Oscilloscopes and signal sources

[elroy]

A couple of questions about feeding signal sources to a Siglent SDS1202X-E oscilloscope:

-- For various tests, I'm planning to input reference signals to the scope from sources such as frequency standard, a frequency calibrator, and a VNA in frequency generator mode. I gather that rather than using the scope's probes, it's common to feed such signals into a scope using cable with appropriate connectors on each end (in the case of the devices I'm working with, this would generally be SMA to BNC). Do I need to worry about anything like voltage or power levels? I assume that scopes like this can handle a range of inputs from devices of this type, but want to make sure I don't overlook anything before starting to plug anything in. Is there a general rule of thumb about voltage or power levels that are and aren't safe for a modern digital scope like this? (I obviously wouldn't, for example, feed the RF output of a 100w ham transmitter directly into a scope.)

-- As for the cable to use for connections like this, I have a lot of RG316 coax with SMA connectors on each end in various lengths, as well as SMA-to-BNC adapters. Does this seem adequate for most oscilloscope test purposes, or should I consider investing in anything else? I've seen mention made of higher-end cables such as Suhner, and wonder if that would be worth the investment.

[KrudyZ]

The SDS1202X-E only has 1M Ohm inputs.
To connect a signal source via a coax cable, you would need to add external 50 Ohm feed-through termination at the input to the scope.
The scope will be able to handle any voltage your generator is likely able to produce, so the termination will be the limiting factor. Do not exceed its power rating.
RG316 will be perfectly fine for any signals that the scope can work with, since its upper frequency limit is 200 MHz, far below the limit of RG316.

[elroy]

To connect a signal source via a coax cable, you would need to add external 50 Ohm feed-through termination at the input to the scope. The scope will be able to handle any voltage your generator is likely able to produce, so the termination will be the limiting factor. Do not exceed its power rating.

Thank you, that's very helpful. Would something like this be appropriate?

https://www.amazon.com/Pomona-4119-50-Terminator-Feed-Through/dp/B00JJ4EWU2/

[rstofer]

Yes, that terminator will work.

Sometimes people put a T fitting at the scope.  The leg of the T goes to the scope and the signal comes in on one arm and a dead end terminator goes in the other end.

Scroll down about 1/5 of the way and you'll see a 'kit' with T's and terminators along with a coax $20
https://www.amazon.com/CablesOnline-Terminator-Adapter-2-Pack-R-3006-2/dp/B00G10MKCO

[HB9EVI]

Thank you, that's very helpful. Would something like this be appropriate?

https://www.amazon.com/Pomona-4119-50-Terminator-Feed-Through/dp/B00JJ4EWU2/

that's ok for signal levels like from a FG, but don't get the idea to feed 100W for a ham tranceiver in it.

[wolfp]

The maximum rated power for this devices is abt 1W. Putting 100W on it from your TRX will cause smoke and fireworks. Use a 40dB power-attenuator and connect the output direct to the scope. If you use a cable between the attenuator and the scope use the feed -through termination at the end of the cable.

[elroy]

Sometimes people put a T fitting at the scope.  The leg of the T goes to the scope and the signal comes in on one arm and a dead end terminator goes in the other end.

Thanks, good to know about that option. I've been spending a lot of time recently on impedance matching between transmission lines and antennas, so it makes sense that this is basically the same thing. Right at the moment I'm not clear on how a dead-end terminator on a T-fitting would prevent reflected energy from going back up the coax, but I'm sure I can do some web searches to find out.

The maximum rated power for this devices is abt 1W. Putting 100W on it from your TRX will cause smoke and fireworks. Use a 40dB power-attenuator and connect the output direct to the scope. If you use a cable between the attenuator and the scope use the feed -through termination at the end of the cable.

I've heard of other ways of coupling an oscilloscope to examine RF power out of a transmitter, but just connecting it straight through using an attenuator is new to me -- interesting to hear about that.

[HB9EVI]

A termination normally gives you nearly perfect impedance matching, so basically a 1.0xxx:1 SWR.
Of course every termination or dummy has a limited frequency range which has to be taken into consideration; such simple, low power bnc terminators are available upto 1-2GHz, sma versions go >4GHz, a simple, not to expensive high power (~100W) dummy or attenuator maybe only goes to 150MHz with a SWR <1,2

If you use a dummy with an attenuated output, you have to recalculate your measurements; with a T-fitting (one side dummy, other side signal source) you have the full voltage swing on the scope

[David Hess]

RG316 works great however a thicker cable and especially one with double shielding like RG142, RG223, or RG400 is tougher.  I really like RG400 for general purpose patch cables because it is so much tougher than RG58.  My RG316 cables tend to get damaged where the cable enters the connectors or at the clamp if present.

For triggering signals, frequency response and amplitude usually does not matter so frequency response limitations of the cable and termination is seldom an issue.  BNC-T terminators work fine at 10MHz and up to at least 100MHz for triggering purposes.

High impedance oscilloscope inputs are very well protected against overload so excessive input signal levels are only very rarely an issue.  This applies to almost any high impedance input like that which can be found on a frequency counter.  50 ohm inputs can be very well protected also but this is rare today. (1)

Ground loops can be a major problem when low noise is a requirement.  Galvanically isolated triggering through a wideband transformer is very helpful with this.

(1) This is a fascinating circuit design problem and if you can protect a 50 ohm input, then the same circuit can be used to protect a 50 ohm output but nobody bothers to do this anymore.  Protection against 50 volts and higher on a timescale of nanoseconds?  Yea, I can do that.

[W0MPM]

Your RG316 should work fine. The best coax to use is the one I have a lot of.     And it's nice and flexible.

If it's a concern Pasternack (and maybe others) sell an RG316 with double shield. RG142 is fine but it's normally solid center conductor so is a little stiff for bench work. But fine if you are not going to move things around.