Homebrew spectrum analyser high-impedance probe - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: April 03, 2013, 12:23:26 am
content: I'm not sure whether this belongs here or in Test Equipment, but here goes anywayA while ago I picked up an old Advantest R3361A spectrum analyser. The old girl works a treat, despite being near enough twenty years old now. Or rather - she works a treatwhen probing anything which provides a standard 50-ohm output impedance. I'm sure you can see where this is headedI'm currently debugging a low-distortion receiver for the phase-modulated time signal on BBC Radio 4 (198kHz) and a separate but similar receiver for the Anthorn / MSF time signal on 60kHz. The output impedance of the ferrite loop is high enough that loading it with 50 Ohms is enough to completely kill any signal it picks up. Probing the output of my first-stage RF amp (a common-source JFET amplifier with Zo of ~10k-ohms) works to some extent, but the JFET clearly isn't happy driving it. Probing with an oscilloscope is a complete waste of time - even the amplified signal from the JFET is well below my oscilloscope's noise floor (although admittedly my oscilloscope is a Tek TDS2000B series machine which doesn't have particularly good noise characteristics to start with...)My question is thus: how would I go about building a high-impedance probe for my spectrum analyser?I've looked into MMICs (I have a couple of MAR-6 and ERA-2SM chips dotted about) but these appear to be internally terminated for 50 Ohms and will load the 'target' circuit down far too much.JFETs are another possibility, but the output impedance of such devices is rather low. They might work as a preamp stage, but for a 50-ohm driver? Forget it.Can anyone suggest anything else I could look into? I'm only after ~150MHz bandwidth at the most - heck, even 20MHz would be a nice start.Thanks,Phil.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 03, 2013, 01:38:26 am
content: Active probes that only need a few hundred MHz bandwidth are easy to make, even with just carefully selected op-amps. If you have the equipment to measure/calibrate its linearity and gain, you should be fine

author: pauln
author_rank: Contributor
timestamp: April 03, 2013, 05:58:27 am
content: Quote from: olsenn on April 03, 2013, 01:38:26 amI made an active probe several years back, using a J310 followed by an MMIC, voltage regulator and a handful of other bits all SMD - its not Agilent but it works well . The original article was from a very early 73 magazine.Happy to send circuit, photo and description via PM if required. I may well have a PCB as well.Let me know.Paul

author: KJDS
author_rank: Super Contributor
timestamp: April 03, 2013, 11:32:46 am
content: The lazy way of doing it is to just use a length of 50ohm cable with a 470R resistor as the probe. This will reduce the signal seen at the analyzer by (about) 20dB but means the circuit is loaded much less.The proper way of doing it is with a FET input stage followed by a buffer amp. The main problems with these are stability, probe near a high Q resonance where the JFET is a little marginal and it can oscillate and also the risk of static damage.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 03, 2013, 12:26:25 pm
content: Maybe the "Poor Man's 1GHz Active Probe" project by Elektor magazine would help you out...http://elektrotanya.com/files/forum/2009/10/e04a036.pdfThis high impedance, active probe is on my to do list (need it for my RF Explorer-3G analyzer).I have the components already, but I need to make the PCB for it.Specs listed in article for the DIY probe:Input-Z: 0.75pf//10MohmOutput-Z: 50 ohmsBandwidth: 100KHz - 1.5GHz (+-2.5dB)Gain: -20dB nominalNoise figure: 1dB (using SOT143 BF998 MOSFET)Regards,FrankEdit: Updated the broken link.

author: philpem
author_rank: Frequent Contributor
timestamp: April 03, 2013, 08:54:17 pm
content: Wow, that's a lot of replies! Thanks, guys -- I'll answer these in one post if you don't mindQuote from: olsenn on April 03, 2013, 01:38:26 amActually the spectrum analyser has a Normalise function. If you connect the TG output straight through to the input via whatever probe you're using, it figures out the correction factors (I think it works by subtracting to the lowest level, though the manual isn't especially clear about that).Quote from: pauln on April 03, 2013, 05:58:27 amWow - that'd be great! Will send a PM shortly. Thanks!Quote from: KJDS on April 03, 2013, 11:32:46 amThe problem is I'm dealing with signals which are quite weak to begin with - losing 20dB would probably push them below the noise floor (or very close to it).Quote from: fpliuzzi on April 03, 2013, 12:26:25 pmNow that's useful!I've actually got a bunch of BF998s in my junk box - I bought them ages ago thinking it'd be useful to have some dual-gate MOSFETs. For once, it seems I was right!The only problem there is that -20dB drop. I wonder if an MMIC (per the 73 Magazine designpaulnmentioned) would be enough to bring that within a few dB of unity. Again, junkbox provides - I have MAR-6 and ERA-2SM MMICs; just need to read the specs and pick the best of the two...

