Sampling head GaAs diodes - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: September 10, 2013, 09:21:38 am
content: Does anyone know a manufacturers part number (or equivalent) for the diodes used in the sampling head of GHz-bandwidth sampling amplifiers such as the Tektronix 1S1 plug-in? The manual for the 1S1 only gives a Tektronix part number along with the descriptor "GaAs diode".I'd ultimately like to find a contemporary equivalent for experimental sampling heads with a -3dB bandwidth of 1 GHz and a sampling gate time in the order of 1/3 nS at repetition frequencies up to ~100 kHz. There are plenty of multi-GHz "RF" diodes in the catalogs of the usual suppliers but these are almost exclusively PIN diodes, which are designed for RF attenuation and signal muting, not ultra-high speed switching. Searching for "GaAs diodes" just turns up some ampere-rated HV diodes intended for SMPS which are completely unsuited for the task.Or perhaps there is an integrated solution?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 10, 2013, 10:07:04 am
content: Originally GaAs was used for GHz range diodes, as Silicon technology wasn't yet up for the task. These days you absolutely can get Si diodes good enough for GHz switching. Check Avago's HSMS-8xxx range for an example, and they do come better than that if needed.As for genuine GaAs GHz switching diodes, thenthese guyshave you covered. They also manufacturea lot of other microwave goodies.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 10, 2013, 10:07:37 am
content: I have a Racal 9303 RF sampling voltmeter. It uses 5082.2815 diodes in the sampling bridge. The manual describes these as "Hot Carrier", or otherwise Schottky. They appear to be HP (Agilent) part numbers out of a series of 5082.28xx parts. This series also include 1N5711 and 1N5712. What sets the 2815 apart from the rest of the series is that they are a matched set of four with a maximum capacitance of 1pF. The other diodes in the series range up to 2pF.Edit: The Racal is specified upto 2GHz.

author: GK
author_rank: Super Contributor
timestamp: September 10, 2013, 10:08:54 am
content: What kind of diodes are used in switching ring mixers such as these:http://www.minicircuits.com/pdfs/SYM-63LH+.pdf?I guess if I got one of those I could hack the package open and retrieve the diode bridge. And the diodes should be nicely matched too.Anyone ever cut one of these open before?

author: GK
author_rank: Super Contributor
timestamp: September 10, 2013, 10:13:23 am
content: Quote from: ElectroIrradiator on September 10, 2013, 10:07:04 amWow; 0.035pF capacitance at zero bias.

author: GK
author_rank: Super Contributor
timestamp: September 10, 2013, 10:15:35 am
content: Quote from: Andy Watson on September 10, 2013, 10:07:37 amOK, thanks. I'll try to track down a service manual for the Racal and check out the schematics.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 10, 2013, 10:20:51 am
content: Quote from: GK on September 10, 2013, 10:08:54 amMini-Circuit's mixers use chip scale diodes, bonded individually, unknown type. You don't want a ring diode configuration for your sampling head as found in these mixers, but a 'bridge rectifier' with two series connected diode pairs, common anode/cathode for the pairs.

author: poida_pie
author_rank: Regular Contributor
timestamp: September 10, 2013, 11:05:33 am
content: I would tryhttp://www.reprise.com/host/tektronix/reference/transistor.aspwith the Tek part no. from the manual. See what comes up.

author: poida_pie
author_rank: Regular Contributor
timestamp: September 10, 2013, 11:11:15 am
content: oh bugger, that's only for transistors.anyway maybe try the TekScopes group on Yahoo.I found this from a quick search"--- In TekScopes@yahoogroups.com, "benj3867" <benj3867@...> wrote:>> Hi Guys,>> It can't believe it has been a couple of years since I had time to devote tomy beloved old Tek gear. Well, I guess life is sometimes hectic. Anyway, it'sgreat to be able to be working on this stuff again.>> I am trying to restore to life my two 1S1 plugins - both, unfortunately, dead.> Careful testing revealed that both have the triggering Tunnel diode D430 dead(open circuit). One of them also has a dead back diode D432 (again, opencircuit), and is missing the memory gate GaAs diodes D110 and D112 (donated tothe first 1S1 sometime in the past). At minimum, I need one D430 to get one ofthem going again, but while I am at it I would really like to get both going.>> By the way, why did Tek use such a fast 10mA tunnel diode as TD253B for D430?It has a typical 68ps rise time (compared to 350ps for the plain TD253), but itlooks like the triggering pulses do not take advantage of this.>> Another curious thing is the following: While repairing the first unit a fewyears ago I have noticed (and recorded :-) that the IV-curve of one of the D430diodes (tested out of circuit) was wider and less high than it should have been:instead of a peak point current of 10mA plus minus 10% it was only 7mA, and thepeak point voltage was 150mV (instead of the typical 90mV). Is this a commonsign of a tunnel diode that is about to fail?>>> To sum things up, I am looking for the following parts:> D110, D112: 1 matched pair of Tek. made 152-0083-00> Two D430: TD253B tunnel diodes (Tek. # 152-0177-00)> D432: BD4 back diode (Tek. # 152-0070-00)>> If anybody has some or all of these diodes for a modest sum, please email me.I would be delighted to be able to restore these beauties to life.>> Thanks,> Benjamin>No NOS Tek parts, but eBay has a Russian seller or two selling heaps of tunneldiodes. I don't know which is which, but some listings have a spreadsheet. Backdiodes too! (NOT cheap, but how much is a working 1S1 worth?)."

