Re: Cool cheap oscilloscope for troubleshooting

Tom, wb6b

On Sat, Aug 3, 2019 at 02:01 AM, David Wilcox wrote:
had a slew of attenuators, tees, and other stuff

As long as you are sticking to QRP levels (12 wats or less) like the uBITX you should be able to probe the RF circuits with a 10X scope probe. That would be about 35 volts P-P at the output into a 50 ohm dummy load. Probing the PA Mosfets might be iffy. 

I have some generic P6100 (100 Mhz) probes available on eBay and such. The voltage rating (RMS?) rapidly drops from 600VAC to around 30VAC above 1 Mhz.

The scope probe will cause some loading to circuits that could affect them at RF frequencies.

For anything above a Mhz or so you want to always use the scope probe in the 10X mode. In the 1X setting scope probes only have a few Mhz bandwidth, while in the 10X mode the have the rated bandwidth (in this case 100Mhz).

What may seem interesting to some is that scope probes handle frequencies of 100 MHZ or more and they seem to violate all the rules about transmission lines and impedance matching. So how does a scope probe work at 100Mhz when it is not terminated at the impedance of the coax cable that runs from the probe to the input of the scope? The scope probably is terminating the probe cable at 1M ohm, not 50 ohm, for example. 

The secret is the center wire in a scope probe cable is a thin high resistance wire. The resistance of the wire, as it runs through the center of the cable, provides a distributed damping of the reflections that would normally be present in a normal coax cable. It may have been some genius engineer at Tektronix that figured this out. If you measure the scope probe resistance from the probe tip to the center conductor of the BNC connector you will measure 100 ohms or more. So much for scope probe trivia.

As you are just starting out, you may want to begin by trying out the low frequency, low voltage examples you mentioned before venturing off the the RF power stages of your radio. On receive, go for it. Probe the IF stages, mixers, audio, output of the frequency synthesizers, whatever you want. Even at 10X the probe may put enough of a load to detune something, or lower the amplitude of a signal, but most likely it will be fine. You will not be able to accurately measure signals above the frequency capability of the scope.

Tom, wb6b

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