April 01, 2018, 12:57:10 am
content: Quote from: tggzzz on March 31, 2018, 11:37:45 amI get it...I did after the first discussion of this subject. If you noticed my latest post never used the aforementioned forbidden term. I did not want to start a new thread however because it contains useful info and I'd likely get criticized for doing that was well. So the thread name will likely cause nit-pickers to beat this dead horse into a fine paste eventually.I'm NOT going to make a new run of PCBs to remove the forbidden name on the silkscreen.Unfortunately, all this nomenclature fussiness has distracted from any real technical discussion of the; Impedance changing amplifier, Delta-Z Buffer or thing-a-ma-jig in question. I do find this item quite useful. It can be put into a spectrum analyzer - tracking gen loop to allow for characterization of scope probes or other 1Meg output impedance items. It is also useful for extending the cable on such items in certain test situations.Can we get past this BS? I get it I really do...but the thread bears the "forbidden" name...so deal with it.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 01, 2018, 01:38:08 am
content: I like your logo. Is that a ceramic thyratron?

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2018, 02:57:30 am
content: Quote from: Insatman on April 01, 2018, 12:57:10 amImpedance converters or buffers are very handy. Another place they can be used is to drive a 50 ohm frequency counter with a standard high impedance passive prove. I have seen them used for low frequency network analysis.With an adapter that includes enough shunt capacitance, an active probe can be used as an impedance converter.

author: Insatman
author_rank: Frequent Contributor
timestamp: April 01, 2018, 03:43:39 am
content: Quote from: ahbushnell on April 01, 2018, 01:38:08 amYes, it is. I was my companies expert on hydrogen thyratrons and ignitrons. Check out the datasheet for one of the tubes I worked with frequently. It cost upwards of $50K each when I retired.

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2018, 08:47:19 am
content: Quote from: David Hess on March 31, 2018, 08:00:27 pmLook at any of their scope front panels before the 24xx series. The timebase has units of mS.I don't cut them any slack for being legibly wrong - because the lower case letters were perfectly legible.There have been many complaints about the SI system; I've even seen some claim some twats claim SI was the devil trying to replace the "natural" pounds/ounces system.QuoteI would rather not appear to be ignorant by using the wrong units. Where appropriate I can give other indications that it is conductance. I too have crossed my 7s and zs since university - but that is principally soIdon't misread my (increasingly illegible) scrawl!

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2018, 03:52:01 pm
content: Quote from: tggzzz onYesterdayat 06:47:19 PMWell, let's see. I have a 2232 and 2247A in front of me. And I have a 2230, 465, various 7000, and a 547 in the other room. All use lowercase "s" except for the 547 which uses all lowercase "sec" on their front panels and except for the 7000 readouts, they also all use lowercase "s" but the later later readouts had better precision.So do you have a specific example?I'm going to keep using mhos just to spite SI; and because it is much more mnemonic and avoids confusion with seconds no matter how marked.Insatman, below is an example of a simple 3 transistor impedance converter used as an active probe. For lower noise where low input capacitance of the FET is actually a problem, a JFET traditionally used for low noise audio (Linear Systems makes these now) can still achieve 100+ MHz. I would probably add a trimmer capacitor to shunt the input so the input capacitance can be trimmed to match your oscilloscope using a normalizer or x10 passive probe.A feedback amplifier will of course have lower distortion at low and mid frequencies. A more complex discrete circuit could add a cascode to the FET for better performance up to at least 100 MHz at the cost of more noise from active devices and a good chance of VHF oscillation without careful layout.I seem to recall that TI has some 8-pin DIP high speed operational amplifiers with a pinout that includes 2 output pins shortening the feedback path for better performance. LT had a part I would have recommended but AD has absorbed their web site and apparently discontinued it.Burr-Brown made some really nice operational transconductance amplifiers (OPA860 and OPA861) which would work well in this application but TI never followed up on them and none had FET inputs so they would require an input buffer. They could effectively implement the x2 gain stage and cable driver shown in the Tektronix P6202A example with shunt termination instead of series termination in a single part.

