My current, old, cheap, scope has the standard 1Meg ohm input. Some scopes also have a 50 ohm input.
What are the applications for this 50 ohm input?
My guess is low power outputs, but I'm not sure.

Hi

Most SS RF use is at 50 ohms. It's good for Impedance matching as having the wrong Impedance will either load the cct under test or give the wrong readings (In voltage and Signal Shape).

See Fig 7 here for a graphic showing the effect of input impedance: http://www.eetimes.com/author.asp?section_id=36&doc_id=1319495

Do watch the caps in the "old" CRO. They fail just the same as radio's. My old Heathkit had 400V ones on the input, when redone, they were upped to 2kV.

Watch the modern test equipment: Some of it cannot withstand Valve radio voltages, nor RF mixed with DC.

I've crashed my digital multimeter plenty of times when trying to measure voltages in the local oscillator.

Okay .... now that I know what I should have been searching for, I'm also getting some good articles on Google. Apparently, a common question .... thanks all.

Therein lies the unseen traps; There are multi-meters & then there are multi-meters. Most of the old analogue meters will not see RF nor AC past a few thousand cycles. Many of them were designed to withstand the voltages found in valve radio.

The better modern ones have a "CAT" rating and some are cheap hobby ones that are incapable of more than just a few hundred volts, not mains, or kV. That is why I often qualify that the meter has to be able to withstand a kV or more. If you dare measure Plate to Plate on a 5Y3: for example, you could be looking at 800V, or more.

For valve stuff often a CAT rated industry type is more appropriate, not the price. Digital meters are a little bit more robust. Accuracy is another matter. Due to loading both types can lie. Where big value resistors are involved, you get a lower reading from the analogue (and that is mostly what they used in early radio) than you would with a digital & that catches the inexperienced out quite often.

One of the things that it is essential to do with Digital meters is to look at their capacity to handle RF. Some of their components can rectify high frequency, this it adds to the rest, or uses to confuse the hell out of itself and you get a false reading. The RF can, due to the close spacing's (a few atoms) actually cause a flash over (like a TIG welder operates) and that can be quite destructive and fatal to the chips.

So you select a meter not just buy it. Price is not always a guide. There is a lot of RF in the oscillator circuits.

50 ohm inputs are for testing transmitters RF output characteristics of power, modulation and frequency. They are used on Communications analysers and should have the maximum RF input power rating stated. A through, high power dummy load is used before the 50 ohm input to reduce the transmitter power to milliwatts before it goes into the device input.

Here is a nice analyser.

http://www.measurement.net.au/p/1067494/r8000b-rf-communications-analyser.htm

Hi Garyoz,
Thanks for the info ...... and your sense of humour .... $30+k .... the Carrycase was over $500 .....