On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote:> I guess, I'm not really sure how to think of 7' of coax.. transmission > line or capacitance?If the termination is higher resistance than the transmission line impedance, it's a capacitance. If it's lower, it's an inductance. With 'correct' termination, it's neutral (current and voltage in phase). An easy way to remember: what happens at DC? Zlow, means inductor. Zhigh, means capacitor. So, that was the familiar scenario of an op amp with a capacitive load, which oscillates.
TIA-PD compensation, by eye
Started by ●October 19, 2017
Reply by ●October 23, 20172017-10-23
Reply by ●October 23, 20172017-10-23
whit3rd <whit3rd@gmail.com> wrote:> On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote:>> I guess, I'm not really sure how to think of 7' of coax.. transmission >> line or capacitance?> If the termination is higher resistance than the transmission line > impedance, it's a capacitance. If it's lower, it's an inductance. > With 'correct' termination, it's neutral (current and voltage in phase).> An easy way to remember: what happens at DC? Zlow, means inductor. > Zhigh, means capacitor.> So, that was the familiar scenario of an op amp with a capacitive load, > which oscillates.It depends on the frequency. An unterminated cable at 1/4 wavelength reflects back as a short. The ringing period can be estimated as Period[ns] = 2/3 � Len[inches]: http://tinyurl.com/yda8nk5x For 7.5 ft coax, 7.5*12*2/3 = 60 ns. This is in the ballpark of what he is seeing. However, the main problem is trying to do a 200 MHz op amp on a two-sided pcb with no ground plane.
Reply by ●October 23, 20172017-10-23
On Mon, 23 Oct 2017 06:45:49 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Sunday, October 22, 2017 at 10:40:39 AM UTC-4, Joerg wrote: >> On 2017-10-21 18:44, George Herold wrote: >> > On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote: >> >> On 2017-10-20 18:09, George Herold wrote: >> >>> On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: >> >>>> On 2017-10-20 14:32, Joerg wrote: >> >>>>> On 2017-10-19 18:17, George Herold wrote: >> >>>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: >> >>>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle >> >>>>>>> <martin_ridd@verizon.net> wrote: >> >>>>>>> >> >>>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com >> >>>>>>>> wrote: >> >>>>>>>> >> >>>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold >> >>>>>>>>> wrote: >> >>>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to >> >>>>>>>>>> gin up an old photodiode circuit. Put it in today, and the step >> >>>>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF >> >>>>>>>>>> across it. >> >>>>>>>>>> >> >>>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 >> >>>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. >> >>>>>>>>>> >> >>>>>>>>>> Here's the uncompensated step. >> >>>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 >> >>>>>>>>>> (note time base change) >> >>>>>>>>>> >> >>>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. >> >>>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... >> >>>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my >> >>>>>>>>>> voltage step isn't fast enough. >> >>>>>>>>>> >> >>>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that >> >>>>>>>>>> was faster than the 'HC14? >> >>>>>>>>>> >> >>>>>>>>>> George H. >> >>>>>>>>> >> >>>>>>>>> Georrggggeeee, have you been using the white solderless breadboard >> >>>>>>>>> again? >> >>>>>>>>> >> >>>>>>>>> Grins, >> >>>>>>>>> James Arthur >> >>>>>>>> >> >>>>>>>> I was going to say ground lead oscillations, but that is a low >> >>>>>>>> frequency. >> >>>>>>>> >> >>>>>>>> Cheers >> >>>>>>> >> >>>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance >> >>>>>>> at the summing point making it ring. A bigger feedback cap, or a >> >>>>>>> cascode thing, can fix that. I don't know why the amplitude should >> >>>>>>> change. >> >>>>>> >> >>>>>> Right, Sorry, classic vanilla TIA-PD. >> >>>>>> I'm just picking the tuning cap by eye... shape >> >>>>>> of the step response. But the shape doesn't look right. >> >>>>>> I've tuned a few TIA's with slower opamps and maybe >> >>>>>> more C on the inverting input. It has this >> >>>>>> classic two pole shape. You can add more feeed back C and pick >> >>>>>> your Q by eye. This doesn't, so it's my light pulse >> >>>>>> or my electronics. I'm mostly stuck with the old pcb layout >> >>>>>> so I'm just trying to make sure my light source is not the >> >>>>>> problem, or fix it if it is. >> >>>>>> >> >>>>>> Instead of an led, I could wiggle a diode laser, that's >> >>>>>> less convenient. >> >>>>> >> >>>>> >> >>>>> LEDs are ok. They can be pulsed blazingly fast. >> >>>>> >> >>>> >> >>>> Which leaves the question which photodiode you used and how much you >> >>>> pre-biased it. That can have a major impact on overall bandwidth. >> >>> osi-optoelectronics 44pin (there might be some letters in there. >> >>> or pin44) >> >>> IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and >> >>> 100uF Al to ground... that's hacked in too... Well, you're going >> >>> to have to wait till Monday for the pics. >> >>> >> >> >> >> The biased side of the PD needs an RF-compliant small ceramic cap to >> >> ground. 