[RFI] Oscilloscope requirements for power line noise locating

Util.RFI.pro utility.rfi.pro at gmail.com
Sun Dec 30 13:19:48 EST 2012

The capabilities of the scope are determined primarily by the frequency of the signal you want to observe. I use a scope only to observe the audio output of the built-in AM/FM radio in my work van. My ears are not able to distinguish the changes in volume that occur as I drive by sources I hear at 1710 kHz. The scope is an effective signal strength indicator. Virtually any o'scope will work if you are just looking at audio. 

If you want to observe the RF envelope, you will need more complex stuff to do that. I don't believe that's either necessary or worth the effort. The only time I want to observe an RF envelope is when I'm transmitting. In that case, I use the CleanRF signal tap system (http://www.cleanrf.com) which is fed to a basic scope to provide envelope and trapezoid images for distortion analysis. But I digress...

I'm a firm believer in keeping things simple...very...very...very simple. All of the tools I use to chase RFI are designed for the purpose and have no fluff. The o'scope I use spends only a fraction of its time in my work van displaying the AM radio audio. Most of the time it's on the work bench doing more difficult jobs. As much as possible I try to buy and use tools that perform multiple tasks.

If I can hear the source on any of my portable receivers that already have a signal strength indicator, the scope is unnecessary. I rarely have to compare the "signatures" of different sources. I can usually tell if I'm chasing the right source just by listening. Purists will poo-poo my claim but I hold up my 100% success rate as evidence that I'm doing something right. 

Essentially, you want to equip yourself with tools that give you the info you need to pinpoint the source. Simple is always better and easier to use. The frequency of the source you are chasing will determine the type and capability of the tool(s) you use. 

Good luck!


Frank N. Haas KB4T
Utility RFI Investigator

Sent from somewhere in Frank's electronic universe

On Dec 30, 2012, at 12:35 PM, Cortland Richmond <ka5s at earthlink.net> wrote:

> Paul,
> Even an audio scope is good enough if you first rectify the RF to recover the envelope. I presume that's what they were talking about WRT the Sadelco (etc.) FS meter.  These old CB meters' sensitivity increases with frequency (until the sampling baod nears resonance, anyway) but you can use a toroidal style meter to get a pretty level response.
> For DIY directional couplers, check out (for example)
> http://www.hoaglun.com/blog/2010/1/16/directional-coupler-version-two.html
> Cortland
> KA5S
> On 12/29/2012 1610, N1BUG wrote:
>> I have some more questions. I have been re-reading relevant sections of Loftness' book on using an oscilloscope with a Sadelco (or similar, presumably) field strength meter but it isn't all coming together for me.
>> For this purpose, what are the minimum scope specifications for acceptable performance (bandwidth and otherwise)? The two scopes referenced in the book appear to be 40 and 60 MHz units. I don't see why that bandwidth would be needed. Is it? ...
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