Signal tracing with a scope

[JEAN-FRANCOIS MENARD-2]

Ok..... I see that on my oscilloscope I can have the "True RMS readout
(Vrms ac)".... is it the same thing as we get when we take a mesure
using the RF probe that we built in the K2 kit ???

The RF probe that we built in the kit should convert RF into Vrms when
we set the DMM in DC 20V-30V.... and in the "Signal tracing" chapter,
we can take many readouts of certain part of the K2. With the RF probe
using the DMM, I have correct values in the range that they mentioned,
but using my scope for the same reading, the values are not same, even
sometime not near of.

Maybe I missed something or maybe I mixed values and unit used... but
"True RMS readout Vrms ac" is not the same as we get using the RF probe
with a DMM ???

Or is there any conversion or formula I could use to have the correct
answer using my scope ???

Thanks for your patience !!!

73


===============================================
Jean-François Ménard / VA2VYZ
[hidden email]

Club d'Astronomie Amateur de Sherbrooke
Club Radio Amateur de l'Estrie

Mon site web personnel : http://homepage.mac.com/jfmenard
===============================================
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[Don Brown-4]

Hi

Your RF probe is calibrated to measure RMS volts, the scope will measure peak to peak voltage. To convert from peak to peak you must first divide by two to get peak volts then multiply the peak voltage by .707 to get RMS volts. This is only valid for sine waves but RF carriers are sine waves so the conversion will work. You can make the conversion in one step by dividing the peak to peak voltage displayed on the scope by 3535. Another way to look at it is your scope will measure 2.829 times higher than your RF probe on the same signal.

When measuring AC signals I set the position to the center division mark with the probe grounded. Set the input coupling to AC. Then connect the probe to the circuit and adjust the volts per division so the trace is on screen. Count the number of divisions above and below the center line (or the maximum to minimum). Multiply by the volts per division setting and multiply by 10 if you are using a X10 probe.

You should almost always use a X10 probe when working around most circuits to keep from loading the circuit under test. A scope without a X10 probe has a 1 meg ohm input resistance with about 20-50 pF in parallel. The X10 probe multiplies the resistance up to 10 meg ohm and reduces the capacitance to 2-5 pF. It actually divides the voltage on the probe tip by 10 before sending it to the scope input. It is called a X10 probe because you multiply the reading on the scope by 10 when using one.

Of course your DC meter and scope will measure the same if you are measuring DC. To measure DC with a scope first ground the input and set the position to a convenient division mark on the scope. For positive voltage I set to one division up from the bottom. Set the input coupling to DC. Connect the probe to the circuit and adjust the volts per division so the trace is on screen. Count the divisions the trace moves up and multiply by the volts per division setting and times 10 if you are using a X10 probe.

Many scopes have a ground position on the input coupling switch. This does not ground the input at the BNC connector but grounds the input of the amplifier with the input disconnected. This is convenient to set your trace to a reference position without needing to disconnect the probe from the circuit. Just switch the coupling switch to the ground position and set the vertical position to a convenient reference mark on the scope graticule then switch to AC or DC to make your measurement.

Don Brown
KD5NDB


  ----- Original Message -----
  From: JEAN-FRANCOIS MENARD<mailto:[hidden email]>
  To: [hidden email]<mailto:[hidden email]>
  Sent: Sunday, June 20, 2004 6:07 PM
  Subject: [Elecraft] Signal tracing with a scope


  Ok..... I see that on my oscilloscope I can have the "True RMS readout
  (Vrms ac)".... is it the same thing as we get when we take a mesure
  using the RF probe that we built in the K2 kit ???

  The RF probe that we built in the kit should convert RF into Vrms when
  we set the DMM in DC 20V-30V.... and in the "Signal tracing" chapter,
  we can take many readouts of certain part of the K2. With the RF probe
  using the DMM, I have correct values in the range that they mentioned,
  but using my scope for the same reading, the values are not same, even
  sometime not near of.

  Maybe I missed something or maybe I mixed values and unit used... but
  "True RMS readout Vrms ac" is not the same as we get using the RF probe
  with a DMM ???

  Or is there any conversion or formula I could use to have the correct
  answer using my scope ???

  Thanks for your patience !!!

  73


  ===============================================
  Jean-François Ménard / VA2VYZ
  [hidden email]<mailto:[hidden email]>

  Club d'Astronomie Amateur de Sherbrooke
  Club Radio Amateur de l'Estrie

  Mon site web personnel : http://homepage.mac.com/jfmenard<http://homepage.mac.com/jfmenard>
  ===============================================
  _______________________________________________
  Elecraft mailing list
  Post to: [hidden email]<mailto:[hidden email]>
  http://mailman.qth.net/mailman/listinfo/elecraft<http://mailman.qth.net/mailman/listinfo/elecraft>
  You must subscribe to post.
  Subscriber Info (Addr. Change, Unsub etc): http://mailman.qth.net/subscribers.htm<http://mailman.qth.net/subscribershtm>
  Elecraft page: http://www.elecraft.com<http://www.elecraft.com/>
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
http://mailman.qth.net/mailman/listinfo/elecraft
You must subscribe to post.
Subscriber Info (Addr. Change, Unsub etc): http://mailman.qth.net/subscribers.htm
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