0.1uF in 0603 package or similar, very short path to the ground >> >> plane. Short meaning a via right next to its ground pad. The RF >> >> impedance of an electrolytic cap is like roulette. >> > Hmm, I can try adding a ceramic to the bias supply. >> > There's no ground plane in this circuit. >> >> >> Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. >> Although a 40MHz scope won't necessarily show you if it does tarantella >> dances. >> >> >> > ... Well the whole thing >> > lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. >> > Inside, capacitance is not your friend at 1 meg ohm. >> > (I've been measuring this outside of the Al box, >> > I wonder if it will change by much inside?) >> > >> >> The RF behavior can't be ascertained well from outside. Or even inside, >> without a ground plane. Trying nanosecond stuff that way doesn't make >> much sense, I'd rebuild that with a ground plane. For prototyping a >> piece of copperclad will do. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > >Hi Joerg, (others) Here's a couple of pics. >https://www.dropbox.com/s/2ivoas0759qz03i/PD-front.JPG?dl=0 >https://www.dropbox.com/s/9ls9kkm7n6i0l6v/PD-back.JPG?dl=0 >https://www.dropbox.com/s/0jxk4vzf66g6xrv/PD-case1.JPG?dl=0 >https://www.dropbox.com/s/p3ox17ngwhu4ruo/PD-case2.JPG?dl=0 > > >That's not a photodiode, it's a solar panel. What's its capacitance? -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●October 23, 20172017-10-23
On Monday, October 23, 2017 at 4:49:11 PM UTC-4, whit3rd wrote:> On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote: > > > I guess, I'm not really sure how to think of 7' of coax.. transmission > > line or capacitance? > > If the termination is higher resistance than the transmission line impedance, > it's a capacitance. If it's lower, it's an inductance. With 'correct' termination, > it's neutral (current and voltage in phase). > > An easy way to remember: what happens at DC? Zlow, means inductor. Zhigh, means > capacitor. > > So, that was the familiar scenario of an op amp with a capacitive load, > which oscillates.Right, It's cool (to me) that 50 ohms on the end cures it. If this ever turns into a product I'll pick a slower opamp, or add a line driver. I've got some opa827's from the same order. slower, less noise. George H.
Reply by ●October 23, 20172017-10-23
On Monday, October 23, 2017 at 3:32:03 PM UTC-4, Joerg wrote:> On 2017-10-23 06:45, George Herold wrote:, *** big snip above pics... ***********> > Hi Joerg, (others) Here's a couple of pics. > > https://www.dropbox.com/s/2ivoas0759qz03i/PD-front.JPG?dl=0 > > https://www.dropbox.com/s/9ls9kkm7n6i0l6v/PD-back.JPG?dl=0 > > https://www.dropbox.com/s/0jxk4vzf66g6xrv/PD-case1.JPG?dl=0 > > https://www.dropbox.com/s/p3ox17ngwhu4ruo/PD-case2.JPG?dl=0 > > > > That is not likely going to work reliably. You can get away with > double-sided PCB but one side has to be a solid or almost solid ground > plane.Joerg! I'm not sure why you are interested in this (minor) project, but I love it! You know it's cool that this pcb almost works with an opamp that's ~50 times faster than the opa124A it was designed for.> > > > > James, yeah the opamp TIA drives the coax too. It was oscillating happily > > when I had only 330 ohms of TIA R. :^) > > > That is one of the telltale signs that the whole enchilada is teetering > on the edge when it comes to stability.Grin, I could stick a Big damping cap across the low end, For this device the only app for low gain is as a power meter, DC. (at least at the moment)> > > > ... But it seems OK when terminated. > > So as you say, the ringing is due somewhat to the cable capacitance.. > > I guess, I'm not really sure how to think of 7' of coax.. transmission > > line or capacitance? > > > > For coax you need a staunch driver. I believe the lowest load resistance > the THS4631 is supposed to drive is 100ohms. > > > > Ground planes and such. Joerg I'm not sure, what I'd like is a big ass > > photodiode at 1 M ohm gain and 1 MHz BW. (typically ~1 uA of current.) > > 1 MHz is sorta border line RF, and with 1 Meg ohm a pF of capacitance > > can ruin your whole day. (I've got hacked pcb's where I learned