[ASK] Scope's 50 Ohm input mode, why cheap scopes don't have it ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: April 21, 2012, 08:25:59 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 09:24:04 am
content: 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.Afixed50 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 21, 2012, 09:39:35 am
content: 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.

author: Unknown
author_rank: Unknown
timestamp: April 21, 2012, 09:51:15 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 01:12:33 pm
content: 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,fixed50ohm inputs reduce the versatility of the instrument,& for normal use ,especially by a beginner,are unnecessary.

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2012, 01:25:21 pm
content: You want to keep the impedance constant to the real input ( the first amplifier )So this means from your pickup point , through the coax, through the bnc or mxc ( hf scopes do not use bnc's , or specially modified bncs with less dielectric / not stupid plastic but torlon or even kapton as isolator . Look at the bnc of a 1 ghz scope.. It does not look like the bnc of a 10MHz scope ... Ill take a picture of mine and post it. Notably different.... ) all the way on the pcb in the scope to the first active element.So ,adding the terminator outside fools you , because the last stretch , in the connector , on the board is not covered.Simply adding a switch is no solution either. It has to be done at the right place and in the right way. In 'real' scopes this is done in the front end hybrid or chip. Wing wang pong scopes... May be a simple through hole relay... Good up to a few 100MHz.. Above that ... Needs self wiping contacts and shielded switches.As for scopes with a fixed 50 ohm input. The machines i have at work are in that category. Once you hit the 2GHz and above machines it is fixed 50 ohm... No high impedance option, not even switchable.... Only to be used with active probes. Amplifier in the tip , 50 ohm all the way in to the digitizer.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 02:12:14 pm
content: The OP,as far as I know,just wants to know why some (most) Oscilloscopes do not have 50 ohm terminations.In my reply,I pointed out the main reasons why.I also conceded your point on continuity of impedance.I still maintain that for most normal work,an ordinary 'scope with an external termination is adequate.The OP is unlikely to require specialist 'scopes like you use at work.Most UHF work outside your special area is done using Spectrum Analysers in the frequency domain.These devices commonly operate up to & beyond 2GHz,they use "N" connectors with plain old PTFE insulation(not "stupid plastic"orkapton) and they are made by HP/Agilent,Tektronix,Anritsu,Rohde & Schwarz,etc,NOT "Wing wang pong"!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 02:51:09 pm
content: even with my wing wang pong scope, long 50ohm coax cable need to be terminated with 50ohm externally (at the scope's bnc input). otherwise the signal will wing ping pong in the coax back to the source. just an ack.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 21, 2012, 03:00:07 pm
content: Quote from: vk6zgo on April 21, 2012, 02:12:14 pmPsst... Your age is showing. 2 GHz is no longer the exclusive domain of RF signal junkies. Most manufactures entry level 'pro' scopes go up to at least 1 GHz, and there are lots of general purpose scopes in the ~ GHz range.But yes, a cheap low performance low bandwidth scope doesn't benefit much from a integrated terminator compared to an external 'tee' with a 50 ohm terminator. Up around 500 MHz, you want integrated terminator. A good switchable termination should remove the 1 MOhm resistor and its compensation cap from the signal path, although I confess I don't know how many scopes meet my definition of 'good' here

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2012, 03:50:51 pm
content: I work in the harddisk industry. all digital stuff. Take the transceiver eye diagram of a usb3 or sata cell ? get the 20Ghz scope out .... it's all controlled impedance , matched line length stuff.Wanna look at the write signal going to the head of a harddisk ? that a 4 Ghz analog signal.. better have a real scope.and mechatrommer also has a valid point.Same goes for people using any kind of active probe ( detector probes, differential probes etc, current probes ) . these things all have built in amplifiers in the head and do need the 50 ohm termination on the back end.So , why fidget with external 50 ohm feed throughs . You are injecting 2 connectors. cable - bnc - feed through - bnc - scope. all the more risk of trouble.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2012, 01:14:57 am
content: Quote from: free_electron on April 21, 2012, 03:50:51 pmBecause they are commonly used by thousands of people doing general Electronic work without any problemsIn Spectrum Analyser use ,external attenuators are commonly used without these problems you are worrying about.I never disputed that Oscilloscopes with responses of the kind you are referring to do need 50 Ohm inputs.Many perfectly adequate 'scopes do not have 50 Ohm inputs,& some lousy 'scopes do,so for general use I stand by my comment that they are "a bit of a gimmick".

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2012, 01:27:48 am
content: Quote from: ejeffrey on April 21, 2012, 03:00:07 pm

author: BravoV
author_rank: Super Contributor
timestamp: April 24, 2012, 04:33:03 am
content: Thanks for the responses, I think I got it now, if the context is for "casual" measurements , basically even the cost or design complexity difference is trivial, its very unlikely the cheap scope owners buying an active probe is almost none as Mike said.