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2018, 04:26:11 pm
content: Quote from: David Hess on April 01, 2018, 03:52:01 pmNow thatreallyannoys me: you are right and I am wrong.Where onearthdidI get that misapprehension - which has lodged in my brain over the decades? I must be channelling Aristotle, who maintained women had fewer teeth than men, but never opened either of his wifes' mouth to count them!QuoteThat is unambiguous, if archaic.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2018, 05:29:13 pm
content: Quote from: tggzzz on April 01, 2018, 04:26:11 pmIt happens to me also. Forget about it.QuoteI had examples at hand to look at first hand so did not have to rely on memory. For all I know, the other 465 and 22xx series models use an uppercase "S".

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2018, 06:50:28 pm
content: Quote from: David Hess on April 01, 2018, 05:29:13 pmSo do I, and I didn't choose to look. Hence Aristotle, and hence it being so damn embarrassing/annoying

author: Insatman
author_rank: Frequent Contributor
timestamp: April 02, 2018, 01:09:44 am
content: Quote from: David Hess on April 01, 2018, 02:57:30 amWhen I designed this buffer, I set up the input to look like a typical scope input ie 1M/18pf specifically so scope probes would work correctly. That would also apply to your active probe I would guess in most cases. Trim adjustment of the probe would probably be advisable for best performance.

author: Insatman
author_rank: Frequent Contributor
timestamp: April 02, 2018, 08:09:19 am
content: I built another version of this type of Impedance Matching Amplifier based on a different op-amp. This version, while more complicated, uses a lower cost chip ($5 vs $15) and has a few other advantages and some disadvantages:Advantages:1) It's gain is 0dB rather than -6dB nominal.2) It has a slighter higher -3dB point and the roll off is slower, resulting in usability over a greater range.3) The 2nd Harmonic at 100MHz is down about -50dB rather than -40dB for the previous circuit.4) Op-Amp is cheaper5) Possible performance improvement through circuit tweaks.6) Increased gain possible.Disadvantages:1) Circuit is more complicated and requires +/-5 volt regulators.2) Previous circuit would tolerate higher input voltages as it used 9V supplies rather than 5V.3) Harmonic data shows some spurious signals between the 2nd and 3rd Harmonic, largest of which is -50dB down from fundamental.This version of the circuit board had an error on the IC pinout so some bodges were required. Also a pot was bodged in to adjust the gain to as close to 1:1 as possible over the frequency range of interest.I believe that further improvement can be obtained from this op-amp, after another run of PCBs to correct errors and add a few elements for flexibility we will try again.Yet another version of this circuit using the VCA821 suggested amplifier is in progress but I've yet to receive the PCBs.

author: David Hess
author_rank: Super Contributor
timestamp: April 02, 2018, 09:48:32 pm
content: Quote from: Insatman on April 02, 2018, 01:09:44 amHigh frequency active probes are designed to have very low input capacitance for obvious reasons so they require added input shunt capacitance to work as an impedance converter for a standard passive probe.I have not seen this so much in modern oscilloscopes but older oscilloscopes usually have input capacitance adjustments for each channel which allows all channels to have a matched input capacitance so that a compensated probe can be moved between channels without affecting the compensation. This also allows different oscilloscopes, or at least those with the same nominal input capacitance, to be adjusted to have the same input capacitance. This input capacitance adjustment on an impedance converter would allow probes to be moved around without extra compensation adjustments.One thing I have thought about making is a differential impedance converter to use a pair of x10 or x100 probes as a differential probe. Old oscilloscope differential amplifiers rely on the probes for DC and AC common mode adjustments but I would want to move these adjustments to the amplifier instead of buying specialized probes; I think LeCroy is the only supplier for these today.