this... > > ground plane sanded off around PD and TIA... ugly.) > > > > You can leave open a small island around IN- but this sort of > requirement asks for Phil's medicine. Cascode and such.Hmm well I keep talking about the stray feed back capacitance, which at the moment is the big problem. I'm not sure a ground plane will solve this problem, but I'll give it a try. (I was thinking more of a driven shield at the minus input, but except for high frequency that's almost the same as ground.) So I took out a blank pcb today and first measured the trace C. ( ~1" bus wires into production's SRS RCL meter, 1.7 pF, it's not all that accurate at the pF level. I built up the circuit with a 1 Meg R lying flat from the switch wiper hole, f_3dB = 150kHz, ~1pF. I then soldered bus wire from the wiper and bent the TH resistor into a hairpin, (see pic above) f= 120 kHz. I've always wondered what the C of a hair pin 1/4 W TH R was, about 0.3pF more than lying flat, is my answer. I took out the bus wire and added the switch f~ 70 kHz, ~2.3 pF, the switch added about 1 pF. Adding in 9 more hair pin R's brought f_3dB down to 30 kHz, ~5.3 pF, each R adds another ~0.3 pF, on average. Tomorrow I'll try some 0805's.> > > > I bought a couple of THS4361's so maybe I'll build up a pcb w/o the switch > > and see what that look like. > > > > Yes, and with a solid ground plane. > > > > Oh the numbers I post upstream are not right. > > I'm a little bothered by the high gain end, cause I think I was > > getting more (HF) gain there with a slower opamp, so I need to redo this > > more carefully. > > > > Gain R 3dB > > 330 20M(Hz) > > 1k 15M > > 3.3k 9M > > 10k 6M > > 33k 3M > > 100k 400k > > 330k 90k > > 1Meg 30k > > > > At 1M it is bound to fall apart. You might also want to look for amps > with lower input noise so you don't need such high feedbakc resistor values.Hmm I need ~1M (or 10M) ohm because of low currents. Not much light, for some apps that's because of a weak source, for other atomic physics stuff it's because light saturates the transitions, (more light, less signal) For some stuff you have to turn off the room lights, or hang black cloth's here and there. you can do lenses and stuff,but for beginners, that's complicated. George H.> > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●October 23, 20172017-10-23
On Monday, October 23, 2017 at 6:32:03 PM UTC-4, John Larkin wrote:> On Mon, 23 Oct 2017 06:45:49 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Sunday, October 22, 2017 at 10:40:39 AM UTC-4, Joerg wrote: > >> On 2017-10-21 18:44, George Herold wrote: > >> > On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote: > >> >> On 2017-10-20 18:09, George Herold wrote: > >> >>> On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: > >> >>>> On 2017-10-20 14:32, Joerg wrote: > >> >>>>> On 2017-10-19 18:17, George Herold wrote: > >> >>>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: > >> >>>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle > >> >>>>>>> <martin_ridd@verizon.net> wrote: > >> >>>>>>> > >> >>>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com > >> >>>>>>>> wrote: > >> >>>>>>>> > >> >>>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold > >> >>>>>>>>> wrote: > >> >>>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to > >> >>>>>>>>>> gin up an old photodiode circuit. Put it in today, and the step > >> >>>>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF > >> >>>>>>>>>> across it. > >> >>>>>>>>>> > >> >>>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 > >> >>>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. > >> >>>>>>>>>> > >> >>>>>>>>>> Here's the uncompensated step. > >> >>>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 > >> >>>>>>>>>> (note time base change) > >> >>>>>>>>>> > >> >>>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. > >> >>>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... > >> >>>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my > >> >>>>>>>>>> voltage step isn't fast enough. > >> >>>>>>>>>> > >> >>>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that > >> >>>>>>>>>> was faster than the 'HC14? > >> >>>>>>>>>> > >> >>>>>>>>>> George H. > >> >>>>>>>>> > >> >>>>>>>>> Georrggggeeee, have you been using the white solderless breadboard > >> >>>>>>>>> again? > >> >>>>>>>>> > >> >>>>>>>>> Grins, > >> >>>>>>>>> James Arthur > >> >>>>>>>> > >> >>>>>>>> I was going to say ground lead oscillations, but that is a low > >> >>>>>>>> frequency. > >> >>>>>>>> > >> >>>>>>>> Cheers > >> >>>>>>> > >> >>>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance > >> >>>>>>> at the summing point making it ring. A bigger feedback cap, or a > >> >>>>>>> cascode thing, can fix that. I don't