author: JackOfVA
author_rank: Unknown
timestamp: April 03, 2013, 09:28:24 pm
content: A BF998 is a rather stout device and will drive a 50 ohm spectrum analyzer without too much loss when configured as a source follower. And by "too much" loss, I would expect something between 3 and 6 dB. Since the BF998 has useful gain around 1 GHz or more, parasitic oscillation is always a concern, and yes, a source follower can oscillate as it has power gain.To check the resonance of a 60 KHz loop antenna, in the absence of a high Z probe, either purchased or home brewed, I would try an inductive pickup. A sensor coil with a turn or two on it, connected to the spectrum analyzer by a meter or two coaxial cable. Keep the inductive pickup a reasonable distance from the 60 KHz tuned antenna and you may be able to pick up enough energy to see the resonance peak.Spectrum analyzers are designed for linearity and have miserable noise figures - 30 to 40 dB is not untypical. You can improve the sensitivity by a low noise high gain preamplifier. You can also pick up 10 dB of usable gain by setting the minimum SA attenuation to 0 dB instead of the default 10 dB. One point of warning ... if you blow up the front end of your SA, you will unhappy, and the 10 dB attenuator is a safety measure to prevent damage to the SA input stage, so bypass it with care. There's a non-obvious key sequence to set input attenuation at 0 dB on the Advantest instruments (I have a pair of R3463s here). It's something like set to manual attenuation, enter 0 dB with the MIN ATTN button and then press the MIN button a second time. Been so long since I've done it that I've forgotten the exact key sequence.I assume you've also done the usual things like use the minimum possible span and narrowest bandwidth on the SA and also cranked down the video bandwidth. You can also turn on trace averaging to reduce the noise more, but it will slow updates way down.You can also make a simple voltage probe with a length of coax cable unterminated with a few cm of the center conductor exposed beyond the braid. Probably not going to help with very low level amplifier stages, however.

author: Marco
author_rank: Super Contributor
timestamp: April 03, 2013, 09:29:00 pm
content: Why is the 20 dB drop for the poor man's probe necessary BTW? (ie. why not use a couple of parallel pF range capacitors for AC coupling or even just DC coupling.)

author: philpem
author_rank: Frequent Contributor
timestamp: April 04, 2013, 12:43:44 am
content: Quote from: JackOfVA on April 03, 2013, 09:28:24 pmThat's interesting - the Elektor article quotes gain as -20dB (minus 20dB) - i.e. a loss of 20dB.Quote from: JackOfVA on April 03, 2013, 09:28:24 pmI'm not trying to find peak resonance on a loop antenna, I'm trying to figure out if my ferrite antenna is picking up a signal, whether that signal is being amplified correctly by the JFET (and following stages) and finally track down any distortion which might be creeping in.I started out with a 198kHz receiver based on a design from EPE magazine, but the output is clipping quite a bit. I found this out when the PLL locked fine from cold, but if the input signal stopped it'd drift up to Fmax then never recover.Quote from: JackOfVA on April 03, 2013, 09:28:24 pmMy Advantest does something similar - COUPLE, ATTEN,. You can't select 0dB attenuation using the spinner - it has to be directly entered.Quote from: JackOfVA on April 03, 2013, 09:28:24 pmAlready done that :)The problem I have is that the 50R load kills the amplifier. It's a relatively high impedance output (a few Kohms or so) compared to the 50R load the SA presents to it. It just gets loaded down and the SA sees almost nothing. With attenuator disabled I can get a bit of a signal -- just high enough above the noise floor for Zero Span to work (the MSF signal is OOK/100% AM so zero-span with Fc=60kHz shows the transmitted data). Sadly this is nowhere near good enough to see if there are any harmonics being introduced, which this early on would be indicative of bad FET biasing (hardly surprising - this is the first time I've used a JFET in a linear application and I haven't found any good, solid references on JFET -- or indeed MOSFET -- biasing for RF).Thanks,Phil.

