Using DSO Scope for RF Measurements

[kiheiman]

For a number of years, I used Tek 2000 series scopes for making RF measurements in transmitters and receivers.  I used scopes that had bandwidths of 60 to 100 Mhz, along with good Tek probes.  The signal levels ranged from a few mv to 100 watts.  I could measure the frequency response of the scope by attaching the probe to the output of my HP 8648 signal generator.  The response was always predictable - very flat up to the -3 db falloff point of the scope.

I now have a LeCroy WaveAce 224 scope that is supposed to have bandwidth of 200 Mhz.  If I attach the 300 Mhz LeCroy probe to my HP signal generator, the frequency response curve is all over the board - mostly flat up to about 30 Mhz, then lots of peaks and valleys up to 200 Mhz.  Signals at 300 Mhz are readable, but are about 10 db down in level. 

So I am wondering if my frequency response problem with my LeCroy is typical for DSO scopes or if my LeCroy is not working right.  I would like to get a lower cost Siglent scope to test - that should tell me if I am seeing something normal or abnormal.

My gut feeling tells me that I should have stayed with the Tek scopes for doing RF work.

Regards
Clint
W7KEC

[rhb]

I suggest using a 50 ohm cable from the SG to the DSO with a 50 ohm thru terminator if needed as a check.  The probe could be the problem.

[kiheiman]

Thanks for the reply.  I tried a direct connect 50 ohm coax cable to a 20 db pad attached to the input jack of the scope.  I set the scope to 10X and 50 ohms impedance.  The frequency response was very flat up to 200 Mhz.

I was able to finally discover the problem.  I had been using the ground lead attached to the end of the probe.  There was a metal adapter is the bag of accessories that went over the end of the probe.  The shell of the adapter made contact with the metal ground band on the end of the probe.  The other end of the adapter mated up with a female BNC jack.  Very cool adapter.  I removed the ground wire and ran a new set of frequency response checks using the probe.  The scope was set for 10X and 1 Meg ohm impedance.  The response was very flat up to 200+ Mhz.  No more peaks and valleys. I am now very pleased with the operation of the scope.

[Wolfgang]

In RF, only a dead probe is a good probe. 

Some hints here:

https://electronicprojectsforfun.wordpress.com/rf-measurement-techniques/

[Beamin]

Thanks for the reply.  I tried a direct connect 50 ohm coax cable to a 20 db pad attached to the input jack of the scope.  I set the scope to 10X and 50 ohms impedance.  The frequency response was very flat up to 200 Mhz.

I was able to finally discover the problem.  I had been using the ground lead attached to the end of the probe.  There was a metal adapter is the bag of accessories that went over the end of the probe.  The shell of the adapter made contact with the metal ground band on the end of the probe.  The other end of the adapter mated up with a female BNC jack.  Very cool adapter.  I removed the ground wire and ran a new set of frequency response checks using the probe.  The scope was set for 10X and 1 Meg ohm impedance.  The response was very flat up to 200+ Mhz.  No more peaks and valleys. I am now very pleased with the operation of the scope.

so the ground of the probe was touching something and acting like an antenna? Was the noise coming from you acting like the antenna?


I always am trying to find ways to use my scope for Rf stuff. I really should have bought a spectrum analyzer because really only rf circuits interest me and 300Mhz isn't a lot.

[jh15]

the ground lead is a wonderful antenna. don't use it much above audio.
 Is it SK jim williams? shows how to make spiral leads to ground probe end to circuit. Or your probe kit should have a thing to go to your probe shield end.