author: GK
author_rank: Super Contributor
timestamp: September 10, 2013, 12:50:28 pm
content: Quote from: ElectroIrradiator on September 10, 2013, 10:20:51 amYes, of course. How small are these chip diodes and how hard would it be to turn a couple around? Alternatively I could gut two mixers and cut their diode rings in half- but then the pairs would not be matched. There are also other mixers in bigger cans that probably have more manageable sized internal diodes.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 10, 2013, 02:55:51 pm
content: Quote from: GK on September 10, 2013, 12:50:28 pmThe diodes I have seen are from the larger mixers in metal cans, like the older SRA-1, they have the circuit laid bare under the can. In these the diodes are about 1x1mm2, or perhaps a bit larger, with one electrode being the chip bonding to the substrate. I wouldn't rule out that you could cut the substrate in one of those mixers, and 'flip' half the diodes, have seen stranger things in my time.You would need areallygood and fine pitched saw, though...Hope you don't mind me asking, but is this a commercial or a hobby project? I have had an idea for a hobby project of making a 'resampling' head for a low BW oscilloscope for repetitive signals. However, my sinister plan was actually to start working at much below the target sampling frequency, to make it easier to work out basic soft- and hardware bugs. Then tackle the GHz technology once the foundation is fundamentally working.At this point in time this is no more than an idea, though.

author: Marco
author_rank: Super Contributor
timestamp: September 10, 2013, 06:11:47 pm
content: Quote from: GK on September 10, 2013, 09:21:38 amI assume you haven't readHubert Houtman's paperabout how to turn an analog scope into a sampling scope? It's pretty vintage, but not as vintage as the Tektronix plugin. The diodes he used are still being produced and cost 15 cents in singles.PS.a stroboscopic converter with a clocked comparatoris an interesting alternative to sampling scopes I think.

author: GK
author_rank: Super Contributor
timestamp: September 11, 2013, 06:02:31 am
content: Quote from: ElectroIrradiator on September 10, 2013, 02:55:51 pmYes, this is a homebrew thing. I intend to initially prototype a functional equivalent to the 1S1 (but using contemporary design techniques and components), refine the design, then add uC-managed digital sampling, memory and waveform storage.That Houtman circuit posted by Marco (thanks!) is a great example of simplicity versus function, but is still rather limited. For example the MAX691 comparator is rather slow and the triggering frequency capability is well under 1GHz. The article mentions that a trigger countdown unit was used for the 100MHz sine wave example.Just to get something initially up and running though, I’ll prototype a circuit based on the Houtman design, with the major difference being the addition of my own timebase/sweep generation and retrace blanking circuitry for exclusive interfacing to any X-Y oscilloscope. The Houtman circuit requires the internal timebase sweep signal of the oscilloscope.That will be a good basis for starting to assess the sampling bridge performance and fidelity. I have a Type 180 trigger countdown unit (https://www.eevblog.com/forum/projects/old-school-tek-porn/msg193842/#msg193842), so that is an easy temporary fix/ workaround for the limited triggering frequency capability for now, without having to stuff around with ECL pre-scaling counters. I’d like to initially assess just how well the basic circuit performs with an input signal approaching 1GHz.After that I can start modifying/upgrading the system with my own ECL/discrete level translator circuitry. Ultimately, I’d like to get the triggering capability to no less than 500 MHz without the need for trigger-input pre-scaling or synchronised oscillator based “count down” methods. There are readily available ECL comparators that should do the trick, and I have some proven non-saturating, discrete level translation circuitry that should adequately boost the ECL voltage levels to those necessary for driving the bridge.My analog computer production line is parked right now, as funds for the next bulk parts/hardware order recover, and I have an audio amplifier to complete and shift out of the way for the rest of the week. My small order of super fast diodes, ECL comparators and RF SMD transistors will take “6-7 working days” to arrive on back order, which works out just fine.I should have a basic prototype up and running in 2 to 3 weeks. Its been quite a while since I’ve played with pS stuff ( “time of flight” laser control the last time). I’m looking forward to getting started.EDIT:I quite like the master and slave track and hold / sampling memory circuit used by Houtman. This is much simpler and a lot more elegant that the complicated 1S1 circuit, that blips the sampling bridge into conduction for an uber brief 340pS at the repetitive sampling rate frequency. In Houtmans track and hold, there is no ridicuously fast sampling pulse: rather the sample is taken when the (rather pedestrian) bridge control transitions from track to hold.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 11, 2013, 08:56:57 am
content: GK, I know this is completely off topic, but you mentioned time of flight laser. Is this possible down to just a few inches? The reason to ask is my nephew is a cabinet maker, and measuring up a kitchen etc is difficult and time consuming on your own. Try holding out a 10' tape measure. So looking at these laser based measures but no real idea how they work, or how accurate, do they measure time of the reflected laser pulse?Thanks

