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| Inexpenive 50 Ohm Feed Through Terminations |
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| David Hess:
--- Quote from: vk6zgo on July 16, 2019, 08:03:05 am ---Just for chuckles, have you tried the 20dB attenuator alone, without the termination? I have played around with these, & found that 20dB attenuators approximate a pretty good termination by themselves. --- End quote --- I have done that when I did not have a feedthrough termination handy and it worked fine. Good feedthrough terminators however place their resistive element as close to the male connector or even make it part of the male connector to minimize the length of the unterminated transmission line so in theory they could perform better. BNC connectors with a built in 50 ohm resistor disc as part of their construction used to be available. --- Quote from: graybeard on July 16, 2019, 08:34:44 pm --- --- Quote from: David Hess on July 16, 2019, 01:19:11 am ---Older oscilloscopes with 3 or even more high impedance input attenuators perform better. --- End quote --- This is not always true. I tested the the inputs to my 200 MHz 7A26 plug-in on my Tektronix 7904A 500MHz CRO and it was worse than the Siglent SDS1104X-E since the 7A26 input capacitance is 22pF vs. 15pF for the SDS1104X-E. The 7A26 measured input impedance with the feed-through termination varied a bit on almost every input attenuator setting. --- End quote --- I meant they perform better because their high impedance buffer can operate over a smaller signal range. The 7A26 has a higher input capacitance so will present a worse match but it should be more consistent between attenuation settings unless the compensation for the attenuators is out of calibration. It would make for boring measurements though. |
| thm_w:
Here is a photo of one of the cheap "plastic" terminators you said were bad. In reality they are all metal, and the plastic is simply a cover. There are two 1210 resistors inside which can take 0.5W each, so the 1W rating should be accurate. I don't have equipment to test the frequency response though. The PCB inside might be useful for making custom filters or attenuators. |
| graybeard:
--- Quote from: thm_w on July 19, 2019, 11:41:07 pm ---Here is a photo of one of the cheap "plastic" terminators you said were bad. In reality they are all metal, and the plastic is simply a cover. There are two 1210 resistors inside which can take 0.5W each, so the 1W rating should be accurate. I don't have equipment to test the frequency response though. The PCB inside might be useful for making custom filters or attenuators. --- End quote --- It appears to be a repurposed housing for a low frequency oscilloscope probe compensation network. My guess is holes in the shielding are to facilitate adjustments. Given the extended ground path for the resistors I would not expect it to work well. My guess is it will work OK up to the high tens of MHz, but look inductive above. The specification of DC to 1GHz is BS. In my experience the plastic outer pieces of BNC connectors do not last. Since you have it you should use it. It will probably be fine below 100 MHz. However I would not buy one. |
| bitseeker:
--- Quote from: thm_w on July 19, 2019, 11:41:07 pm ---Here is a photo of one of the cheap "plastic" terminators you said were bad. In reality they are all metal, and the plastic is simply a cover. There are two 1210 resistors inside which can take 0.5W each, so the 1W rating should be accurate. I don't have equipment to test the frequency response though. The PCB inside might be useful for making custom filters or attenuators. --- End quote --- Thanks for the peek inside. As graybeard said, it does look like a re-purposed scope probe compensator. |
| vk6zgo:
--- Quote from: thm_w on July 19, 2019, 11:41:07 pm ---Here is a photo of one of the cheap "plastic" terminators you said were bad. In reality they are all metal, and the plastic is simply a cover. There are two 1210 resistors inside which can take 0.5W each, so the 1W rating should be accurate. I don't have equipment to test the frequency response though. The PCB inside might be useful for making custom filters or attenuators. --- End quote --- As an "even greyer beard", you had me going for a while with "1210"! I thought "120 Ohms--- what the heck?", until I realised that it was the size of the resistor package :-[ The older Tektronics 75 Ohm feedthrough terminations used two leaded 150 Ohm resistors in parallel. These terminations had a habit of becoming unscrewed after years of use, & detaching one or more resistors. We routinely soldered them back. The externally physically identical 50 Ohm ones never seemed to have this problem, (probably because they weren't used as much as the others), so I wasn't able to check if they used the same type of internal parts. They both used a brown plastic "sheath" with the impedance value & Tek part number, slid over what looked like an assembly made up of standard BNC fittings. We also had a larger 50 Ohm one, which had a higher wattage rating & had a bigger plastic body, with no obvious way of dismantling it. |
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