Pure Signal & external amplifier

DL5RBW
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Joined: Tue Sep 25, 2018 5:37 pm

Pure Signal & external amplifier

Postby DL5RBW » Fri Feb 15, 2019 11:45 am

2 weeks ago I built an rf-sampler to include my external amplifier in the predistortion loop. Without the amplifier in the loop but driving it with ANAN´s Pure Signal enabled I have seen 3rd order IMD down about 45 dB which is good enough any time however I wanted to push the envelope and find out what is possible. The rf sampler is designed for -60 dB and gives me a 0 dBm sample (for 1 kW) which is pretty much in the ballpark of what I measured at the coupler output on the ANAN (200 W output give me approx. + 4 dBm). The coupling factor of the rf sampler is flat from 1 to 58 MHz.

I didn´t see any improvement with the amplifier in loop or not so the question is whether there is any limit in phase and/or amplitude that once exceeded the software cannot correct anymore? With limit I mean the difference in (electrical) line length of the interconnecting cable (transmitter to amplifier) and the length of the cable from the amplifier output to the point where the rf sample is taken. Should they be equal if possible? In NR0V´s presentation it is pointed out that this, among other, is a critical factor which makes me believe that very different lengths of these two cables make it impossible for Pure Signal to work properly. Otherwise it should not matter -UNLESS there is a certain limit- as the software simply looks at the output sample and correct the input signal accordingly to make the ouptut signal maximum linear. I only ran a test on 20 m and didn´t try lower frequencies where at least the phasedifference between the cables could be within range again.

Another explanation could be that -45 dB is the maximum that can be achieved with the combination radio & amplifier in the loop. However as I get the -45 dB already with a clean driving signal and the amplifier out of the loop I think there should be some headroom for improvement left.

Any ideas or suggestions?
K9RX
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Re: Pure Signal & external amplifier

Postby K9RX » Fri Feb 15, 2019 2:06 pm

I have an 8000DLE, Expert 2K amp with a Xytronics coupler and routinely see -55 to -60.
Your findings seem low to me ...

Gary
K9RX
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w-u-2-o
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Re: Pure Signal & external amplifier

Postby w-u-2-o » Fri Feb 15, 2019 3:08 pm

First, read this post on PureSignal external feedback levels:

viewtopic.php?f=20&t=2384

At 1KW max. I suggest you want a -50dB coupling factor for optimum operation, not -60dB.

Second, differential line lengths will make no difference (hah, a pun!) It will merely cause a constant phase offset at the output. One does not care what that constant is, just as long as the amp response is phase linear. If all phases are linear plus 10 degrees then they are all still linear. I apologize if that is as clear as mud, but I hope this short answer works ;)

73,

Scott
DL5RBW
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Re: Pure Signal & external amplifier

Postby DL5RBW » Sat Feb 16, 2019 10:58 am

I was aiming at a +10 dBm level at maximum output however wanted to start on the low side that´s why I included the additional 10 dB pad. The + 10 dBm are nothing magical but I found this level somewhere in wikidot/anan or the HPSDR website. Anyhow to make sure I measured the output level of the coupler on the ANAN-8000 before starting and found

+ 1,17 dBm for 100 W output
+ 4,17 dBm for 200 W output

As Pure Signal already works fine with 50 W output (equaling a level of -2,17 dBm) I was striving for 0 dBm @ 1 kW which is also what I measured at 1 kW output. The automatic attenuator adjusted itself to about 7 dB which I thought should be fine and verifying there is some headroom. Removing the 10 dB pad would raise the automatic attenuator level to 17 dB - even more headroom but maybe not necessary (?)

I assume that Apache Labs level calculation is correct (as is proven by the effect of Pure Signal on IMD) so with 0 dBm I should be right in the ballpark although 1 or 2 dB on the conservative side. Obvisouly the ANAN itself is doing fine with levels at least from approx. -2 dBm to + 4 dBm.

As I have a switchable attenuator around I will remove the 10 dB pad and go for 4 or 5 dB additional attenuation which will make it 49 dB (the actual coupling attenuation of the rf tap) + 5 dB. This should be plenty for Pure Signal to work.

