Electronics > Beginners

[ASK] Scope's 50 Ohm input mode, why cheap scopes don't have it ?

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BravoV:
Just a noob question, understand the objective to reduce the cost, but from technical point of view, does it really need expensive front end components and / or advance electronic design/build that can make huge or significant difference in the cost of making it ?

Appreciate any comments or technical insight why they don't provide it at low end models.

vk6zgo:
A switchable 50 Ohm termination,is in my opinion,a bit of a gimmick,although some people are very taken with them.
Most older Oscilloscopes don't have them.
Unfortunately you can't just stick a toggle switch & a resistor across the input,as a switched termination needs a bit of finesse in design at high frequencies.
Special switches must be designed,or relays used,all of which adds mechanical complexity & creates possible reliability issues.
A fixed 50 Ohm termination would severely limit the versatility of your 'scope.

Through terminators are available for both 50 & 75 Ohms,which are,again in my opinion,a more versatile way of using a terminated input.
The external terminations usually have a higher power rating,but if you do cook one,you don't have to replace any bits in your 'scope.

mikeselectricstuff:
The normal reason for 50R inputs are for use with active probes, which are expensive, and for high frequencies, so users of cheap scopes don't have any need for it, and the additional cost is not justified.

alm:
The reactance of the 1 Mohm inputs becomes a problem at high frequencies, so an external terminator works just fine at 50 MHz. I don't agree that it's a gimmick. With an external terminator, there's still the input capacitance of the scope in parallel, which screws up your VSWR. Most fast scopes might have 1+ GHz BW at 50 ohms, but only 500 MHz at 1 Mohm.

vk6zgo:
The value of Xc for a 1 Mohm input is going to have a lot less effect when the 1Mohm is shunted by 50 Ohms.
That said,yes,an input stage designed from the outset for 50 ohm impedance should be capable of better frequency response.----conceded!

From what I have seen,some of the "50 ohm" inputs are,as I suggested above,just switched terminations,still using the 1Mohm input.
I still regard these as "gimmicks".

As I said earlier,fixed 50ohm inputs reduce the versatility of the instrument,& for normal use ,especially by a beginner,are unnecessary.

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