author: KJDS
author_rank: Super Contributor
timestamp: September 11, 2013, 10:08:45 am
content: Macomtech make decent diodes.https://www.macomtech.com/schottky-mixer-and-detector-diodes

author: GK
author_rank: Super Contributor
timestamp: September 11, 2013, 12:01:12 pm
content: Quote from: woodchips on September 11, 2013, 08:56:57 amSorry, I have no idea! This was in a university chem lab. I just designed the hardware for a programmable multichannel laser timing controller - had nothing to do with setting up the experiment(s). IIRC the laser firing repetition rate was in the order of 10Hz and the phase of each channel with respect to any other could be programmed from 0 to 360 degrees in 250pS increments. There were 20 installed channels and the trigger pulses to the lasers were via fiber optics. I was a 3rd year apprentice at the time and some aspects of the design (particularly the fiber optic receivers) are a bit inelegant to my eyes now, but it worked! I'm pretty sure it's still in use too, as not long after I left at least one duplicate "Kleinschmidt" box was built and commissioned to my knowledge. I remember the biggest laser in the lab was rated at 8W, but I can't remember any longer if that one was controlled directly from my unit or not.

author: Marco
author_rank: Super Contributor
timestamp: September 11, 2013, 02:10:49 pm
content: Quote from: GK on September 11, 2013, 06:02:31 amI don't get it ... why would a GHz bandwidth sampling head need 500 MHz triggering capability? You're going to try to look at a 2 ns repetitive signal at 1 GHz bandwidth? (Actually less if you take into account the reset time for the ramp generator and the propagation delay of the comparator.)

author: GK
author_rank: Super Contributor
timestamp: September 11, 2013, 11:57:01 pm
content: Quote from: Marco on September 11, 2013, 02:10:49 pmThe "equivalent" sweep time is not related to the triggering circuit delay or reset time. The Houtman sampler goes down to 1nS per division. Should I wish to view a 500MHz sinewave from my RF signal generator I'd like the display of the trace to be locked, without the requirement for external pre scaling or down counting. An ability to trigger on signals up to 500MHz in a sampling system does not imply a triggering system that resets itself and is ready for the next trigger 2nS later.EDIT; the tunnel diode triggering circuit of the Type 1S1 plug-in was capable of triggering on input signals of "1 GHz or more".

author: GK
author_rank: Super Contributor
timestamp: September 12, 2013, 01:52:50 am
content: Those super low Cj GaAs diodes:http://cp.literature.agilent.com/litweb/pdf/5989-6228EN.pdf.... don't look very suited to a sampling bridge, unfortunately. The "small signal parameters" tabulated on page 3 give an abysmally low value for Rj (junction resistance) when reverse biased.

author: edavid
author_rank: Super Contributor
timestamp: September 12, 2013, 02:33:53 am
content: Quote from: GK on September 12, 2013, 01:52:50 amThat's because they inexplicably measure it close to breakdown (-4V). I wonder what it is at -2V.

author: GK
author_rank: Super Contributor
timestamp: September 12, 2013, 02:49:09 am
content: Well, somewhere between 3000 and 440 ohms. Now why couldn't they just provide a graph? Incomplete datasheets are annoying!

author: edavid
author_rank: Super Contributor
timestamp: September 12, 2013, 03:08:47 am
content: Quote from: GK on September 12, 2013, 02:49:09 amOr is it higher than the zero bias value???Agreed...

