old tek 453 input impeadance - Page 1
Forum section: electronics / repair

author: m3vuv
author_rank: Super Contributor
timestamp: March 21, 2020, 11:28:07 pm
content: hi all,ive a tek 453 circa 1966 with the fet front end,i just wondered if there would be any way to mod it so the input impeadance is 50r?,any ideas,cheers in advance.

author: duak
author_rank: Super Contributor
timestamp: March 22, 2020, 03:06:18 am
content: Were you thinking of permanently modifying the 453? I don't know anything specific about the 453 but I'll bet there's shielding around the input connector. I can't imagine there's much room in there.Perhaps a BNC feedthrough terminator? They can be bought or built:https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2020, 02:28:56 pm
content: In the old days, we put a BNC F-M-F tee connector at the input, with a 50 ohm terminator at one F and the M to the input connector, but a feed-through terminator is a much better idea and takes up less space.

author: m3vuv
author_rank: Super Contributor
timestamp: March 22, 2020, 04:42:53 pm
content: so is it basicaly a 50r resistor between the bnc center pin and ground?,i only need it for hf

author: m3vuv
author_rank: Super Contributor
timestamp: March 22, 2020, 04:50:18 pm
content: at the moment the input is 1Mohmx20pf

author: duak
author_rank: Super Contributor
timestamp: March 22, 2020, 06:06:45 pm
content: m3vuv, yep, that's all it is! Although the way it's done is important for high frequency performance.

author: magic
author_rank: Super Contributor
timestamp: March 22, 2020, 06:48:57 pm
content: I tested this DIY feedthrough terminator (using two generic 0805 100Ω resistors) on my 200MHz 20pF scope with nanosecond risetime squarewaves, it looked okay. I also tried a BNC tee with cheap 50Ω "plug" terminator, pretty much no difference.That being said, SPICE simulation of 1 meter 50Ω cable terminated with 50Ω|20pF showed that some frequency response variation already appears around 100MHz and things get pretty bad above 200-300MHz. So don't expect miracles

author: m3vuv
author_rank: Super Contributor
timestamp: March 23, 2020, 09:36:57 am
content: well its only a 50 Mhz scope anyway.

author: med6753
author_rank: Super Contributor
timestamp: March 23, 2020, 10:03:40 am
content: Quote from: m3vuv on March 23, 2020, 09:36:57 amCorrect. And these for your application will work fine. 50 ohm feedthru terminators. Available on Amazon. I have several and they work well.

author: bd139
author_rank: Super Contributor
timestamp: March 23, 2020, 10:53:20 am
content: Important note here. It really depends what you're doing. If you're connecting a transmitter to the front end of the radio, DONT use an inline terminator. It will blow up instantly and turn into a pretty nasty mismatch. Bye bye finals. Also possibly your scope front end if you get some nice voltage peaks. The little inline terminators can only handle 0.5W or so.Best approach here if you're doing radio stuff is get a BNC T piece and plug it into a scope channel. Connect your signal to the left half of the BNC T piece. Connect a properly sized 50 ohm dummy load to the right hand side of the T piece. Terminate AT THE SCOPE always.You DO NOT need to modify the scope at all. Don't even think about it!If you want to measure higher powers it's probably safer to build a power tap which will knock 40dB off the signal amplitude and not scare your poor little scope to death:http://on4khg.be/wordpress/wp-content/uploads/2015/02/Simple-RF-Power-measurement-W7ZOI-W7PUA.pdf(see later in the PDF)

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2020, 11:51:35 am
content: Quote from: m3vuv on March 22, 2020, 04:50:18 pmAnd afterwards it will be 50ohm//20pF.That non-ideal termination will, in the usual way, introduce a frequency-dependent VSWR. The exact VSWR at any frequency will depend on the cable's impedance (many are 52ohms), the length of the cable and, to a small extent, the cable's loss.There are online calculators for VSWR, or you could run a quick simulation.BTW, bd139's points are sensible.

author: bd139
author_rank: Super Contributor
timestamp: March 23, 2020, 12:09:31 pm
content: BTW I measured input VSWR with a crappy RS inline terminator and my NanoVNA and it's under 1.1 for 1M/18pF input for entire HF spectrum which M3VUV is interested in. Same with the T termination and larger load (under 1.2 because I smoked the load a little bit)Things get a lot more "interesting" around 70MHz or so which is where that scope is useless anyway. Probably better with a spectrum analyser and some hefty attenuators or taps then.