author: Insatman
author_rank: Frequent Contributor
timestamp: May 24, 2018, 07:33:08 am
content: Quote from: Insatman on April 02, 2018, 08:09:19 amI finally built and tested the VCA821 IC based impedance matching amplifier as suggested by another member of this blog (thanks MasterT). This IC is surface mount only and the version I bought costs about $9 from Digikey. The BW is the best so far at over >300Mhz. This op-amp is different from most and the datasheet is confusing to say the least. I spent most of a day getting it to work this well and further tweaking is probably possible. If particular importance is R5,R12 and C6 to tweak high end frequency response.I will order a revised PCB eventually and see if I can improve things next version. Like my previous posted version this one has variable gain and is tested at 1:1 into 50 ohms.Advantages:1) It's gain is variable and tested at 0dB not -6dB as in my first version2) It has a significantly higher -3dB point at ~330MHz and the roll off is reasonable.3) Op-Amp is cheaper than 1st version.4) Probable performance improvement through circuit tweaks.Disadvantages:1) Circuit is more complicated and requires +/-5 volt regulators.2) Original circuit would tolerate higher input voltages as it used 9V supplies rather than 5V.3) Harmonic data at 10Mhz 2nd/3rd is down >60dB but at 100MHz 2nd/3rd is down 35dB/53dB.All three version of the PCB share identical BNC connector spacing (2") so one housing design works for all of them. All can use 9V batteries as on board voltage-regulators are incorporated.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 24, 2018, 09:44:33 am
content: Quote from: David Hess on April 01, 2018, 03:52:01 pmMeh? Not mine! Indeed, not in any picture I could find, they all have SEC in upper case. And I leafed through Stan Griffith's book and looked at tons of old Tek stuff.

author: tggzzz
author_rank: Super Contributor
timestamp: May 24, 2018, 09:51:56 am
content: Quote from: Alex Eisenhut on May 24, 2018, 09:44:33 amI refer you to my post that immediately followed David Hess' post:https://www.eevblog.com/forum/projects/1meg-to-50ohm-trans-impedance-amplifier-for-general-instrumentation-use/msg1467894/#msg1467894(plus the ensuing conversation between David an me)

author: IDEngineer
author_rank: Super Contributor
timestamp: May 24, 2018, 04:03:44 pm
content: Quote from: David Hess on March 31, 2018, 08:00:27 pmAs do I.I also write my dates in DDMMMYYYY format, thus: 24May2018. This completely eliminates the possibility of confusion between the day and month, since depending upon the writer and reader the two formats MM/DD/YYYY and DD/MM/YYYY are not definitive for days <= 12. The four digit year might seem unnecessary, except to those of us who write software and/or lived through the whole Y2K thing.In my opinion, the goal of written communication is to communicate. Details that reduce ambiguity, such as slash-throughs and dash-throughs and choice of formatting, are a good idea.I do have a story about this, though. I adopted the slash-zero in junior high school (grades 7-8). One day a student was asked by the teacher to grade a test, and I got something like 50% wrong (?!?). Turned out the student thought my slash-zeroes were where I'd changed my answers, so she marked every single one incorrect. I pointed out that every single zero on the page had a slash through it, none of the ones had a circle, and asked her the odds that I'd "correct" literally every single zero on the page to a one. She didn't relent. Grrrrr.

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2018, 04:55:55 pm
content: Quote from: IDEngineer on May 24, 2018, 04:03:44 pmI write out my dates in some form of numeric YYYYMMDD to match what I type into computers so they may be easily sorted. This is essentiallyISO 8601and I refuse to conform to the US civilian standard. For convenience I may include the text month.I also set all of my clocks to 24 hour time if possible.And I swap the control and caps-lock keys on my keyboards so that the control key is next to the A key where God intended it. This has occasionally resulted in hilarity when someone tries to use my computer.

author: IDEngineer
author_rank: Super Contributor
timestamp: May 24, 2018, 05:51:15 pm
content: Quote from: David Hess on May 24, 2018, 04:55:55 pmGood point, I too do that with file and directory names (e.g. "2018-05-23_mnemonic_info.ext") so they will sort by date. I tried that for a while in written communications but it made things worse, not better. Try dating a signature with that format at a bank or the DMV and settle in for a long afternoon....QuoteI used to design keyboards in a previous life. Strictly speaking, there's no way to follow historical standards with today's keyboards. Caps Lock should be SHIFT Lock, for example (though frankly Caps Lock is more useful). And the -Lock key is what belongs to the left of the A key.There was no Ctrl key on traditional keyboards, so it was just tacked on someplace. Later the same was done with the Alt key. Note the singular... early 80's keyboards only had a single Ctrl and single Alt key. Later designs started having two, like shift keys (which they functionally resemble). I bear some responsibility for that.I just purchased a new ThinkPad X270. They reversed the Ctrl and Fn keys (Fn is to the outside on the extreme left). Get this: They know it's so non-standard that there is an option in the BIOS to flip those two keys! Seriously, if what you're doing is so contrary to societal norms that you ALREADY know you need a bailout option, why not just do it properly in the first place? Or at least make the standard the default, so the keys are silkscreened properly?