author: Marco
author_rank: Super Contributor
timestamp: April 04, 2013, 01:41:42 am
content: Quote from: philpem on April 04, 2013, 12:43:44 amThat's almost entirely because of the decoupling capacitor (and it's effect as a divider with the source follower input capacitance).

author: JackOfVA
author_rank: Unknown
timestamp: April 05, 2013, 02:57:50 pm
content: I build and sell to the amateur radio and shortwave listener market a voltage probe active antenna. It's basically a high input impedance source follower, with careful attention to maximizing dynamic range and minimizing noise figure. The input stage is a BF996 dual gate FET, which drives a BFQ19 follower, and the output is a BFQ19/BFQ149 CS follower. Frequency coverage is 10 KHz - 30 MHz where an input filter starts rolling off the coverage to avoid FM broadcast band overload. Without the input filter, it's usable well above 100 MHz. This is, excepting that low frequency response does not extend to 0, an FET scope probe. (It's also a bit physically large to use as a probe.)In developing the antenna, some early experiments involved driving a 50 ohm load directly with the BF996 and I found it does a credible job, based purely on amplitude. It does not provide the best intermodulation performance, so a more complex circuit is required, but you should not see more than 6 dB loss into 50 ohm load.The input capacitance of a BF996, plus stray trace capacitance and, in the case of my active antenna, a disconnect relay, is on the order of 8-10 pF. To prevent the input blocking capacitor from being a limiting factor (voltage divider effect) it's necessary to make it considerably larger - I use a high voltage (630V) 1000 pF C0G capacitor.If you don't want to fiddle with a physically small SMD part like the BF996, a TO-92 style J310 is nearly as good and much easier to throw together in a one-off arrangement. For minimum distortion, set the gate bias to provide about 20 mA idle current through the J310. That's a lot more than most people think of when an FET is used, but the "sweet spot" for the J310's intermodulation distortion is in that range.

author: philpem
author_rank: Frequent Contributor
timestamp: April 05, 2013, 10:30:55 pm
content: To be honest, I prefer working with SMD -- it means I don't have to get the Dremel out to drill the PCBsAt the moment I'm reconstructing the CAD files for the Elektor probe, to which I'll add a MAR-6 as a test. It turns out I have almost all the parts I need scattered amongst my spares boxesChances are I'll build up the Elektor probe as-is, then add the MAR-6 and see what (if anything) happens. I'd like to do a board shrink-and-split - move the voltage regulator into a metal can mounted on the back of a bulkhead N plug, then run a cable to the Probe Power socket on the SA. That'd mean the probe board was just the probe, meaning I can machine a nice highlighter-pen sized case for it.I sense a weekend project coming onI'll let you know how I get on - it should be interesting.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 06, 2013, 02:05:54 am
content: Hi Phil,Your idea of adding a MAR-6 to the Elektor probe sounds very interesting. Please keep us updated.Regards,FrankP.S. When I went to purchase the tiny $20 PCB from Elektor to build their "Poor Man's 1GHz Active Probe", they wanted to add an additional $30 to mail that puny thing to me, here in the US. $50 total is unreasonable.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 06, 2013, 10:18:52 am
content: Quote from: pauln on April 06, 2013, 06:05:19 amThanks for the schematic. It's very helpful to see how the MMIC can be incorporated into my active probe (hopefully built soon).Regards,Frank

author: KJDS
author_rank: Super Contributor
timestamp: April 06, 2013, 12:32:07 pm
content: How stressed is that 78L05.7V across it, and at a guess 40mA? A quick check suggests it'll get warm enough to worry me enough to either lower the supply voltage if practical or if you're going to mount it away from the probe then use a 7805 instead.