author: GK
author_rank: Super Contributor
timestamp: September 12, 2013, 08:44:08 am
content: Quote from: edavid on September 12, 2013, 03:08:47 amThat would make for a rather weird diode characteristic, I would think. I'm almost certain the value would be south of 3000 ohms.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 12, 2013, 09:24:02 am
content: Marco,Thanks for the links, very interesting. I had not seen those before.GK,Depending on whether you intend to publish your work, then I'd certainly be interested in keeping an eye on your progress on this project. At the very least I can offer happy noises of encouragement, though it is quite likely I'd like to build some experiments of my own. My lack of experience with UHF range triggering circuits is what caused me to want to start at 'LF', then work my way up.The Houtman schematic is very interesting as a starting point:*) I suspect the 1M and the 15M resistors of the sampling head ought to be swapped. Would probably also be worthwhile to double check the input+stray capacitances of the opamp configuration used.*) The Enable pin on the MAX961 seems to be abused. In the current configuration the Enable pins are driven far below the negative supply rail, as the inverting outputs go low. Might be worth rethinking that part of the timing circuit, so as not to rely so heavily on those RC time delays.*) The diodes used by Houtman seem very well suited for the job. At 5V reverse bias their leakage current is very low, and the minority carrier lifetime of 10-15pS is hilarious. On the other hand their 'big' parallel capacitance can be worked around, as none of them are connected directly in parallel across the 50 ohm load resistor. I suspect stray capacitances will end up being the limiting factor. The MMDL301T1G, also from ON Semi, is a readily available SOD-323 substitute for the MBD301. Using it would allow the building of a very symmetric layout, with a minimum of stray capacitance.*) A common mode choke/inductor between the differential bridge driver and the 150 ohm bridge load resistors might be worth considering. The choke will help limit the influence of the common mode diode capacitances, as it will be in series with them, and the choke will also help to reduce any 'clock skew' from the differential driver. Making a choke with no nasty series resonances in the UHF range may be tricky, though. A short piece of bare enameled wire twisted pair transmission line might do the trick.

author: Unknown
author_rank: Unknown
timestamp: September 12, 2013, 09:28:04 am
content: Quote from: GK on September 12, 2013, 08:44:08 amSomediodes for microwave applicationshave rather weird diode characteristics. Although granted, this was when forward biased.

author: GK
author_rank: Super Contributor
timestamp: September 13, 2013, 08:51:40 am
content: Quote from: ElectroIrradiator on September 12, 2013, 09:24:02 amWell, OK. I'll post up details of my prototype progress once I have something of substance up and running.1) The 15M and 1M resistors form a frequency compensated divider (in conjunction with the 15pF op-amp input capacitance and the 1pF capacitor) specifically included to reduce the load capacitance to the bridge (by the same ratio as the division). The design would not work as intended if the resistors were swapped.It's important to keep the bridge load capacitance low, as the rise time of the sampler is determined by the time constant formed by the characteristic impedance (50 ohm) series with the bridge series resistance and the total load capacitance.Incidentally, the reason Houtman gets away with such a low frequency op-amp for the sampling amplifier is because the op-amp only needs to be fast enough to slew its output to the charge/potential maintained on its 15pF of input capacitance during the "hold" interval. The "hold" interval is long enough for the output to slew and settle to the correct amplitude, but short enough so that there is negligible discharge of the 15pF capacitance through the parallel 1M resistor - a JFET-input op-amp is obviously mandatory here.Houtmans circuit is nice and simple, but with off-the shelf fixed values and without calibration I doubt that the frequency compensated attenuator would be particularly flat. I have some better ideas for implementing a low input-C sampling amplifier anyway.2) I agree. However this is immaterial at this stage as I won't be using Houtmans circuit in the long term. It needs a lot of work to make it a serious piece of calibrated test equipment. Just for example, a much quicker comparator is required (ECL) to get the triggering frequency capability to an acceptably high level, the fast ramp generator needs to work in discrete, calibrated timebase settings (as the rate of the fast ramp generator is what determines the "equivalent" sweep time) and the triggering comparator lock-out needs to be manually variable/controllable over a large range ganged to the "timebase" rates for the "trigger hold-off" control.3) I've got the MBD301's on order.4) The sampling bridge requires a few other mods to make it really serious - DC offset and charge injection balancing trims, etc.