author: ebastler
author_rank: Super Contributor
timestamp: May 24, 2018, 06:14:11 pm
content: Quote from: IDEngineer on May 24, 2018, 05:51:15 pmYour first Thinkpad, I reckon? They have been doing this since 1992:https://forums.lenovo.com/t5/ThinkPad-T400-T500-and-newer-T/Why-is-the-FN-key-on-the-outside-left-instead-of-CTRL/ta-p/359099(Which still doesn't make it a good idea, in my book...)

author: IDEngineer
author_rank: Super Contributor
timestamp: May 24, 2018, 06:30:27 pm
content: Quote from: ebastler on May 24, 2018, 06:14:11 pmAmazing. I wonder who thought that was a good idea in the first place.Yes, my first ThinkPad. I have zero brand loyalty, I just start with a list of required specs. I build my own home machines but you sort of have to buy laptops.So I start with specs and winnow the field.High on my list this time around was native Win7 so Win7 drivers were downloadable from the manufacturer's support site for all of the onboard peripherals. I just can't stand the post-Win7 user interfaces. Why Microsoft insists on forcing their users to constantly relearn how to use a tool is beyond me. Computers aren't a hobby for me, they are a TOOL and I resent having to relearn where they've hidden all the important stuff every couple of years. Remember when Office already had 95% of its market, yet they completely redesigned the UI?Here's an idea: Let's put the steering wheel in the back seat and have the driver sit sideways. No, really, it's better because they can look forward AND backward with just a turn of the head! Trust us, we know better than you.It's notable that, despite there being provably better keyboard layouts (example: Dvorak), we continue to raise new generations of typists on QWERTY whose reason for existence died along with typewriters that had typebars instead of the Selectric's spherical element. If the marketing research says to leave the keyboard layout alone, why doesn't that apply to the GUI?The X270 was one of the last laptops to have a replaceable battery, RJ-45 Ethernet port, and a good array of USB and other comm ports. I'm often in a hotel room late at night, revving code based on customer meetings that day in preparation for more customer meetings the next day, and I usually have a backpack development environment with me (portable scope, power supply, ICE/debugger, various sniffers and other USB-interfaced tools, etc.). The hyper-thin laptops are emotionally appealing but lack Ethernet ports or enough USB's to do that job. The X270 is still an ultraportable, amazingly compact by the standards of just a few years ago, which is thin and light enough (~3 pounds) for me. More importantly, it has ports for everything and I get 20+ hours of battery life before I have to swap in another battery or plug in somewhere. Not even Lenovo's X1 Carbon can do that (if it could, I'd own that instead because it really is a cool design).Interesting about ThinkPads swapping those keys so long ago. That was back when they were owned by IBM, and then Motorola, which means they would have had a more US-centric viewpoint (instead of China today). What could have made them buck an industry standard like that?EDIT: I just read the comments in that link you provided. Here's one that sums it up: "My business partner standardized our laptops on Lenovo. Let me tell you, I remedied that in less than a year. Why, and what would make me switch back to these otherwise great machines? This button arrangement. Love the explanation, and I wish the others had all followed suit, but they didn't. We've swapped the buttons in BIOS but it causes unfamiliar users tons of pain as they keep raising issues thinking the button is broken because they're pressing the wrong one now."

author: ebastler
author_rank: Super Contributor
timestamp: May 24, 2018, 07:46:56 pm
content: Quote from: IDEngineer on May 24, 2018, 06:30:27 pmThat's how I have selected notebooks, ever since parametric search engines were made available by a few vendors and price comparison portals. But despite the fact that various generations of Thinkpads had attractive specs (both back in IBM days and later Lenovos), I never bought one for personal use since the keyboard layout did put me off.QuoteMy employer just went through an extended phase of buying Lenovo machines as a standard. (Just switched back to DELL after > 5 years, whew...) I have activated the Fn/Ctrl BIOS swap on my corporate notebook, and it works for me. But it sure confuses others who have to type something on my machine, e.g. during a meeting, and I struggle and curse when I have to use theirs. Why couldn't Lenovo at least make the two keycaps the same size, so you can physically swap them?Regarding quotes on that web page, my favorite one is from the main author of that Q&A: "Every other notebook personal computer manufacturer that I know of has the Fn and Ctrl key positions swapped."

