I originally intended to title this "How to measure 3rd order distortion on your RXTX". But my reason for doing this work was to find an objective way to determine how much power can be pulled from the RXTX without causing objectionable interference and distortion. It turns out the most accepted way to do this is to adjust for proper 3rd order distortion. If you look in the ARRL handbook it says you need a spectrum analyzer to do this
before you move on to the next post
you have one! At least if you have a Softrocks you have one that is good enough for this exercise.
Here is what you need:
A two-tone test generator: wait!
.. Rocky works just fine!
A receiver that can interface with HDSDR on a second computer (WHAT? You don't have an RXII LF? You are missing one of the best diagnostic tools for Softrocks and other ham equipment as well
. I recommend you order one up from Tony pronto! You wanted another project anyway, right?
Basically you set your RXTX up to transmit into a dummy load and the HDSDR receiver to tune the signal. Using the zoom control on the HDSDR spectrum display, zoom in until you can see only about 10 KHz on either side of your received signal. In Rocky, transmit a single tone test and note the level of the signal on HDSDR. Note you can zoom the vertical scale too so you can get 1-2 dB resolution in the level. Then in Rocky transmit a two-tone test signal. Note the level of the two center tones and then also the level of the two next tones going left and right from the center. These second tones going left and right on the display are the "3rd order distortion" products. ITU says these should be 30 dB less than the single tone level. Note the two center tones are normally 6 dB less than the single tone.
Your transmitter is properly adjusted with respect to 3rd order distortion if the second tone on either side of center is 30 dB less than the single tone. All other things considered this would be the "safe" operating level for an RXTX. It can be argued that this does not apply to single tone modes like CW and PSK. However, for SSB operation, especially if you intend to amplify the signal, you probably want to find this "correct" operating level.
I have four RXTX and one Peaberry at the moment. Using this tool I measured 3rd order products at maximum output ranging from -22 to -26 dB. Not good enough! You should reduce the drive level on the transmitter, by reducing the audio output from the PC or by reducing the "IQ level out" in whatever software you are using, until the difference between the single tone level and the second tones on the two tone display is 30 dB.
On my five rigs which range in max output power from 0.8 to 2.5 (!) watts, I found the 30 dB point to be generally around 1Watt
. higher than I actually expected.
I hope this helps. Happy to answer questions or accept input for where i messed up.
Stealth Amateur Radio from Beijing
CW, BPSK31, and WSPR on 40/30/17/15/12/10 Meters