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Post by gregstv on Jan 11, 2017 1:44:50 GMT -5
gjaky, Can you post a picture of the multiburst pattern on your oscilloscope please?
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Post by gjaky on Jan 11, 2017 2:49:18 GMT -5
This is taken with my XG-VNB mod, at G1 output. HP 1120A FET probe (with 100:1 divider), HP 1720A oscilloscope, 20v/div, 1080P 60Hz signal out of a GeForce 6600GT video card (the output of the card was measured here) With analog scopes the intensity grading is also carrying information, which you loose on a digital scope (unless it has digital phosphor feature...) The point is when I started measuring the XG neckboads chasing the bandwidth problems I did not have the FET probe yet only the HP scope along with Tektronix P6105 and P6106 probes. I estimated the bandwidth of the neckboards to be around 30MHz based on the resolutions I tested on it -by eye, measuring on the output with the Tek 6106 probe showed 5ns rise time (~70MHz), later I got the HP FET probe and remeasuring the the same board gave 12ns (~30MHz) rise time then. And this was pretty much the case with every other stage when I used the Tek probe... On ebay you can find 3-4-500MHz passive probes with one single HF trimming element, the Tek 6106 has 5 of them, yet the tip capacitance make too much disturbance to make it useless for most HF measurements. The HP FET probe in its bare form can take only +/-0.5V signal maximum and has a tip capacitance of 3pF shunted by 100kOhm, you need passive dividers (10:1-> +/-5V, 100:1 ->+/-50V) for higher voltages, using the dividers makes the input to 1MOhm shunted by <1pF. You may wonder why is this high impedance head if it only 100kOhm or 1MOhm, because the capacitance is low. It has so high input impedance that often I don't even have to touch the head to the circuit, it pick up the signal even from a few mm distance! You can find good deals about FET probes and good analog scopes (like Tek 485, Tek 2465, or HP 172x) on E-bay if you are serious about these. If you look for a FET probe always make sure the dividers are included, because if you miss them it is basically impossible to find a spare... I bought the HP 1720A scope with the Tek probes locally for about 140USD, the HP FET probe was another 85USD... I briefly had a Yokogawa DL1740 4x500MHz scope as well, but it wasn't suiting well for CRT measurements despite its awesome bandwidth, I bought it for 280USD as faulty, repaired and then I sold it for 680USD, I regret it only a little
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Post by gregstv on Jan 11, 2017 4:27:55 GMT -5
I have made an offer on a HP Agilent 54615B. The case is damaged but working well. He has not accepted my offer $300. What would be a fair price on this unit?
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Post by gjaky on Jan 11, 2017 6:18:35 GMT -5
Honestly I don’t know… It is a nice scope except for one thing, it has very little memory, only 4kpts, so at slower timebases you can’t exploit the full sampling speed of the ADCs. As I see you have a modern digital scope already that is perfect for general measurements. In this case I’d recommend the Tektronix 2465 series to get while keeping the current scope as well, it is an analog scope with digital readout, the base model is 300MHz, but Tektronix was very generous with these specifications, there are several reports that they far outperform their advertised bandwidth, not to mention the later models (2465A: 350MHz, 2465B: 400MHz). There are loads of threads how to keep those scopes running flawlessly, so there are a lot of experts as well.
I went with the HP 1720 because this was the only one I could afford (and was available) at that time, this is a 275MHz scope and I’m sure it is indeed not more not even by a slight. Mine is already 40 years old and while it still holds up certainly show some sign of its age. The same vintage would be the Tek 485, which has 250MHz bandwidth in 1M Ohm input mode, but is 350MHz in 50 Ohm mode, but as I pointed out the 1MOhm input really does not matter much, (FET probes are working at 50 Ohm). I hacked my HP scope so at channel A I replaced the 50 Ohm termination resistors to 75 OHM, so I can directly connect video coax to it, using low impedance probing is an other way to go for high frequency measurements, but it is obviously not letting you to take real in-circuit measurements.
Later the day I might be able to post some pictures how the different probing techniques matters on the same signal/circuit.
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Post by gjaky on Jan 11, 2017 16:15:24 GMT -5
So here are some measurement results. Rise time out of my pulse generator into the scope's input measures ~1.5ns, this is essentially the same with the HP 1120A FET probe, Testec 250MHz 10:1 probe about 1.8ns (but see later), Tek P6106 about 2ns or little more. 2ns/div uipper trace source directly to scope input 50 Ohm terminated, lower trace HP 1120A FET probe sniffing with a BNC Tee connector on scope's input, note the signal delay. Same as above 10ns/div upper trace the source (50 Ohm fully terimnated), lower trace Testec 250MHz 10:1 passive probe with the shortest ground possible, 20ns/div, note the ugly overshoot, this however can be adjusted on the probe but it seriously affects rise time. Upper trace source (50 Ohm fully terminated), lower trace Tek P6106 250MHz 10:1 probe with typical 5cm ground clip, note the ringing which affects the source signal as well (hard to see on the picture). 20ns/div The same as above (Tek 6106) but with the shortest possible ground connection. These are the most ideal conditions. Also note that one pixel at 1080P 60Hz is 6.7ns wide...
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Post by gregstv on Jan 12, 2017 3:57:17 GMT -5
Dragged out the Tektronix TDS210. Input to CAG board Output from MOOME
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