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 24, 2018, 08:49:55 pm
content: Quote from: David Hess on April 01, 2018, 02:57:30 amwhy dont just build an active probe (Hi-Z in 50 ohm out) with selectable 1x 1/10x input? less one Hi-Z probe to take care about. cost should be less than $100 alltogether.

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2018, 09:48:21 pm
content: Quote from: Mechatrommer on May 24, 2018, 08:49:55 pmI suppose I could but x10 passive probes are cheap and rugged and all of my frequency counters have 1 megohm inputs and some of my frequency counters lack 50 ohm inputs. Internally 1 megohm input frequency counters have an impedance buffer driving the low impedance circuits where amplification and comparison is made.When working above 200 MHz, an active probe and 50 ohm input become a better option and very few frequency counters have high impedance inputs much above 100 MHz. My fastest high impedance input is 225 MHz while 50 ohm inputs work into the microwave range of course. Somewhere above 100 MHz you would want the lower input capacitance of an active probe (or low-z probe or 50 ohm connection) anyway so this all makes sense.

author: IDEngineer
author_rank: Super Contributor
timestamp: May 24, 2018, 10:37:33 pm
content: Quote from: ebastler on May 24, 2018, 07:46:56 pmThat would just be further admission (beyond the BIOS option) that they made a mistake. And continue to refuse to correct it.QuoteComic genius!

author: Insatman
author_rank: Frequent Contributor
timestamp: May 25, 2018, 02:01:01 am
content: Quote from: Mechatrommer on May 24, 2018, 08:49:55 pmBecause often it's not simply circuit probing I'm using the buffer amp with. For instance, maybe I want to look at the frequency response of a standard 1M probe on the spectrum analyzer. Put the buffer in the tracking gen loop, normalize, then put a 50ohm feed-thru term on the tracking gen cable and the probe tip w/ adaptor connected to it. The output of the probe then goes to the buffer input....now you have the frequency response and attenuation of your 1M probe displayed on your SA. Lots of applications for this kind of buffer.

author: HVUser
author_rank: Newbie
timestamp: August 20, 2020, 10:40:24 pm
content: I just found this thread while searching for a product to do this. Would you consider selling these? If not, maybe posting masks/Gerber files?

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2020, 11:14:53 pm
content: I just found this thread. I have a Marconi 2374 high-Z probe (specified bandwidth 200 MHz) for my Marconi 2380/2382 400 MHz spectrum analyzer (50 ohm input).I tested the THD of the probe at 500 kHz (half the maximum frequency of my Wavetek 98 DDSvgenerator, which has low THD at its 50 ohm output.Without probe: 50 ohm output to 50 ohm input. THD = 0.12% for the naked generator (-63 dB 2nd, -61 dB 3rd, and -70 dB 4th) at 0 dBm/50 ohms = 0.22 V rms.With probe: 50 ohm output to 50 ohm terminator to probe to 50 ohm input. THD = 1.5% (-37 dB 2nd, -46 dB 3rd, and <-72 dB 4th) at same generator level.I quote these values as what can be accomplished with a simple circuit with discrete devices; obviously, the probe distortion is much worse than the generator's THD.The probe comes with a set of input attenuators, which I haven't tested.

author: Insatman
author_rank: Frequent Contributor
timestamp: August 21, 2020, 02:09:13 am
content: Quote from: TimFox on August 20, 2020, 11:14:53 pmI think you are right that a circuit with discrete devices could be designed/built for this purpose. I chose to use op-amps simply because I am more familiar with them than discrete transistors. It might be a nice learning project to build such a circuit someday.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 06, 2022, 07:25:29 am
content: 1MEG TO 50ohm Impedance Amplifier Revisited. 2/2022 dwmRecently after nearly 4 years I decided to take a fresh look at this project of mine an see if I can better characterize the performance and make some improvements.The revised board/circuit is based on the VCA821 design described earlier. Mechanically the new board has identical dimensions and can replace the old version except the connectors are reversed. The prior version had the input on the right and output on the left. This was a mistake on my part as I prefer the input on the left and output on the right, so that is corrected in this layout.Overall performance is similar; however, I have made much more extensive measurements this time around.Power consumption for the board is approx. 35 to 40mA for the +/- 5 regulators. +/-9V was supplied from a bench supply to mimic the 9V batteries normally used to power the circuit.In general I found that THD increases quickly at higher frequencies when the input is above 0dBm. Operation at between -10dBm (70mV RMS 50ohm) and 0dBm (.707mV RMS 50ohm) is recommended.Frequency Response was measured at +0.85dB, -3dB from 1MHz to 280MHz at 0dBm input. The peak value of +0.85dB occurs around 100MHz. Instrument used was Siglent 3021X with tracking genFrequency Response was measured at +1.44dB, -3dB from 1Mhz to 340MHz at -10dBm input. The peak value of +1.44dB again occurs around 100MHz. Instrument used was Siglent 3021X with tracking gen. Low Frequency Response was measured using ARTA, a laptop and an external USB sound “card” device. I estimate the response to be < +/- 0.3dB.THD was measured at 1KHz, 10KHz, 2.5MHz, 45MHz, 100MHz and 190MHz. the 45, 100, and 190MHz measurements were performed at two power levels. The reason for the odd frequencies is that I lack a clean signal source for frequencies above 10’s of KHz. Both my Chinese signal generator (FY6800) and vintage HP8654A both have significant 2nd and 3rd harmonics. So the 2.5 thru 190MHz clean tones were generated my placing a lowpass filter on the generator output. For example the 2.5MHz signal used a 1.9MHz low pass filter. This results in an attenuated but clean fundamental tone. Some examples of the drive tones are shown below.I do not believe that THD measurements below 0.3% can be counted on a accurate, so that is the lowest value I will state.1KHz THD <0.3% at 0dBm (ARTA)10KHz THD <0.3% at 0dBm (ARTA)2.5MHz THD <0.3% at 0.5dBm (3021X)45MHz THD 2% at 0dBm, 45MHz THD 1.0% at -10dBm (3021X)100MHz THD 1.3% at 0dBm, 100MHz THD 0.8% at -10dBm (3021X)190MHz THD 2.6% at 0dBm, 190MHz THD 0.9% at -9dBm (3021X)Time domain SCOPE overlays are also shown below. Scope used as TEK784D (1GHz), Signal gen was FY6800. Signal was split using a power divider with one leg going to the DUT and the other to scope channel 3. The DUT is displayed on CH4. Both channels are 50ohm terminated.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 06, 2022, 07:27:44 am
content: Additional post to add more JPG data

author: Insatman
author_rank: Frequent Contributor
timestamp: February 06, 2022, 07:29:36 am
content: additional post to add more JPG data

author: Insatman
author_rank: Frequent Contributor
timestamp: February 08, 2022, 12:01:20 am
content: Schematic and photos of build of unit. Performance data is shown previously.

author: Gyro
author_rank: Super Contributor
timestamp: February 08, 2022, 10:01:45 am
content: It's never too late to go back to your first post and edit the thread title from 'Trans-impedance' to 'Impedance matching'. It might actually generate more / fresh interest.

author: magic
author_rank: Super Contributor
timestamp: February 08, 2022, 10:33:21 am
content: I think a variable TIA with 1:20000 range of gain could be interesting too.I came here wondering what application somebody might have for such a thing in "general instrumentation use"

author: David Hess
author_rank: Super Contributor
timestamp: February 08, 2022, 04:41:07 pm
content: Quote from: magic on February 08, 2022, 10:33:21 amSome specialized oscilloscope probe amplifiers use a transimpedance configuration. The Tektronix type 134 current probe amplifier is an example which can also be used with low impedance probes.Achieving a wide gain range with constant bandwidth would be a challenge.