know why the amplitude should > >> >>>>>>> change. > >> >>>>>> > >> >>>>>> Right, Sorry, classic vanilla TIA-PD. > >> >>>>>> I'm just picking the tuning cap by eye... shape > >> >>>>>> of the step response. But the shape doesn't look right. > >> >>>>>> I've tuned a few TIA's with slower opamps and maybe > >> >>>>>> more C on the inverting input. It has this > >> >>>>>> classic two pole shape. You can add more feeed back C and pick > >> >>>>>> your Q by eye. This doesn't, so it's my light pulse > >> >>>>>> or my electronics. I'm mostly stuck with the old pcb layout > >> >>>>>> so I'm just trying to make sure my light source is not the > >> >>>>>> problem, or fix it if it is. > >> >>>>>> > >> >>>>>> Instead of an led, I could wiggle a diode laser, that's > >> >>>>>> less convenient. > >> >>>>> > >> >>>>> > >> >>>>> LEDs are ok. They can be pulsed blazingly fast. > >> >>>>> > >> >>>> > >> >>>> Which leaves the question which photodiode you used and how much you > >> >>>> pre-biased it. That can have a major impact on overall bandwidth. > >> >>> osi-optoelectronics 44pin (there might be some letters in there. > >> >>> or pin44) > >> >>> IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and > >> >>> 100uF Al to ground... that's hacked in too... Well, you're going > >> >>> to have to wait till Monday for the pics. > >> >>> > >> >> > >> >> The biased side of the PD needs an RF-compliant small ceramic cap to > >> >> ground. 0.1uF in 0603 package or similar, very short path to the ground > >> >> plane. Short meaning a via right next to its ground pad. The RF > >> >> impedance of an electrolytic cap is like roulette. > >> > Hmm, I can try adding a ceramic to the bias supply. > >> > There's no ground plane in this circuit. > >> > >> > >> Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. > >> Although a 40MHz scope won't necessarily show you if it does tarantella > >> dances. > >> > >> > >> > ... Well the whole thing > >> > lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. > >> > Inside, capacitance is not your friend at 1 meg ohm. > >> > (I've been measuring this outside of the Al box, > >> > I wonder if it will change by much inside?) > >> > > >> > >> The RF behavior can't be ascertained well from outside. Or even inside, > >> without a ground plane. Trying nanosecond stuff that way doesn't make > >> much sense, I'd rebuild that with a ground plane. For prototyping a > >> piece of copperclad will do. > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > >Hi Joerg, (others) Here's a couple of pics. > >https://www.dropbox.com/s/2ivoas0759qz03i/PD-front.JPG?dl=0 > >https://www.dropbox.com/s/9ls9kkm7n6i0l6v/PD-back.JPG?dl=0 > >https://www.dropbox.com/s/0jxk4vzf66g6xrv/PD-case1.JPG?dl=0 > >https://www.dropbox.com/s/p3ox17ngwhu4ruo/PD-case2.JPG?dl=0 > > > > > > > > That's not a photodiode, it's a solar panel. > > What's its capacitance?Grin ~120pF at 10V. (according to the data sheet) But it's the stray feedback C of ~5pF that is my problem, (at least that's where I am.) George H.> > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●October 24, 20172017-10-24
On Monday, October 23, 2017 at 5:34:23 PM UTC-4, Steve Wilson wrote:> whit3rd <whit3rd@gmail.com> wrote: > > > On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote: > > >> I guess, I'm not really sure how to think of 7' of coax.. transmission > >> line or capacitance? > > > If the termination is higher resistance than the transmission line > > impedance, it's a capacitance. If it's lower, it's an inductance. > > With 'correct' termination, it's neutral (current and voltage in phase). > > > An easy way to remember: what happens at DC? Zlow, means inductor. > > Zhigh, means capacitor. > > > So, that was the familiar scenario of an op amp with a capacitive load, > > which oscillates. > > It depends on the frequency. An unterminated cable at 1/4 wavelength > reflects back as a short. > > The ringing period can be estimated as Period[ns] = 2/3 в Len[inches]: > > http://tinyurl.com/yda8nk5x > > For 7.5 ft coax, 7.5*12*2/3 = 60 ns. > > This is in the ballpark of what he is seeing. However, the main problem is > trying to do a 200 MHz op amp on a two-sided pcb with no ground plane.That formula's wrong--propagation velocity is about 2nS/ft on FR-4, not 2/3rds of a nS per inch! 'c' in vacuum is 3x10^8 m/s, and about 2/3rds that in coax, right? A two-way trip through 7.5' of coax is roughly 5 meters, so 5m / (2/3 * 3x10^8 m/s) = 25nS. George's ringing is about 62-ish nS, too slow for the cable to be resonant. Cheers, James Arthur
Reply by ●October 24, 20172017-10-24
