IM improvements using DPD with a SSPA

The question that a lot of users seem to have on their mind is "What benefits, if any do I get by using ICOMS DPD in conjunction with a Solid State Power Amplifier (SSPA)"

Well, I measured a 6dB improvement. 

Measurement setup:

• SPECAN: SDR-IQ
• TAP: VK5HW (50dB)
• ATT: 20dB
• PWR: 1000W
• ANT: Dummy Load (RL 28.3dB)
• Software: SpectraVue V.3.44 Beta 0

DPD off

Two tone test:

Result:

1st Value    -31.311dBc

2nd Value    -31.455dBc

3rd Value    -52.398dBc

4th Value    -53.545dBc

IM Max     -22.2dBc

IM Avg     -21.6dBc

NOTE: 1st and 2nd value are the two tones (fundamentals) and 3rd and 4th value are IM3.

Look at IM3 and IM5, they are nearly the same in strength.

And the below using my Voice Caller calling CQ:

Result:

1st Value    -23.996dBc

2nd Value    -31.168dBc

3rd Value    -49.526dBc

4th Value    -53.689dBc

IM Max    -29.7dBc

IM Avg    -24.0dBc

DPD ON

Two tone test:

Result:

1st Value    -31.168dBc

2nd Value    -31.311dBc

3rd Value    -58.996dBc

4th Value    -59.139dBc

IM Max    -28.0dBc

IM Avg    -27.8dBc

Well, we do see about a 6dB improvement on the two tone test which is better than a Mosquito fart. But I believe the below is quite a good example on how well it improves your VOICE signal!

And below, using my Voice Caller calling CQ:

Result:

1st Value    -20.840dBc

2nd Value    -27.582dBc

3rd Value    -57.750dBc

4th Value    -60.143dBc

IM Max    -39.3dBc

IM Avg    -34.7dBc

On average we see an improvement of 7dB again better then a Mossi fart.

So I'd say any reduction in IMD, and be it only 6dB, is an improvement. As such, I would recommend enabling DPD whilst using an RF-Amplifier however, YMMV ....

ICOM IC-7610 Firmware Ver. 1.42 DPD IM measurements

A quick update on my IM measurements for the ICOM IC-7610 with the new Version (V1.42). This Version is the third installment for the '7610 and this one has the I&Q output fixed. I've not tested this but I believe that quite a few forum members have done so and are over the moon.

Here are a few IM measurements I made after the Firmware upgrade.

TRX: ICOM IC-7610

SPECAN: SDR-IQ

SOFTWARE: SpectraVue 3.44 Beta 0

TRX Power: 35W

The above shows the output without DPD. About 23dB 

NOTE: This is 6db below PEP. 

Here we see the same two tone signal with DPD enabled, a nice 54dB down on IM3 and all other IM products are below -140dB. Again, this is 6dB below PEP. So pretty darn good if you'll ask me.

And so the question remains, is it worth to enabled DPD even if we would use an amplifier? I'd say definitely. Using an Amplifier with a much cleaner exciter would give you a cleaner signal (yes yes it depends on a few other things but hope prevails ....)!

Remember GIGO (Garbage in = Garbage out)! This is also true for RF amplifiers not only for Computer Systems.

Nice going ICOM.

See the below pictures how my signal looks over the air. Solid state Amp and DPD enabled. 

• 20m
• 40m

ICOM IC-7610 Digital Predistortion

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)

Picture 5

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: 

Garbage into the Amplifier means a lot of Garbage out of the Amplifier.

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.

Who would have thought that that is an option. 

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)

shows that a slight reduction in power makes a significant difference in the cleanliness of the transmitted signal, improving the third-order IMD performance to –45 dB PEP.

Update: 

The below is my signal on 20m:

And on 40m: