Ok Guys, I’ve finally got round to testing a scope probe with the NFX Oscilloscope and SpecScope. This uses the headset input which makes it super convenient.
I bought a premade scope cable from http://www.hmb-tec.de/HMB-TEC/Scope_Cable.html which seemed pretty good quality really. I did have to modify it slightly for my HTC Hero. There was a load resistor which went from the mic (signal) input to ground on the jack end, this is for the iPhone to recognise an external source has been plugged in. This seemed to attenuate my signal far too much, although looking at other post I do no think this will be required on most newer phones. I will try and confirm this when the new Galaxy nexus comes out!!!!
There is other alternatives to this scope cable though, you could buy an adapter cable from https://www.kvconnection.com/Articles.asp?ID=165, these adaptors have there own load matching in them, so you could just convert from a standard BNC connection to the 3.5mm TRRS with adaptors.
I would always suggested checkout your headset pin assignment on your phone before buying anything. https://www.kvconnection.com/Articles.asp?ID=165 and http://pinoutsguide.com/pin_HeadsetsHeadphones.shtml are the best sources I found, but I think they have not got all the newest phones. Although I do know that most phones line up to the iPhone standard or T-L R1-R R2-GND S-Mic . I have seen the at the galaxy S does not though!!!!!!! Basically what I am saying is go careful guys, I am not responsible for blowing up you phones or buying the wrong product.
As we are on this note, please please be aware of the voltage level you are inputting into your phone! If you are making your own device I would always suggest putting some diode protection in there. I would say something at 4V4 would do it. But that is only my phone, please do not quote me on this.
It is important to note that there is DC blocking in mic input of a phone, therefore you can only analyse AC transient signals. Another thing to note is that we know the sample frequency is only 44k1, therefore anything above 22Khz will be unreadable by the oscilloscope. So those guys wanting to look at higher frequency square waves, sorry it wont be for you.
To finish it off, there’s a little video I made last night, which shows a couple of sine wave inputs. It has also shown me a a few places I can update the Oscilloscope as well, so nice updates are on thier way!!!!