EMRFD Message Archive 11980

Message Date From Subject
11980 2015-12-09 20:57:03 mosaicmerc Probing RF with a 'scope
Hi all:

Since impedance/reactance  can seriously impair 'scope probe accuracy in a circuit, how does one use a 'scope to probe, say, an RF amplifier running at 250Mhz?

If the probe impedance at 250Mhz does not match the test point impedance, then how does one accurately sample what's going on?

11984 2015-12-10 08:04:54 Graham / KE9H Re: Probing RF with a 'scope
Scope probes are intended to be bridging inputs, with impedances so high that they do not load the circuit being measured.
This is also true of Voltmeters, etc.
The assumption may not always be true in high impedance circuits.
It is generally true for 50 Ohm RF circuits.

It is not a matching of impedances for transfer of power problem. Just the opposite, you don't want to transfer any significant power into the front end of the scope.

Beware the Voltage limit of scope probes.  Most 10X probe inputs are around 40 Volts maximum. Any transmitter over a few Watts output can destroy a regular 10X scope probe.  You might need to go to a 100X scope probe, which are typically rated for 1000 Volts.

--- Graham

11985 2015-12-10 09:04:31 Rod Re: Probing RF with a 'scope

One method that can be used at  HF is to use a 20 db coupler, with the low-Z side facing the scope.
Add a 50 ohm termination at the scope input so the coupler 'sees' 50 ohms.

The high Z side can be used to probe many circuits without loading them drastically.
If you put a blocking cap at the hi-z input, you can even probe active circuits which
have DC present, like the collector of power amplifier, or driver stage.

20 db couplers are easy to make, and are non-critical, with many examples
on the internet.



11986 2015-12-10 09:21:45 dixonglennb Re: Probing RF with a 'scope
A simple way that works at 250 MHz is use a very small capacitor in series with your probe tip.  The size of the cap can depend on the impedance of the node you are probing but 1 pF will work in most cases.  This forms a capacitive divider with the capacitance of the probe so reduces the sensitivity.  You can probe a known voltage to find the gain reduction (a simple with/without capacitor on a signal generator). An insulated wire wrapped around the probe tip (without the 'witches hat')will work too.  The probe loading will be essentially the value of the little capacitor.
Be careful that your ground lead is very short...use the little spring ground that fits around the probe tip.

Glenn AC7ZN
11987 2015-12-10 09:45:30 Nick Kennedy Re: Probing RF with a 'scope
As others have said, you don't want to match the circuit impedance; you want a very high probe impedance so as not to disturb the circuit. Typical probes have plenty of series resistance but too much shunt capacitance for very high frequency circuit probing.

Sometimes active circuits using FETs are added ahead of the probe. A simple method from a Win Hill posting looks interesting. I'll just paste in the message rather than trying to paraphrase and getting it wrong:

PJO wrote...
> I'm looking to build a simple buffered low capacitance scope probe
> for looking at low voltage signals up to about 100MHz. Ultimate
> accuracy is not so important as preserving the wave shape and not
> loading down the signal being tested - this would mostly be used
> for troubleshooting 3.3V and 1.8V digital systems. Although these
> things exist from manufacturers such as Tektronix and Lecroy they
> are fantastically expensive!
 They can be expensive.  I suggest a simple 500-ohm low-capacitance
 probe made from a resistor and some thin RG-174 coax cable.
.        ___    ________________BNC
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11988 2015-12-10 15:00:58 Brooke Clarke Re: Probing RF with a 'scope

It only works if the probe impedance is much higher than the circuit. This assumes proper terminations.
A 1 Meg Ohm probe is much higher resistance than most circuits (not not all, for example crystals), but the capacitance
of most passive probes is typically large compared to many circuits, hence the FET active probes, which are pricey.

Have Fun,

Brooke Clarke, N6GCE

emrfd@yahoogroups.com wrote:
> Probing RF with a 'scope
11991 2015-12-10 16:36:28 Andy Re: Probing RF with a 'scope
The input resistance of typical X10 scope probes starts to become much smaller than ~megohms when you get up into RF, because the dielectric loss of the probe comes into play.  So it's not only the capacitive load.

Believe it or not, the moderately low impedance (by comparison) of a 450+50 ohm homemade passive probe, can beat the X10 "hi-Z" probes for less loading due to both capacitance and input resistance.

Have you ever wondered how a X10 scope probe gets a flat frequency response despite the fact that the cable itself is not terminated, which should make it have serious reflections?  Not talking here about the compensating trimmer cap in the divider.  The cable itself is made special and uses an unusual arrangement which I think includes cable loss and a varying inner lead diameter, to try to be "flat" without end termination.  The 450+50 ohm passive divider is a better known quantity (well, at least it is easier to understand) because the cable is properly terminated in its characteristic impedance.