EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: peetDup on July 13, 2020, 10:51:58 pm
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Hi,
I picked up an old analog oscilloscope (metrix OX712D), since it's my first scope i'm trying to learn how to use it and how it works.
I found the schematics, and there is something that bother me : on both inputs there is a 51Ohm resistor which does not appear on the schematics[attachmini=1].
What is the purpose of those resistors ?
Thank you
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Many modern oscilloscopes (both analog and digital) have a switchable 50 ohm resistor to terminate the input for use with coaxial cables. Perhaps the previous owner added them permanently for that purpose? The switch at the lower left is the normal switch to switch in a series capacitor for AC coupling, or short it out for DC coupling. Be careful about the 51 ohm resistor: the normal maximum voltage for your oscilloscope (5 V/cm times 8 cm or so) would fry that small resistor: 40 V into 50 ohms is 32 watts. 20 V is only 8 W, but the part you photographed is probably rated at 0.5 W.
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That makes sense. But why 51ohm and not 50ohm?
Since I have an 50ohm terminator, can I safely remove remove them ?
Thank you
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That makes sense. But why 51ohm and not 50ohm?
Since I have an 50ohm terminator, can I safely remove remove them ?
Thank you
51 ohms is a common value while 50 ohms is mainly available in precision resistors that are more expensive and harder to find. The difference is basically meaningless.
If you're planning to use regular probes, the added resistors could be a big problem, especially with a X10 probe. You'd probably be better off removing the 51 ohm resistors.
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That makes sense. But why 51ohm and not 50ohm?
Since I have an 50ohm terminator, can I safely remove remove them ?
Thank you
51 ohms is a common value while 50 ohms is mainly available in precision resistors that are more expensive and harder to find. The difference is basically meaningless.
If you're planning to use regular probes, the added resistors could be a big problem, especially with a X10 probe. You'd probably be better off removing the 51 ohm resistors.0
^This, 51 Ω is a standard value for ±5% and better tolerance resistors and are as stated quite commonly available. The resistors you have are ±2% tolerance so their actual value could be anywhere from 49.98 to 52.02 Ω.
TimFox's caution re: power rating is spot on...
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The OP’s first photo link clearly shows the resistor is series, not in parallel with the bnc input pin. So, power dissipation, probe error, etc is all meaningless.
Based on the schematic, it looks like an a owner modification. The red tubing used to insulate the lead - do you see that used elsewhere in the scope? If not, it supports the owner mod theory. The schematic does show R1, what appears to be a 10 ohm resistor in series with the input. But, that is after the ac/dc/gnd selector switch.
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I removed the resistors to match the schematics, everything seems to work. I'll take a closer look just in case there are other mods
Thank you all for your help
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That makes sense. But why 51ohm and not 50ohm?
Since I have an 50ohm terminator, can I safely remove remove them ?
Thank you
51 ohms is a common value while 50 ohms is mainly available in precision resistors that are more expensive and harder to find. The difference is basically meaningless.
If you're planning to use regular probes, the added resistors could be a big problem, especially with a X10 probe. You'd probably be better off removing the 51 ohm resistors.
Only with a *10 "high" impedance probe (which isn't high at, say, 100MHz - do the calculation!).
With a simple *10 resistive-divider probe (which has a higher impedance of 500ohms), then you would need the 50ohm termination at the scope.
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The OP’s first photo link clearly shows the resistor is series, not in parallel with the bnc input pin. So, power dissipation, probe error, etc is all meaningless.
Based on the schematic, it looks like an a owner modification. The red tubing used to insulate the lead - do you see that used elsewhere in the scope? If not, it supports the owner mod theory. The schematic does show R1, what appears to be a 10 ohm resistor in series with the input. But, that is after the ac/dc/gnd selector switch.
A physically small 51 ohm carbon composition resistor is often added in series directly after the BNC to suppress resonances and act as a fusible link.
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Oops--that was my mistake about the 51 ohms shunting the input--I had a problem opening that photograph.
With a 51 ohm resistor in series with the input, and < 20 pF input capacitance, that will limit the bandwidth to roughly 150 MHz, which is perfectly reasonable for such an oscilloscope, which I assume has a bandwidth of no more than 50 MHz.
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Oops--that was my mistake about the 51 ohms shunting the input--I had a problem opening that photograph.
With a 51 ohm resistor in series with the input, and < 20 pF input capacitance, that will limit the bandwidth to roughly 150 MHz, which is perfectly reasonable for such an oscilloscope, which I assume has a bandwidth of no more than 50 MHz.
Some of the input capacitance is before the series resistor as part of the BNC connector.