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I just recently acquired a slightly functional Icom IC-735 HF radio, and am trying to resurrect it.

The service manual outlines procedures that specify the use of an RF Voltmeter, which I do not own. I do have a Tektronix 2236 dual channel 100MHz analog scope. Is there any reason I cannot use the scope to obtain the Voltage measurements (at least, the ones that don't approach the BW limit of the scope)?

AI7OW
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  • If the frequency is high enough and the measurement assumes a high impedance (usually) expected from scope measurements, you might be in for a surprise. Check out this interesting video: https://www.youtube.com/watch?v=Pk7pMguQDy4. – Buck8pe Feb 01 '21 at 08:22
  • I too have a IC-735. Quirks aside, it was a good investment. It should be worth it to fix it. – AI7OW May 16 '23 at 03:20

2 Answers2

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One of the functions of an oscilloscope is actually to measure RF volts, just make sure to use the x 10 function to minimize loading on the circuit you are measuring, and keep in mind that some high impedance circuits will still be affected even if you use the x 10 function.

Andrew
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    I might add that you'll also need to apply some conversion factor for peak-to-peak to RMS, which is easy when the voltage is sinusoidal but more difficult for arbitrary waveforms. Fancier digital scopes can do RMS calculation automatically. – Phil Frost - W8II Jan 31 '21 at 23:03
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Maybe.

If measuring a circuit output that expects a load impedance, such as a 50 Ohm coax or antenna, then to measure the RF voltage at that point with an oscilloscope, you should terminate that circuit output with the expected impedance (a dummy load), and measure the voltage waveform across that impedance load with a much higher impedance probe (x10 or x100 or resistor divider).

Also make sure the peak voltages and currents are well within the specified limits for your scope input. If you exceed the power input limits, then you may end up with smoke instead of a RF voltage measurement.

hotpaw2
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    Hotpaw2, IMHO if you added something like 'Yes it can be used as an RF voltmeter' to the beginning, it would look more like an answer. – Mike Waters Feb 01 '21 at 17:49
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    I don't agree that this answer is "more like a comment" — it is definitely not asking for clarification of the question — but “Maybe” is rather vague. Perhaps you could replace “Maybe.” with explaining what conditions must be met? If the remaining paragraphs are those conditions, then say so, like: “Only if the impedance and power are considered:” – Kevin Reid AG6YO Feb 02 '21 at 00:27
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    I voted to reject an edit to this question. To the would-be editor, I applaud your willingness to help, but your edit only makes slight formatting changes, and in my opinion that isn't enough of a difference to be worthwhile, sorry! – rclocher3 Feb 08 '21 at 19:23