[RTTY] RTTY spectrum analysis article

[Kok Chen]

On Jan 10, 2013, at 8:52 AM, Dave Greig wrote:

> Question. Is there a simple RF sampling device and software that will allow
> the RTTY RF signal to viewed?

Both K0SM and I have used a directional coupler for our separate work.

I think Andy uses some professional directional coupler.  I myself use the directional coupler kit from Elecraft.  I wired my coupler for -30 dB.  If you want to take this route, the Elecraft CP1 kit is $40.

The loss of the directional coupler is small enough that you can keep it in line all the time if you run low power (or wire it between your transmitter and the amplifier).  Think of it as a SWR meter without the meter :-).

You then send the tap through an attenuator to the receiver.  

Remember that -30 dB of 100 watts is still 100 milliwatts, and will overload pretty much any receiver but perhaps the Hilberling :-).  So, you need to have plenty of RF attenuation on hand.

I use a bunch of cheap F-connector attenuators that are used in the cable TV world -- they are meant for 75 ohm termination, but works perfectly well for this type of application.  A couple of resistors in a T- or Pi topology will work as well too as long as you take a little care to shield the input from the output.

The most cost effective receiver ("detector") to use is actually the SoftRock. It costs a couple of Happy Meals plus an evening of soldering and coil winding), which you feed into a decent sound card.  

If you are only watching your own signal, a stereo sound card that gives you 90 dB of dynamic range is sufficient.  For monitoring your own signal, you also do not even need a high sampling rate.  48000 samples/second will give you more than 40 kHz of bandwidth.  (If your signal spatters more than 40 kHz, you really have problems :-) :-).

There are lots of software that will display wide and deep spectrum from an SDR.  A Google search for "SDR software" should turn up many.  

73
Chen, W7AY