author: GK
author_rank: Super Contributor
timestamp: September 13, 2013, 12:42:49 pm
content: Quote from: ElectroIrradiator on September 12, 2013, 09:24:02 amJust discovered another serious infelicity. While the MAX961 comparator has a common mode input range that extends to +/-100mV beyond the rails, it has a limited differential input range, which is diode-clamped at +/-2.1V by two internal strings of 3 Si diodes each in anti-parallel. This means that the charge on the fast ramp generator timing capacitor will be screwed up whenever the voltage between the inputs of the sampling gate comparator exceeds +/-2.1V - the timing capacitor will receive charging current through the conducting clamp diodes via the non-inverting input through to the inverting input. I can only assume that this is why Houtman put a 1k resistor in series with the non-inverting comparator input - to limit the unwanted charging current; otherwise putting a high impedance (such as 1k) in series with a input to an ultra high speed comparator is generally a definite no-no. Really, the 82pF timing capacitor needs to be buffered from the comparator input, if the fast ramp generator is expected to operate with adequate linearity. Sometimes designers go overboard with the KISS principle.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 13, 2013, 01:30:45 pm
content: Quote from: GK on September 13, 2013, 08:51:40 amSounds great, looking forward to seeing what you come up with.Quote from: GK on September 13, 2013, 08:51:40 amYes, not my finest hour when I wrote that. Not sure where I kept my head at the time...I will spend the time trying to wrap my head around a few of the concepts involved.

author: edavid
author_rank: Super Contributor
timestamp: September 13, 2013, 05:33:38 pm
content: Quote from: GK on September 12, 2013, 08:44:08 amBased on the IV curve, I was thinking that the low zero bias resistance is mostly due to forward current, in which case a little bit of reverse bias should increase resistance.

author: GK
author_rank: Super Contributor
timestamp: September 19, 2013, 11:06:41 am
content: I'm considering ordering one of these for my trigger circuit take-off:http://www.minicircuits.com/pdfs/ZFRSC-42+.pdfMini Circuits also do SMD, PCB-mount splitters/power dividers, but unfortunately not of the "resistive" variety which have the desired response down to DC. I'd prefer a PCB-mount splitter. Does anyone know of any alternatives?Also I have tentatively settled of SMA connectors and RG58 coax. The smaller diameter, more flexible coax cables would be more convenient, construction wise, but they seem to much too lossy at 1Ghz and beyond. I will need a few meters of the stuff to act as a delay line for the vertical input and I of course would prefer a flat as possible/practical frequency response.What is a good, trusted brand/specification of RG58, that isn't too cheaply made and excessively lossy at HF? There is an awful lot of rubbish out there.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 19, 2013, 11:19:26 am
content: Quote from: GK on September 19, 2013, 11:06:41 amBelden RG-142.(Note: 0.70 velocity factor, compared to 0.66 for most RG-58 stuff).What does the divider have to do? They are easy enough to calculate, if you are happy with the performance of purely resistive dividers.

author: KJDS
author_rank: Super Contributor
timestamp: September 19, 2013, 11:25:00 am
content: Just lay out three 51R resistors in a triangle for your splitter. If you use 0603 resistors and thin the 50R lines down to about 10 thou for 40 thou before each junction then you'll match out most of the junction capacitance and it should be good to a few GHz.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 19, 2013, 11:35:24 am
content: Quote from: KJDS on September 19, 2013, 11:25:00 amYou can also use three 16.7 ohm resistors for a three way star splitter, one resistor in series with each input port. This may give less of an impedance bump due to the smaller geometry. You will have to live with a more uneven power distribution between the resistors, though.

author: GK
author_rank: Super Contributor
timestamp: September 19, 2013, 11:43:02 am
content: The input signal needs to be fed to both the trigger circuit and the sampling bridge, hence then need for a splitter. I know splitters are simple but a commercial solution requires zero R&D effort for instant results.

author: krenzo
author_rank: Regular Contributor
timestamp: September 19, 2013, 03:07:44 pm
content: No R&D needed:http://www.microwaves101.com/encyclopedia/Resistive_splitter2.cfmMinicircuits sells the same circuit as model ZFRSC-123-S+. I've used the same layout myself with good results.

