Well it was only a matter of time (a long time though) that one of the big three Amateur Radio Suppliers would follow the ANAN "PureSignal" crase.
So what is this Digital Pre-Distortion or DPD for short anyway.
Well, a tiny winey bit of techo talk first.
Every radio has an RF amplifier. This is a component which does add to the performance, and as such to the communication distance of the radio. Sometimes we use an external amplifiers to increase the performance of our radio and even though we call our amplifiers "Linear-amplifier", the RF amplifiers is quite a non-linear device. This nonlinearity creates spectral growth (IM) which leads to interference of adjacent channels.
Please note that the above is a very, and I really mean a very simplified view.
DPD is a technique to linearise an amplifier, which is basically a mechanism to improve the cleanliness of one's transmission, i.e. to reduce the spectral growth, also known as intermodulation (IM) products.
If you like to get a better understanding of DPD, checkout this description, an excellent read.
To enable DPD on the IC-7610 we need to update the Firmware of the radio to the latest firmware version, Ver. 1.40 available from ICOM Japan.
And before we go any further I'll answer the most important question first!
"So is it working?"
Yes!
To show you the difference between DPD and non-DPD I've created a few screenshots using 100W into a RF Dummy Load and using my SDR-IQ as a Spectrum Analyser. The transmitted output power of the radio has been set to 100W for these tests.
Pictures 1 and 2 are using a two tone signal recorded and installed onto the IC-7610 TX Voice memories.
Picture 1: 2-tone audio signal, DPD OFF
Picture 2: 2-tone audio signal, DPD ON
And Pictures 3 and 4 are plain old voice signals.
Picture 3: Voice signal, DPD OFF
Picture 4: Voice signal, DPD ON
As can be seen the "crud" (IM) has been reduced quite a bit. I'd say about 20dB more cleanliness. Quite an improvement.
So onto how to set this up. But first a note:
Before we can enable DPD we need to add a RF Dummy Load to one of the Antenna Ports of the Radio.
This Dummy Load needs to have a reasonable low VSWR of at least 1.5:1 across all the bands that we would like to use with DPD.
So if we go into the SET Menu we should see a new Menu entry. The DPD Adjustment Menu as depicted in Picture 5. (Only after the installation of the Firmware Ver. 1.40)
If we select the DPD Adjustment Menu we have the option of selecting three different adjustments. Since I don't have an ICOM PW-2 I did select the IC-7610 Single Adjustment.
Picture 6
If you are the owner of an ICOM PW-2 you would follow the setup/adjustments as per the PW-2 manual.
NOTE: At this stage I have not found a way to integrate my Amplifier into the DPD loop. It is fairly simple using only a TAP to feed an AC (RF) voltage back to an ANAN radio for DPD (Pure Signal) however, this doesn't seem to be the case with the ICOM. It looks like a little bit more trickery is involved to make the feedback loop work with a non-ICOM amplifier.
However, it is still a worthwhile improvement to utilise DPD only on the Radio.
Remember:
So by cleaning up the signal to feed your amplifier you will gain lots of brownie points within the community.
The next step is to run the adjustments per band you'd like to operate and use DPD on.
Picture 7
As you can glean from Picture 7, I have run the DPD adjustments successful on most of the band I will operate on. This is indicated by a tick and pawn icon next to the Band designator.
And you're done.
Try it and be one of the "cleaner" signals on our bands.
Additionally, if you don't have DPD the following most likely will help you to become one of the cleaner signals on the band. To reduce this nonlinearity, we could operate our amplifier at a lower power so that the active device in your (Linear)-amplifier (the power transistor or valve) will operate within its linear region.
Oh, and if you do not believe me maybe the below quote from the ARRL-LAB will convince you.
Excerpt: (Page 38 - Lab Notes: RF-KIT RF2K-S)
Update:
The below is my signal on 20m:
