Need help designing the input stage for a frequency counter - Page 1
Forum section: electronics / projects

author: void_error
author_rank: Frequent Contributor
timestamp: February 01, 2017, 12:37:25 pm
content: You read the title right, I'm pretty much clueless.I'd like to measure frequencies up to 100MHz and have the input both AC or DC coupled.Input level should be 100mV or above and there should be some kind of overload protection on the input. It would be nice to have a schmitt trigger with adjustable thresholds too.In other words I have no idea what to do between the BNC input connector and the frequency divider input.Suggestions will be greatly appreciated.

author: danadak
author_rank: Super Contributor
timestamp: February 01, 2017, 01:00:44 pm
content: Use a current mode (norton) OpAmp or one of the new process widebandV mode OpAmps. You will need to pay attention to duty cycle distortion,slew rate distortion effect on duty cycle. To preserve pulse fidelity in caseyou want to measure duty cycle on pulses.Go to TI website -http://www.ti.com/lsds/ti/amplifiers/amplifiers-overview.pageAnd look at selector tool. They also have webench so you can sim response. Not'sure their models will cover all the above effects however.Regards, Dana.

author: void_error
author_rank: Frequent Contributor
timestamp: February 01, 2017, 01:28:04 pm
content: Quote from: danadak on February 01, 2017, 01:00:44 pmYou mean look at whether the op amp slews symmetrically both ways?Wouldn't it be easier just to feed the signal into a comparator?

author: voltsandjolts
author_rank: Unknown
timestamp: February 01, 2017, 02:40:08 pm
content: http://andybrown.me.uk/2016/02/21/nanocounter/

author: sahko123
author_rank: Frequent Contributor
timestamp: February 01, 2017, 03:15:53 pm
content: You could use an amp going into a Schmitt inverter. and this will add the Schmitt capability. This could go between the coupling and the input

author: void_error
author_rank: Frequent Contributor
timestamp: February 01, 2017, 05:02:32 pm
content: Quote from: voltsandjolts on February 01, 2017, 02:40:08 pmUnfortunately that won't work for very low frequencies since it uses a transformer.

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2017, 05:02:58 pm
content: Study how high input impedance oscilloscope and frequency counter inputs are designed. They are essentially immune to overload damage yet allow high sensitivity at high bandwidths. Another advantage of this type of design is that it can use standard x10 oscilloscope probes. They use a JFET or MOSFET amplifier, usually a gain of 1 source follower, to convert the high input impedance to a low impedance which then drives the comparator.A fast JFET or CMOS input comparator might work for a very simple design down to 100mV and below but offhand I do not know of any and I suspect the reason they do not exist is that a high input impedance is incompatible with a fast comparator do to oscillation. The same would go for a CMOS differential line receiver.

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2017, 06:06:32 pm
content: void_error, here's a link for you to a very old magazine:http://www.e-pages.dk/audiomedia/557054423/Go to page 32. There you'll find the input amp for a frequency counter.So why rely on an old DIY article? Well, there's a story behind that. The author is an old friend of mine and couldn't get his own input amp to work properly before deadline.So he copied the input amp from a Hewlett-Packard frequency counter 1:1 instead. (he confessed this later)This is what you see in the article, and it works beautifully (as you'd expect from HP).Replacing the active parts with higher frequency-capable devices (eg, MC10H115 instead of MC10115) should bring operating frequency up to where you want to be. IIRC, the original amp was good up to around 50 MHz.

author: eeFearless
author_rank: Contributor
timestamp: February 01, 2017, 06:36:25 pm
content: Here's a design from the ARRL archives:http://www.westmountainradio.com/pdf/Article_Fernandes.pdf

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2017, 06:47:49 pm
content: QuoteInteresting how the HP and ARRL designs both use the JFET/BJT combo for the input stage.

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2017, 08:26:41 pm
content: Quote from: Benta on February 01, 2017, 06:47:49 pmThey are very similar to oscilloscope designs except AC only operation allows for some simplifications. void_error mentions DC coupling as a requirement so attention would need to be paid to DC errors.Probably the simplest way to do it today with modern parts is to use a fast JFET or CMOS operational amplifier as a buffer to drive a fast comparator with the level either applied to the comparator or used to add offset at the input or output of the amplifier; I have seen all three configurations used. A buffer with feedback like this would be unacceptable for an oscilloscope because of poor transient response but a frequency counter is less demanding in this respect.The above design would work well with digital signals but not with analog ones. Slowly varying analog signals require voltage gain before the comparator for good performance.

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2017, 08:48:02 pm
content: David, I agree absolutely with your comments.But what fascinates me is how good the "old" HP designers were at thinking outside the box and producing brilliant designs.Using ECL line receivers for an input amp isn't obvious, but two stages each provide 6...7x gain, which is spot on. And stability or compensation is a non-issue.Using the HP counter was really just: connect the probe(s) and you have a result. Great stuff.I'm sure there are other solutions today, but I'm a fan of ECL :-)Cheers.

author: KhronX
author_rank: Frequent Contributor
timestamp: February 01, 2017, 11:55:58 pm
content: For what it's worth:Disregard the parts that deal with Nixie tubes though

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2017, 01:09:03 am
content: Quote from: Benta on February 01, 2017, 08:48:02 pmECL is cool.Tektronix started using ECL line receivers for the same thing up to 125 MHz, they liked the MC10216, in every design after 1978 or 1979. Before that they used differential bipolar amplifiers. Later their designs at 135 MHz used discrete bipolar cascode differential amplifiers and at 350 MHz used a 1.05 GHz bandwidth oscilloscope vertical amplifier ASIC and a 1.25 GHz differential schmitt trigger ASIC.I am a little dubious about shunt feedback with an ECL differential line receiver used as an amplifier but 10k series ECL parts are not *that* fast so I assume they found them to be stable in their linear range. That is something to watch out for in a modern design trying to do the same thing; at some point, a faster differential line receiver *will* oscillate when abused this way.I think the simplest useful design today is a fast JFET or CMOS operational amplifier used as a buffer followed by a fast comparator. Clamp the input of the buffer with fast low leakage diodes in the form of transistor base-emitter junctions to keep the buffer out of saturation; this is why input gain needs to be limited with an operational amplifier. It will achieve better than 100mV sensitivity limited by the comparator which was typical of discrete designs before cascaded differential line receivers were used.Update: Removed clever video link because the FUCKING FORUM SOFTWARE SCREWS IT UP.

author: void_error
author_rank: Frequent Contributor
timestamp: February 02, 2017, 05:41:09 am
content: Thanks for the replies everyone.I'll probably go with whatDavid Hesssuggested and use a jfet input high speed op amp, cascade them in case the GBW is not enough. Currently looking for oscilloscope input circuits for input protection.

author: awallin
author_rank: Frequent Contributor
timestamp: February 02, 2017, 06:50:58 am
content: keygilent 53123A, page 18 has the inputhttp://literature.cdn.keysight.com/litweb/pdf/5989-6308EN.pdf

author: void_error
author_rank: Frequent Contributor
timestamp: February 02, 2017, 07:22:36 am
content: That looks quite complicated but it'll give me a few ideas.I'm thinking about using 2 or 3 op amps for a total gain of 10 powered from +/-5V supplies, with the last amplification stage adding DC offset and feed into a single supply comparator input. If those op amps turn out to be too expensive I'll lower the maximum input frequency to 50MHz.Op amp slew rate will matter more for the last amplification stages.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 02, 2017, 07:35:57 am
content: I've just done this. I don't have the schematic at hand but I used a MOSFET follower (RCA 40673) and a discrete diff amp (2x 2n3866) with a TTL switch on the output (2n2219a). Works well up to about 120MHz probably limited by the 2219. 1Mohm input impedance and about 30pF of capacitance at the input. AC coupled so only good down to about 10KHz but it's for RF. I will add a separate front end for DC coupled. Sensitivity around 40mV (can be improved).The output is integrated and fed back to the second gate on the MOSFET - this sorts out automatic threshold/biasing. The diff amp has a tiny bit of positive feedback which in most relatively sine-like waves gives it some hysteresis.Will detail it when I'm 100% happy with it. The feedback loop needs adjusting slightly and it testing with a selection of transistors, not just one.Transistors are driven with relatively high collector currents to peak the transition frequency. Whole lot takes about 80mA so don't expect efficiency. I like the smell of warm transistors thoughYou could use a nice modern op amp here or some modern SMD dual gate MOSFETs but I used what I had on hand which is ancient and obsolete, but the idea works quite well.

author: void_error
author_rank: Frequent Contributor
timestamp: February 02, 2017, 08:02:48 am
content: Quote from: SingedFingers on February 02, 2017, 07:35:57 amNice!I've looked at some discrete input stages and they're perfectly fine for AC coupling but I want DC coupling too so I think the easiest way is to go with op amps. Yet again the AD8039 pops up as a candidate which is good since I'm using it in another project too, however, it seems to be lacking in slew rate but then again I haven't done any calculations either. There's going to be a DAC for the last amplification stage, the one which adds the required DC offset to bring the voltage to positive values for the comparator, and it will be used to trim out any unwanted offset added by the previous stages.I don't mind high current consumption as long as nothing gets too hot. I assume a lot of the high speed op amps will run quite warm. Never used dual gate MOSFETs and I know absolutely nothing about them.Just curios, is there any chance I can get my input stage right without experimenting? I don't have the right tools for the job so I'll be relying mostly on simulations.

author: Gyro
author_rank: Super Contributor
timestamp: February 02, 2017, 10:12:59 am
content: Quote from: void_error on February 02, 2017, 05:41:09 amI used inverse parallel HP 5082-2835 diodes for input protection on my prescaler, lower capacitance (1pF), lower Vf and better current capability than most other signal schottkys.https://www.eevblog.com/forum/projects/my-600mhz-prescaler-(from-the-scrap-box)/

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2017, 11:49:35 am
content: Quote from: void_error on February 02, 2017, 07:22:36 amYou have to be very careful using operational amplifiers for this except on the input stage and even there you have to be careful. You have to avoid driving the operational amplifiers into saturation because unlike comparators, they take a relatively long time to recover which is fatal to a frequency counter. That is why discrete differential amplifiers or ECL line receivers, basically the same thing in integrated form, are commonly used.That is why I mentioned clamping the input and using an operational amplifier configured as a gain of 1 buffer. But also note that oscilloscope input clamp circuits use low leakage but *not* fast diodes and this is also a problem. You cannot use schottky diodes because of their high reverse leakage but low leakage diodes are usually slow which does not matter too much on an oscilloscope. The solution is to use base-emitter bipolar transistor junctions as diodes as they are both fast and low leakage albeit with a low reverse breakdown voltage which is not a problem here.If you use multiple operational amplifier gain stages, then clamps will be needed there also to prevent saturation of each stage.If you have gain before the comparator, then the offset will need to be added before that to make a level control or the level range will be tiny.Quote from: Gyro on February 02, 2017, 10:12:59 amSchottky diodes have too much leakage for a high impedance clamp. They work fine of course for stages after the high impedance buffer.Your circuit has a low impedance input so this is not a problem.

author: Gyro
author_rank: Super Contributor
timestamp: February 02, 2017, 12:26:17 pm
content: True enough. I was forgetting that the OP had a low max input frequence requirement.

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2017, 12:37:05 pm
content: Quote from: void_error on February 02, 2017, 08:02:48 amThe AD8039 will not work as an input amplifier because of its high bias current and you really do not need any additional amplification to get to 100mV sensitivity. The fast comparator handles that internally.Using an operational amplifier for the high input impedance buffer has the advantage of allowing DC operation without a separate way to control DC offset and drift.QuoteI think you can. The signal input is shunted to ground with a 1M resistor. Then a 1M resistor with low value bypass capacitor, 0.001uF or smaller, goes to the buffer input which is clamped with a pair of fast low leakage diodes to prevent both saturation and overload. I would probably include atiny 28 volt grain of wheat incandescent lampas a fuse just before the diode clamps. The lamp might need to be bypassed with a very small capacitor. Then include a low value resistor in series and close to the non-inverting input for dampening to keep the fast operational amplifier from oscillating; they do not always behave when driven with a low AC impedance.The buffer drives the comparator and the level adjustment also goes into the comparator. If you want to trim the hysteresis, then that can be done also at the comparator.Let's see ... you need a low input current buffer or operational amplifier with a stable gain of 1 to use as a buffer and you like Analog Devices ... the ADA4817-1 or AD8065 are suitable. A discrete buffer using an RF JFET or MOSFET would be higher performance and cheaper but more complicated.The input range is limited by the diode clamps which keep the buffer out of saturation. There should be enough sensitivity so a compensated input attenuator in place of the 1M input shunt would increase the input and trigger level range considerably.

author: void_error
author_rank: Frequent Contributor
timestamp: February 02, 2017, 03:04:18 pm
content: Quote from: David Hess on February 02, 2017, 12:37:05 pmDid I get this right? Op amp decoupling caps not shown. I assume R3 should be low enough so there isn't any attenuation due to the input capacitance.What would be a good choice for the diodes? Fast response / low leakage / low capacitance I assume. Replace them with transistor B-E junctions?For the comparator I'm thinking about the ADCMP606. No idea if it's a good choice. Run it off +/-2.5V and find a way to shift the output to 0-5V or run it off +5V and do the level shifting at the input?

author: David Hess
author_rank: Super Contributor
timestamp: February 02, 2017, 03:38:02 pm
content: That is about right.You will probably want a wider input range than two back to back diodes to ground will allow so instead of connecting them to ground, connect them to low impedance potentials above and below ground like +/- 1.0 volts since the input common mode range of the AD8065 only extends to +1.7 volts on a +5 volt Vcc. Is roughly +/- 1.5 volts of input voltage range enough? My DC503 frequency counter has exactly that much trigger level range. With a x10 oscilloscope probe attached, this becomes +/- 15 volts.2N3904 base-emitter junctions make good diodes for this. 2N4401 base-emitter junctions would be a little tougher. There is also some advantage to using two diodes in series for each shunt; this will reduce the capacitance to 1/2 that of a single diode. Or use 2 or 3 diodes in series for each shunt directly to ground to get roughly +/- 1.2 or +/- 1.8 volts.

author: void_error
author_rank: Frequent Contributor
timestamp: February 02, 2017, 04:22:01 pm
content: Two diodes in series sounds good, diodes are cheap anyway. I'll play in LTspice a bit to see if 1N4148s are any good. I assume a low value resistor in series with C1 will help limit the current through the diodes. B-E junctions have higher capacitance, around 30pF. I can live with 2VP-Pinput range if I add an input attenuator. x10 scope probe it is then.How about level shifting for the comparator powered by a single 5V supply? Having a resistive divider tied to +5V means that the AD8065 has to sink current all the time. I could balance that out by adding the same amount of resistance to -5V from the op amp's output...? It would be nice to have the ability to adjust the positive and negative going thresholds independently and I'm thinking this way: replace LM393 with something suitable and find NAND gates with low propagation delays, replace pots with DAC outputs:

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 02, 2017, 04:29:40 pm
content: Quote from: David Hess on February 02, 2017, 03:38:02 pmInteresting. Thanks for the tip. Will add that to my counter design

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2017, 12:07:25 am
content: Quote from: void_error on February 02, 2017, 04:22:01 pmThe datasheets I checked say 8pF maximum for the 2N3904 base-emitter junction so 2 in series get down to a reasonable 4pF. Some RF transistors are suitable and have even lower capacitance but offhand I do not know of any specific part numbers. Transistors are not specified or tested for low leakage and gold doped transistors are no better than gold doped diodes like the 1N4148.Advantages of a discrete JFET input stage include wide input voltage range, fast recovery from overload, higher frequency operation, and simplified input protection because the JFET gate diode protects it in one direction so only one external diode is needed. Unfortunately a dual JFET or matched JFETs are needed for fast overload recovery in a DC balanced design; even buying a bunch of JFETs and selecting two which match would be cheaper than a fast JFET operational amplifier though.QuoteSome of the Tektronix designs do this by adding bypassed zener diodes or diode forward voltage drops in either the form of diodes or bipolar emitter followers.QuoteThat seems excessive to me and the LM393 is way too slow for a 50 or 100 MHz counter. You need more like a 10ns comparator.Check out the HP 5315A/B, one of my favorite fully documented "cheap" counters, for an interesting take on the controls; it operates with either level or sensitivity. It uses a simple JFET buffer followed by an AMD AM687 ECL comparator but abuses one of the latch enable pins to finely control sensitivity with the level fixed at 0 volts; I doubt AMD had that in mind.If you want to try something new, look at the adaptive trigger Jim Williams shows in figure 97 on page 44 ofLinear Technology application note 47. This duplicates the function of "automatic peak-to-peak" trigger mode in Tektronix oscilloscopes. Buffer both of the signals from the peak detector and use a potentiometer to set a level which is a percentage of the peak-to-peak values of the waveform. My Racal-Dana 1992 has a mode which sort of does this but digitally and slowly.

author: void_error
author_rank: Frequent Contributor
timestamp: February 03, 2017, 05:15:46 am
content: Quote from: David Hess on February 03, 2017, 12:07:25 am30pF was for the 2N4401.Quote from: David Hess on February 03, 2017, 12:07:25 amI did run intothis... any good?Quote from: David Hess on February 03, 2017, 12:07:25 amWasn't actually going to use a LM393, it's way too slow. I've been looking for a <5ns comparator. Add a fixed amount of hysteresis and have something to adjust the trigger level with?

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2017, 07:20:22 am
content: Quote from: David Hess on February 03, 2017, 12:07:25 amWasn't actually going to use a LM393, it's way too slow. I've been looking for a <5ns comparator. Add a fixed amount of hysteresis and have something to adjust the trigger level with?That is how it is usually done and it works well enough. I mentioned the HP 5315A/B because it is an interesting exception.QuoteNo, that is another feedback amplifier and duplicates what is inside of a JFET operational amplifier. Some high performance frequency counters use differential pairs like this without feedback for their input circuit as shown in the third example below.Below are some examples.The first example shows the single diode needed to protect a JFET buffer.The second example shows a JFET buffer and how to divide the input *and* add an offset to the input which could be the trigger level control with the output driving a comparator making a comparison with ground or some other fixed reference. The division and adding of the offset would also work well with a buffer made from an operational amplifier and is probably what I would do for the simplest possible "good" implementation. There is no input protection other than the attenuator on this one but it could be added with a single diode.The third example shows the differential input design driving a chain of differential line receivers. In a simplified lower performance design, the entire chain could be replaced with a single comparator driven differentially since the trigger level is added before that point.

author: void_error
author_rank: Frequent Contributor
timestamp: February 03, 2017, 02:44:56 pm
content: I like the simple approach and I'll go with the AD8065, level shifter and comparator with fixed hysteresis. I'll have to figure out how to do the trigger level adjustment. The comparator could be a LMV7219. B-E junctions used to limit the input voltage before the op amp will be 4 BCW60s.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2017, 05:28:25 pm
content: If a comparator is used, then the trigger level can be applied to the other comparator input.If a logic gate like a schmitt input gate is used (1), then the level can be added/subtracted to/from either the input or output of the buffer. This works with a comparator as well and might be desirable in some cases.The dual stacked JFET can be simplified by replacing the lower JFET with a resistor but this causes the operating point to change with input voltage. This works well if the negative bias supply had a greater negative potential so for a given current, the resistor is a larger value.The example below shows this with a single JFET but notice the higher supply voltages. The operational amplifier acts as a DC correction loop but is not strictly needed; in this case it also controls the trigger level by adjusting the input offset because the comparator is just a single ECL gate. Unusually there is no gain but this is a low performance design; replace the bipolar stages and ECL gate with a comparator for better sensitivity.(1) After amplification. A comparator essentially does all of this in one package.

author: jimgiordano
author_rank: Newbie
timestamp: January 03, 2020, 02:13:43 am
content: void_error--What did you final solution look like? I'm also making a frequency counter and it looks like you were on to something usable

author: void_error
author_rank: Frequent Contributor
timestamp: January 04, 2020, 06:46:02 am
content: Ended up withthis, see page 2. With those values it can do about 50MHz at -3dB according to LTspice.

author: jimgiordano
author_rank: Newbie
timestamp: January 05, 2020, 01:55:07 am
content: Wow. Thanks. It'll take a bit to digest this. I appreciate it.