author: branadic
author_rank: Super Contributor
timestamp: April 06, 2013, 04:59:33 pm
content: Here is another FET probe based on TI's JFET OpAmps from DC to 500MHz:http://www.mikrocontroller.net/topic/188227#newand another AC FET probe (page 112):http://www.bartelsos.de/index.php?dl_file=3TNJFF9YR3

author: philpem
author_rank: Frequent Contributor
timestamp: April 07, 2013, 11:27:53 am
content: Quote from: KJDS on April 06, 2013, 12:32:07 pmI'll probably use a normal 7805 anyway - I don't tend to keep 78L05s in stock.Exceeding voltage regulator specs seems to be Elektor's SOP - I've seen them do it in a few designs. That and using solder pads barely larger than the component pins...Just patching together an EAGLE library for the Minicircuits MAR-6, then modifying the Elektor design and etching a PCB!

author: philpem
author_rank: Frequent Contributor
timestamp: April 09, 2013, 11:46:15 pm
content: Well, this is what I've got so far -- schematic and PCB attached. Would be interested in any comments, criticisms or suggestions for improvement you might haveDimensions of the board: around 14 x 50 mm. In old money, a whisker over half an inch wide by two inches long.My eventual plan is to (hopefully!) build up some form of shielded case for the thing. That's still on the drawing board, though.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 10, 2013, 02:03:17 am
content: Thanks for posting the schematic and board layout (I'm assuming that you're using Elektor's original bottom layer layout as is). Nice job on the PCB layout. I also like your idea of moving the 5V regulator off of the probe board to minimize the probe's dimensions.I am happy that I held off building Elektor's original probe board design. I like your implementation much better. I'm sure that you cannot wait to test drive your new probe.Regards,Frank

author: vk6zgo
author_rank: Super Contributor
timestamp: April 10, 2013, 02:16:42 am
content: Quote from: philpem on April 07, 2013, 11:27:53 amI thought that was just a generic European thing,like through holes barely larger than component leads.Great for flow soldering,but a nightmare to rework!

author: BravoV
author_rank: Super Contributor
timestamp: April 10, 2013, 02:23:08 am
content: Poor Man's 500 MHz Active FET Probe ->http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdfIts written in German though, but with lots of photos especially tons of results of the probe's profiles from the spectrum analyzer, it should be easy to interpret the probe's performance.

author: philpem
author_rank: Frequent Contributor
timestamp: December 25, 2014, 11:53:06 pm
content: Should probably update this...I eventually did build one of the probes -- the Elektor original. The EAGLE CAD files are here:https://bitbucket.org/philpem/rf_probeEnjoy.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 26, 2014, 02:36:53 am
content: My favorite is the P6032.http://w140.com/tekwiki/wiki/P6032Or you can look into thishttp://w140.com/tekwiki/wiki/282

author: philpem
author_rank: Frequent Contributor
timestamp: December 26, 2014, 01:24:38 pm
content: Interesting!Apparently my spectrum analyser (Advantest R3361A) has a Lemo probe-power socket which matches the pinout of the Tek 492 and 492P connector. The matching probe is apparently the P6201. Sadly they're pretty thin on the ground in working condition; the majority I've seen for sale are either overpriced, broken or in some far-flung locale being sold by someone who'd rather not sell internationally.I'm sure there's a later replacement for the P6201 but I'm not enough of a Tek guru to know what that would be. And I'll bet there's a HP/Agilent/Keysight equivalent too.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 26, 2014, 05:37:13 pm
content: There's a high speed probe on page 96 of this Jim Williams LT app note-http://cds.linear.com/docs/en/application-note/an47fa.pdf

author: jaxbird
author_rank: Frequent Contributor
timestamp: December 26, 2014, 06:19:46 pm
content: Quote from: Conrad Hoffman on December 26, 2014, 05:37:13 pmThe honorable Jim Williams was such a casual, maybe have a look at the work of Bob:http://www.ti.com/ww/en/bobpease/assets/www-national-com_rap.pdf

author: dadler
author_rank: Unknown
timestamp: April 27, 2015, 05:37:26 pm
content: Quote from: fpliuzzi on April 03, 2013, 12:26:25 pmThis link is dead.I found a similar link-is this an archive of the same article?http://elektrotanya.com/files/forum/2009/10/e04a036.pdf

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 27, 2015, 10:26:13 pm
content: @dadlerSorry about the late reply, but yes the article in your link is the same Elektor article that I was referring to in my post above from 2013.