On Monday, October 23, 2017 at 4:49:11 PM UTC-4, whit3rd wrote:> On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote: > > > I guess, I'm not really sure how to think of 7' of coax.. transmission > > line or capacitance? > > If the termination is higher resistance than the transmission line impedance, > it's a capacitance. If it's lower, it's an inductance. With 'correct' termination, > it's neutral (current and voltage in phase). > > An easy way to remember: what happens at DC? Zlow, means inductor. Zhigh, means > capacitor. > > So, that was the familiar scenario of an op amp with a capacitive load, > which oscillates.Yes, we agree on that. Ye olde small resistor Rs in series with the output, and the feed-forward feedback cap he already has fixes that. Cf .------||----. | | | |\ | >--+--|-\ | | >------+--[Rs]-----> to coax Cheers, James Arthur
Reply by ●October 24, 20172017-10-24
On Tuesday, October 24, 2017 at 1:56:43 AM UTC-4, dagmarg...@yahoo.com wrote:> On Monday, October 23, 2017 at 4:49:11 PM UTC-4, whit3rd wrote: > > On Monday, October 23, 2017 at 6:45:53 AM UTC-7, George Herold wrote: > > > > > I guess, I'm not really sure how to think of 7' of coax.. transmission > > > line or capacitance? > > > > If the termination is higher resistance than the transmission line impedance, > > it's a capacitance. If it's lower, it's an inductance. With 'correct' termination, > > it's neutral (current and voltage in phase). > > > > An easy way to remember: what happens at DC? Zlow, means inductor. Zhigh, means > > capacitor. > > > > So, that was the familiar scenario of an op amp with a capacitive load, > > which oscillates. > > Yes, we agree on that. Ye olde small resistor Rs in series with the > output, and the feed-forward feedback cap he already has fixes that..--------[Rfb]-------. | Cf |> +------||----. | > | | | > | |\ | | > >--+--|-\ | | > | >------+--[Rs]-+---> to coax >I sketched in Rfb, in case that wasn't clear. There's still the problem of that solar panel loading the summing junction, but this fixes the capacitive output loading problem. Cheers, James Arthur
Reply by ●October 24, 20172017-10-24
dagmargoodboat@yahoo.com wrote:>> The ringing period can be estimated as Period[ns] = 2/3 в Len[inches]:>> http://tinyurl.com/yda8nk5x>> For 7.5 ft coax, 7.5*12*2/3 = 60 ns.>> This is in the ballpark of what he is seeing.> That formula's wrong--propagation velocity is about 2nS/ft on FR-4, > not 2/3rds of a nS per inch!> 'c' in vacuum is 3x10^8 m/s, and about 2/3rds that in coax, right?> A two-way trip through 7.5' of coax is roughly 5 meters, so > 5m / (2/3 * 3x10^8 m/s) = 25nS.> George's ringing is about 62-ish nS, too slow for the cable to be > resonant.> Cheers, > James ArthurDid you read the article? Eric Bogatin is a well-respected SI expert. He gives the explanation of ringing coax in the article. I copy it here: ------------------------------------------------------------------------- The root cause of the ringing is reflections from the mismatch at the RX between the impedance of the line and the high input impedance of the RX, and the mismatch when the reflected signal makes its way back to the low output impedance of the TX. The reflection coefficient is close to 1 at the RX, but is a negative value at the TX. Let�s trace the signal path on its journey. When it first reaches the RX, it reflects. The initial voltage at the RX is high. The reflected signal makes its way back to the TX, where it sees a high to low impedance and reflects. But the reflected voltage is negative. This negative reflected signal makes its way back to the RX, where it again reflects, but since it is negative, pulls the signal at the RX down. When it reflects from the RX, it is still negative, makes its way back to the TX, changes sign when it reflects, and travels back to the RX as a positive signal. From the initial positive signal at the RX, the signal takes one TD (time delay) to make it back to the TX, another TD to reflect back to the RX (that�s 2 � TD from a peak to a valley at the RX), another TD to get to the TX again, and another TD for the reflected signal to make it to the RX as a positive signal. This is a rather surprising result: four time delays between successive peaks due to reflections in an unterminated line. ------------------------------------------------------------------------- The difference between Eric's analysis and George's photos is I believe that George is measuring the ringing at the transmitting end, not at the unterminated receiving end. However, the fact there is no ground plane and the photodode has such high capacitance could turn the multiple reflections into damped ringing, as shown in the pictures. Adding a termination resistor to the coax stops the reflections. This could stop the ringing by eliminating the reflections and also by loading the op amp with a resistive load.
