Pure Signal (pre-distortion)

To Pre-distort or not to Pre-distort .... 

update 2025: More details can be found here

A few pictures about the effect of Pre-distortion (PureSignal) using an ANAN-100D. The first two pictures show IMD measurements using the in built two-tone generator of PowerSDR-mrx.The tones are at 700 Hz and 1900 Hz and the carrier is at 7.090 MHz (not visible, remember SSB). As we are using the lower side band (LSB) F1 and F2 are at 7.0893 MHz and 7.0881 MHz. The IM3 products are at 2*F1-F2 and 2*F2-F1 respectively, i.e. 7.0905 MHz and 7.0869 MHz

At Pic 1, Pre-distortion has been switched off, IM3 (the third order distortion products) approx. 30 dB down on the fundamental(s) (the two long center lines).

Pic 2 shows us the effect of Pre-distortion, IM3 is now down by about 50 dB. A nice 20 dB improvement. Let's have a look at an SSB voice signal.

Without Pre-distortion at Pic 3 we can see that the SSB signal has an audio bandwidth of approx. 3 kHz (100 - 3100 kHz) with IM3 down to about 30 dB. However, remember -103 dBm equals an S4 so we might step on a few users toe's if we would work a bit of DX next to the local ragchewer. Not a bad signal, but a pretty average signal, but lets switch Pure Signal on and have a look  .....

At Pic 4 we see the full effect of the Pre-distortion. Look at this signal, 55 dB down on IM3. Splatter reduced to about -120 dBm that is close to the MDS of the average HF Receivers Noise Floor. This should keep those RagChewers with their splatter boxes happy. Looking at  RX Signals - the good, the bad and the ugly ... some of those signal could do with pre-distortion, maybe turning the mic gain down for starters might help. Anyway, as far as I know there are two Amateur transceivers that have the ability to pre-distort, the ADAT-200A and the ANAN-[1/2]00D.. 

Next is to use Pre-Distortion with my Amplifier, an SPE1K for that I need to build a TAP/RF-Sampler.