author: GK
author_rank: Super Contributor
timestamp: September 20, 2013, 05:36:53 am
content: Quote from: ElectroIrradiator on September 19, 2013, 11:19:26 amAttenuation wise, Belden RG142 specs only marginally better than Belden RG58. RG142 also seems stupid expensive. E14 sells "Nexans" brand RG142 by the meter, for over $44 per meter.What's the real benefit of RG142 over RG58? Electrical or mechanical? More flexible, robust? The armed forces must be paying that premium for a reason.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 20, 2013, 06:30:26 am
content: Not sure if we are looking at the same data, but for Belden RG-142B/U ('83242 Coax'), versus Belden RG-58-A/U ('8259 Coax'), I have attenuation at 1 GHz for 100 ft / 30m as:RG-142B/U : 13.1 dBRG-58A/U : 22.8 dB.The RG-142 advantages are:*) High temperature operation, 200oC, due to Teflon+FEP construction.*) Much lower losses at GHz frequencies, due to stricter mechanical tolerances, tight double screens, Teflon insulation and silver plating of all metal surfaces. Belden doesn't even bother specifying the losses of RG-58A/U above 1 GHz.*) Mechanically rugged, though admittedly also a bit stiffer to handle. Minimum bending radius is the same for both cable types though, it just takes slightly more force to bend RG-142.*) Unlike on RG-58 the double screens actually provides good shielding at GHz frequencies. IE. much, much lower problems with RF crosstalk if you bundle cables, or send a signal through a delay line on a spool.$44, whether AUD or USD, is a ridiculous price for 1m of RG-142. By comparison Mouser wants $24 /m (Australian) if you buy 30m / 100 ft, and I would consider that to be expensive. The 'real' price is half this again, or even less, depending on the source.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 20, 2013, 06:40:02 am
content: Come to think of it: Depending on how long you want the delay line, then it might be an advantage to use thicker - yet less expensive - coax, like Belden H1000. This is used extensively by radio amateurs, so should be readily available, is a lot less expensive compared to RG-142, and even has slightly lower losses. The downside is that this is *quite* stiff and a lot thicker/heavier as well, of course. You'd also need to use N connectors for GHz frequencies.

author: GK
author_rank: Super Contributor
timestamp: September 20, 2013, 06:52:00 am
content: I was considering a larger cable, but I will need to coil a few meters of it inside the case of my sampler for the signal delay line. An aim in the design of the triggering circuit will be to get the triggering delay down as low as possible, to minimise the length of cable required.Belden attenuation specs for RG58/U are 47.6dB per 100m @ 1GHz, which works out to 14.5dB per 100ft - so only fractionally worse than RG142 @ 1GHz.Datasheet here:http://www.belden.com/techdatas/metric/9201.pdf

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 20, 2013, 07:33:51 am
content: That figure is almost certainly a conversion error, done by whomever converted the original US imperial units to SI. The six significant digits is a clue to this.Worse still, the cable on that datasheet is the RG-58/U type, without the 'A' (for 'A'rmy), which is a low quality version of the RG-58A/U. The screen coverage on the cable in your link is only 80%, while it is 95% for RG-58A/U. RG-142B/U has screen coverage of two times 95%.I'm fairly confident the 13 vs. 23 dB figures are correct, as I have had those numbers rattling around inside my head for years as part of my 'rules of thump' collection.I just looked up the exact numbers to make certain. Few people would use RG-58 (any type) above 500MHz, if even that, for any remotely critical application. It really is low cost consumer carp.By comparison, then H1000 has an attenuation of 14 dB/100m at 1 GHz (I was thinking per 30m/100ft in my previous post). I'd still consider using that, even though I had to mount it outside the main chassis.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: September 20, 2013, 07:56:50 am
content: Using H1000 - while cumbersome - has another advantage: If you need a similar delay as that used by Mr. Houtman, then 10m of H1000 does the job, considering the 0.83 vs. 0.66 velocity factors. At 1 GHz 10m of H1000 has a loss of only 1.4dB, which is almost tolerable. Meaning that at first blush you won't have to bother with frequency compensating the delay line cable losses. A simple resistive divider at the scope input should serve well for an experiment, and 1.4dB @ 1GHz is a lot easier to compensate, should you eventually decide to do so.If you can craft a better triggering circuit, making a shorter delay acceptable, then this advantage gets even better, of course.

author: GK
author_rank: Super Contributor
timestamp: September 20, 2013, 09:55:21 am
content: H1000? Pfft. My first job out of school was as a production assembler, assembling and wiring RF gear including antenna switching frames. A pic of one of my once completed jobs is attached - there is some decent diameter coax for you and some nice power dividers too. We called it (the "coax") "rigid line". Copper tube inner and outers, air dielectric with the inner tubes supported by teflon spacers and joined at the bends with silver plated aluminum spigots. I put a lot of effort into getting all those hose clamps nicely and neatly aligned and pointing in the same directions.

author: GK
author_rank: Super Contributor
timestamp: September 20, 2013, 10:28:12 am
content: The inconsistency between in the Belden spec sheets is a little disappointing. I did think the number of decimal places in the attenuation charts was a bit stupid.However it's not necessary in the early stages of development to actually implement the delay line. I'll just wait and see how good/quickly I can make the triggering of the fast ramp (equivalent sweep time) generator before making a final decision on the type of coax to use. I probably won't be going for the rigid line though

author: GK
author_rank: Super Contributor
timestamp: September 20, 2013, 12:01:53 pm
content: Quote from: krenzo on September 19, 2013, 03:07:44 pmThat's a nice link; thanks.

author: mathias
author_rank: Regular Contributor
timestamp: April 19, 2014, 06:39:58 am
content: Hi,I just want to see if anyone has still been working on this idea. I'am also building the Houtman sampler, as is.Greetings!