author: adinsen
author_rank: Regular Contributor
timestamp: December 26, 2022, 08:59:15 am
content: I picked up this thread when I researched adding a PIC-based frequency counter to a simple XR2206 based function generator I made many years ago. Somehow I got the idea that I should add an external input to the box tooI quickly realized that was not as easy as it sounded, but the idea and the problem stuck with meThis thread made me stubborn... It's possible!And after several weeks of thinking and tweaking on the bench and in LTSpice, this is the circuit I've arrived at...The JFET input stage is a direct copy of the 1980's HP-type input stage suggested by Benta in the beginning of this thread except using components I could obtain from my local components vendor. The original ECL comparator circuit is replaced with single, modern TTL comparactor: The MAX912 is a dual comparator similar to the single LT1016, but my local vendor has loads of the MAX912's (I think they were popular in switch mode PSU's once) and sells them rather cheaply, so I chose that.I modeled a few circuits in LTSpice, but getting the actual circuit to work took some learning experiences on my part comparing LTSpice theory (using the LT1016 model) and reality: The real world is far more noisy than my initial simulations. Learning point: Remember to add at least one -6 dB voltage source with some higher friquency than the intended input signal to simulate real noise with precision, fast comparactors like theseDuring tinkering, I came over one of the legendary Jim Williams' application notes (and learned about him as I wondered who the person was who had written the excellent article i digested):High Speed Comparator Techniques. It's available at the LT1016's product page at Analog:https://www.analog.com/en/products/lt1016.html.He presents a similar trigger input circuit in the note, but with a different FET input stage, which I have more or less copied except that my circuit uses an average of the output signal from the input buffer as a virtual trigger point like the NY Elektronik input stage, where his circuit has an adjustable, fixed trigger.In the article, Willams makes a strong point about the LT1016 needing a ground plane. Thats hard to make on prototype board, but I've been able get my circuit work without, perhaps because the MAX912 is less current consuming and therefore possibly less noisy. Also I haven't tested my circuit at frequencies above 900 kHz. I did add some wires on the prototype board to ensure some shielding of signals. The schmitt trigger of course filters the noise... but it could also cause oscillation. It doesn't, fortunately.And here's a picture of my prototype triggering perfectly on a 70 mVpp 65 kHz sine wave output from the XR2206:I'm hopeful that my design is able to handle at least the 50 MHz that the PIC-counter in theory handles, and I am considering having a small 5x5 cm PCB made with a proper ground plane to ensure that. I'm still in doubt whether that will make sense, though, as I have to order 10 pcs, and that seems a waste so I might just fit a somewhat cleaned up version of my prototype on aluminium ground plane.Thanks to everyone, especially void_error and Benta for inspiration! I hope some might find my experiences useful

author: mino-fm
author_rank: Regular Contributor
timestamp: December 26, 2022, 12:35:28 pm
content: If you like to keep it simple you can use logic inverter as a linear amplifier followed by a schmitttigger.Sensitivity:15 mVeff @ 100 kHz to 1 MHz20 mVeff @ 30 MHz50 mVeff @ 100 MHz70 mVeff @ 160 MHz (upper limit of counter)

author: adinsen
author_rank: Regular Contributor
timestamp: December 26, 2022, 01:58:52 pm
content: Thanks!That would indeed have been sufficiently simple to satisfy my needs, except I would have had to employ SMD to use the 74AUP's. I wonder if HC's would work too...Have you actually built this one?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 26, 2022, 02:46:48 pm
content: The normal 74HC04 don't like the linear operation very much. The 74HCU04 is kind of the slower version of the 74AUP and usually OK with linear operation.

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2022, 05:35:20 pm
content: Many of the older but simple frequency counter designs just use limiting amplifiers to directly drive a logic input. Some oscilloscope trigger circuits work this way also.

author: mawyatt
author_rank: Super Contributor
timestamp: December 26, 2022, 05:51:33 pm
content: Yep, Tek famously utilized ECL gates for the Scope trigger inputs, recall they used Motorola MECL types.Best,

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2022, 09:39:54 pm
content: This is not to suggest that a limiting amplifier followed by a logic gate is the way to go. There is nothing wrong with using a fast comparator like an LT1016 as a trigger comparator all in one stage. I have an old Optoelectronics frequency that does exactly this up to 50 MHz.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2022, 10:40:59 pm
content: Just one question: why does it have to work down to DC? If it's going to a frequency counter, then only the rising/falling edges of the input waveform are important. I don't see any reason why DC coupling would be required for a frequency counter.

author: fourfathom
author_rank: Super Contributor
timestamp: December 26, 2022, 10:56:53 pm
content: If you are measuring very slow sinewaves, or events, then a DC response may be a reasonable approach.In my freq counter design I am using the TLV3501 comparator (4.5ns Tpd, six-pin SOT-23), with switchable AC/DC input and switchable 50 Ohm / 1M Ohm input termination. The 3501 has about 6mV input hysteresis. It has decent input over-voltage protection, but I have external resistors and a TLV diode for extra immunity.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2022, 12:25:20 am
content: Quote from: Zero999 on December 26, 2022, 10:40:59 pmWith just a frequency counter that is the case, and such frequency counters are commonly AC coupled.DC coupling is required for many of the measurements that universal timer/counters support besides frequency, and they usually include slope and level controls.