author: signality
author_rank: Newbie
timestamp: September 17, 2015, 02:32:43 pm
content: If anyone is still looking for a design for a DIY high impedance probe, there's a version of Bob Pease's JFET active probe here:https://easyeda.com/andyfierman/Bob_Peases_High_Impedance_Active_Scope_Probe_PCB_Design-g2QNnljhfwith simulations:https://easyeda.com/andyfierman/Bob_Peases_High_Impedance_Active_Scope_Probe_simulations-gsmEyQ8qj

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2015, 04:58:32 pm
content: What are you probing?I'm building a tube radio right now. I've been doing stage-by-stage tests by terminating their outputs into 50 ohms directly. This has two advantages: the dynamic range is more than enough to get a useful signal out (typical tube currents are around 10mA, giving on the order of 4dBm; my HP 8590A has about -70dBm noise floor at typical settings), and I don't have to worry about the output parameters of the stage (compared to the resistance of kohms+ and reactance of ~300 ohms, typical of the output terminal at this frequency).If you're testing tuning/matching networks, it can be very useful to build them to match to 50 ohms. For example, you might have an impedance of ~1kohm from a transistor collector/drain, which obviously cannot be observed with a 10x resistor probe, nor is it practical to use a scope probe (with ~10pF of capacitance that will completely mess up your frequency response) -- even if you had the 1M to 50 ohm amplifier box to use such a probe on the spec.If you're matching one stage into another, with absolutely no need to touch 50 ohms inbetween (it would be a waste of materials and time, and you'd have to re-tune it anyway to optimize out the 50 ohm link), it might be better to wire up the following stage as normal, except leaving its output shorted into 50 ohms. What better probe than the device itself? And you get gain for free, besides!In general, transistors are quite happy being shorted into a low impedance. The output characteristic is largely capacitive, so you only need to check that the baseband bandwidth is sufficient (Fo = 1 / (2*pi*(50 ohms)*Cout) for the frequency you're monitoring.That said, because transistors operate with voltages much lower, and have much higher gain, than for my case with vacuum tubes, you won't be able to assume so easily that the output is acting like a true short. Miller effect and all that will still be relevant. But with it being so easy to build grounded-base/gate or cascode stages, you can use those to your advantage, for this reason and others.Tim

author: Marco
author_rank: Super Contributor
timestamp: September 17, 2015, 05:26:53 pm
content: Quote from: T3sl4co1l on September 17, 2015, 04:58:32 pmIf you are willing to decouple your circuit from subsequent stages for the coax probe then why not do the same for a resistor probe? In this case a 20x resistor probe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2015, 07:07:09 pm
content: That's an option too. Actually, I even did that the other day: testing the local oscillator output, I used a 51 ohm series resistor into the 50 ohm load, to represent the 100 ohm mixer load in my circuit. Just a matter of adjusting 6dB mentally when measuring oscillator power.For interstage measurements, you have two problems: 1. the saturation level may be low to begin with, so you don't have much room to work with before driving it into clipping, or not being much above the spec's noise floor. (Of course, if its noise floor is that bad, a LNA can be used to help.) 2. The following stage probably has reactance too, so you can't simply disconnect it anyway.This stage-wise process obviously works better in design and construction than in troubleshooting...Tim

author: xzswq21
author_rank: Frequent Contributor
timestamp: May 05, 2017, 03:50:28 pm
content: Quote from: philpem on December 25, 2014, 11:53:06 pmQuote from: signality on September 17, 2015, 02:32:43 pmQuote from: fpliuzzi on April 03, 2013, 12:26:25 pmDearI have a small signal within 50mVpp and 10KHz to 20MHz.I want to measure the signal with a high-impedance probe and with a gain of 10V/V (20dB). the amplifier should has an high linearity.you uploaded some useful circuits inthe below link:https://bitbucket.org/philpem/rf_probewhat about linearity and second and third haronic distortions?Please help me to build a good amplifier.thanks

author: harrimansat
author_rank: Regular Contributor
timestamp: December 15, 2019, 08:20:35 pm
content: My DIY BF998 fet probe, I will do more tests with tracking and pulse generator.