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 12:49:25 am
content: Quote from: ElectroIrradiator on September 10, 2013, 10:07:04 amThe Avago RF schottky diodes will almost certainly work and are readily available although you may have to put two in series for each of the original GaAs diodes. One individual on the Yahoo Tektronix list rebuilt an S-2 sampling head using them and improved the performance considerably. Some of them have higher breakdown ratings than the original Tektronix diodes which will make the sampler more rugged.I have not tried this but the base-emitter junction of a bipolar transistor makes an excellent high speed low leakage low capacitance diode and might also be suitable. A 2N3904 can have a reverse recovery time in the 100 picosecond range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2014, 01:03:58 am
content: Quote from: David Hess on April 20, 2014, 12:49:25 amReally? I'd be surprised if it were much better than regular old storage time.The capacitance certainly won't be low. I think 2N3904 is around... 20pF or so? RF types will be better, but still disproportionate compared to Ccb, and the reverse voltage is even worse.I know the reverse (DC) characteristics suck, but I haven't measured dynamic. I'll have to give that a look (unfortunately, I'm not at the bench right now!).Unless you meant B-C, which should have a higher voltage drop but low capacitance (especially in RF types), and a useful reverse voltage (Vcbo or so).Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 01:31:49 am
content: Quote from: T3sl4co1l on April 20, 2014, 01:03:58 amIt is the base-collector junction which is slow while the base-emitter junction is fast which surprised me as well. In both cases, the reverse leakage current can be very low, in the picoamp range or lower, unless gold doping is used but you have to qualify the transistors yourself by type, manufacturer, and process since this characteristic is not tested below 50 nanoamps typically. As you point out, the base-emitter breakdown voltage is low which is no different than existing super fast and/or low leakage diodes.The capacitance is not nearly that bad but I agree, for high performance you would want a small junction RF transistor.Bob Pease mentioned the above in his book and I have made great use of it since.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2014, 08:46:08 am
content: Now I know you're pulling my leg: I recently tried using a 2N3904 emitter follower as a signal clamp, so that normally the B-E is reverse biased, until it forward-biases B-E and the transistor grabs it. Source impedance circa 10kohm. I only got about 3V linear range before it started compressing; the last 2V (Vclamp at 5V) leaked excessively. Wait, that doesn't make sense. I must be remembering the 5V zener diode. Why can't I remember the transistor's curve? Aggh, I'll figure it out when I get back...You win this time, Inspector Gadget!....Tim

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2014, 02:22:35 pm
content: It may prove useful (or at least interesting) to look at the circuits and components used in the old HP8405A VVM. This old voltmeter measures gain/phase up to 1GHz and the 1GHz probes use a set of 4 (schottky?) diodes for the sampling gate. Each of these diodes is in a conventional package which is a bit of a surprise. I guess they match them before fitting them.The service manual gives a good description of how the sampling operates and also some background into the choice of good/bad components to use in a sampler and associated pulse generator like this.I bought one of these voltmeters a while back to allow me to measure negative impedances at HF/VHF and I had to spend a fair bit of time repairing it so I have a reasonable appreciation about how it operates. It is very similar to an old school sampling scope.There's some useful third party info about the probes and the diodes herehttp://www.davmar.org/pdf/HP8405A-probe-repair.pdfSome background from when it was first introduced in 1966http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-05.pdf

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 03:07:59 pm
content: Quote from: T3sl4co1l on April 20, 2014, 08:46:08 amThis is operating as a diode and not a transistor. The emitter or collector has to be left disconnected depending on which junction you are using. The base-collector junction is slow but of course supports the Vcbo which is high.I tried testing this using my fast sampling oscilloscope but none of my fast pulse generators are up to it yet. I need to build something new.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 03:57:11 pm
content: Quote from: G0HZU on April 20, 2014, 02:22:35 pmLike I said, the Avago schottky diodes will certainly work in place of the original GaAs diodes but they may need to be used in series pairs for good blowby performance and it is not entirely clear why. This may be leakage related or have something to do with the strobe pulse. Avago Technologies *is* HP's old semiconductor division so it is not surprising that they make a suitable diode. Even better, their schottky diodes are readily available as matched networks in a good and easy to use form factor.Thanks for linking the HP sampling voltmeter documentation. It is a fascinating subject. There was a good discussion on the Yahoo Tektronix list a couple of years ago about similar repairs on Tektronix S-2 and S-3 sampling heads where GaAs diodes were originally used. The S-3 uses a sampling probe and has a lot of similarities to the HP8405A.The Racal-Dana 9301A True RMS Millivoltmeter also uses a probe with a sampling input and may be worth investigating as well.

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2014, 05:00:29 pm
content: If the HP8405A circuits do look useful then it's definitely worth downloading and reading the service manual because it describes how to get the best efficiency from the sampler and how to optimise the reverse bias on the 4 diodes used in the sampler. The biasing is critical for correct operation and the manual covers all this.I had to go through all of the alignment procedure for my meter to get the sampling probes to work correctly with the required efficiency and flatness up to 1GHz.QuoteYes the old 8405A is a very versatile instrument. It also has rear panel IF output ports A and B to allow the 20kHz IF signal to be viewed on a conventional scope. I also feed these IF ports into a soundcard so I can look at relative phase, amplitude and noise on a PC.I think the 8405A meters also get used (even today) to verify the relative accuracy/tracking of frequency standards by measuring the relative phase drift over time. It can measure to <0.1degree phase error and so can prove the frequency tracking down to a few parts in 10e-13 if you test over several minutes.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 05:38:05 pm
content: Quote from: G0HZU on April 20, 2014, 05:00:29 pmOne of the more interesting uses for sampling that I have run across is high frequency RMS measurements like the mentioned Racal-Dana voltmeter makes. It almost seems like cheating to run the low frequency output of a sampler into a low frequency RMS converter. Good oscilloscopes including fast sampling oscilloscopes can be used to make the same measurement visually. Crest factor of course is a problem at high input levels.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 21, 2014, 03:25:36 am
content: Alright, back at the lab.Setup:50 ohm func gen, square wave, 2.8Vpk (open circuit voltage), 2N3904 B-C junction (E open), 51 ohms to ground (all in series). Resistor shows 1V ~= 20mA forward current, and the same for reverse recovery current. The reverse recovery transient is clearly visible. From the point when current crosses zero during the recovery spike, until it returns to within about 10% of zero, is the recovery time.B-C: 87nsB-E: 34nsHowever, B-E junction with C shorted to B is difficult to measure (I need a faster pulse source), and B-C with B-E shorted is already half (52ns).Perhaps it was a diode-strapped transistor, not the junction alone? That would make sense, as the transistor Baker-clamps itself. A low capacitance RF transistor would do very well in that mode, even for high frequencies. Still probably not schottky level, but should give a good run for the money, so to speak.Capacitance I can't get a good read on, not with the setup I just tried anyway. Cbe seems to measure less than Ccb, which seems inconsistent with the junction (heavier doping than B-C) and relative areas (B-E smaller, but not by a large factor).Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2014, 06:21:24 pm
content: Those results do not look reasonable to me but my best and almost suitable pulse generator is in pieces at the moment. Here is the paragraph Bob Pease wrote discussing this:One “diode” that does turn ON and OFF quickly is a diode-connected transistor. A typical 2N3904 emitter diode can turn ON or OFF in 0.1 nsec with negligible overshoot and less than 1 pA of leakage at 1 V, or less than 10 pA at 4 V. (This diode does, of course, have the base tied to the collector.) However, this diode can only withstand 5 or 6 V of reverse voltage, and most emitter-base junctions start to break down at 6 or 8 V. Still, if you can arrange your circuits for just a few volts, these diode-connected transistors make nice, fast, low-leakage diodes. Their capacitance is somewhat more than the 1N914’s 1pF.

author: Fank1
author_rank: Regular Contributor
timestamp: April 21, 2014, 11:35:48 pm
content: I didn't look close but real TEK parts here:http://www.sphere.bc.ca/test/tekparts.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 22, 2014, 06:02:47 am
content: See, base tied collector. I thought you said junction alone, and was like..

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2014, 04:57:24 pm
content: Ya, I remembered wrong but that is why I provided the exact text from my source.When I originally tested this, I found that my fastest high output pulse generator with a transition time faster than 3.5 nanoseconds was not fast enough. I planned on revisiting it after acquiring or building a more suitable pulse generator or test circuit. I already have a suitable oscilloscope. Since that time, I have occasionally used them to replace fast diodes, trr<500 picoseconds, that were suspect in high performance circuits with excellent results.