I too think that phase difference should not be a problem as with diversity the phase can be shifted any amount between 0 and 360 degrees and probably the same or a similar algorithm is used for the phase correction in Pure Signal. Actually what is important is to have a sample of the output signal, a reference how it should look like and correct the amplitude and phase accordingly.
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w-u-2-o
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Re: Pure Signal & external amplifier

Postby w-u-2-o » Sat Feb 16, 2019 11:56 am

Again, you want +10 out at max. power. Read point #3 in the link I posted above.
K9RX
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Re: Pure Signal & external amplifier

Postby K9RX » Sat Feb 16, 2019 1:30 pm

just to be sure since this discussion got off to the coupler and not the IMD results ... the -55 to -60 I posted is my IMD results in db.

Gary
DL5RBW
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Re: Pure Signal & external amplifier

Postby DL5RBW » Sat Apr 06, 2019 5:48 am

Finally I found the time to roughly translate my measurement notes for the following summary:

The directional coupler in the ANAN has a coupling factor of about -49 dB (as measured with lab grade equipment). With 100 W output the automatic attenuator levels itself at -10 dB while transmitting which means the “perfect” or “tailored” coupling factor would be -59 dB (for 100 W). A -59 dB coupler would need 0 dB attenuation for 100 W however output powers <100 W would eventually not develop enough level for PureSignal to work correct or at all.The 100 W output power level therefore can be used as a reference and every other power level could be scaled to this level. In other words if you want to include your 1 kW amplifier in the predistortion loop (a 10 dB increase referred to 100 W) you have to add 10 dB of additional attenuation. There are 2 ways to accomplish this: external to keep the full dynamic range of the automatic attenuator or internal (= auto attenuator) with the fact that you limit your dynamic range should you decide to run as much output power (approx.. 12 kW!!) to exceed the maximum automatic attenuation level of 31 dB. I somewhere read the maximum permissible level for the predistortion input is +13 dBm – whether this is true or not I couldn´t verify. However accepting this information and taking a little bit of a safety margin I aimed for a maximum level of +10 dBm at the predistortion input port under any circumstances. The automatic attenuator level range of the ANAN is 31 dB which means a power ratio of 1258. In other words if you use an external -49 dB coupler you restore the situation as it would be by using the ANAN´s internal coupler. However the external coupler could (and should) be located at the output of an amplifier as this includes the amplifier in the predistortion loop and makes it really what it pretends to be: a LINEAR amplifier! Going back to the -49 dB coupler as explained above with 100 W the needed 10 dB of attenuation comes from the automatic attenuator of the ANAN. That means there are still 21 dB of attenuation range left (31 dB – 10 dB). 100 W + 21 dB means about 12 kW output maximum power. The minimum power that predistortion will work with is 10 W because we can sacrifice the 10 dB of attenuation that are in line with 100 W. So with a -49 dB coupler one is ready for employing predistortion from the 10 W to 12,6 kW level without any additional circuitry. The predistortion circuit won´t be able to differentiate whether a certain level comes from a -49 dB coupler + 10 dB attenuator or a -59 dB coupler. I personally use a -49 dB coupler and a switchable (1 dB & 10 dB steps) attenuator to keep the input level at between 0 dBm and +5 dBm for maximum power output. As I very seldom run 10 W or less there is always plenty of level for the predistortion to work properly without the risk of damaging anything should I forget to manually increase the attenuation with increasing power – the remaining 20 dB+ headroom of the automatic attenuator takes care of it! The result is that 3rd order intermodulation with the amplifier is down by 55-60 dB now supporting that the calculations are correct.
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w-u-2-o
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Re: Pure Signal & external amplifier

Postby w-u-2-o » Sat Apr 06, 2019 1:17 pm

You results match up with those presented here:

https://apache-labs.com/community/viewtopic.php?f=20&t=2384

With three exceptions.

First, it is better to drive the feedback signal as high as possible within the bounds of safety not just for the sake of achieving a wider range of output power levels that can be corrected, but also to obtain the highest feedback-to-internal crosstalk ratio possible.

Second, and more important, using a 49dB coupler at power levels in excess of 1500W could be dangerous. The reason is that one cannot rely on the step attenuator to reliably protect the preamp. At 12KW, if the step attenuator is at 0dB for any length of time, for instance if PureSignal is turned off, the preamp will see over 21dBm. This could damage the preamp (although it is fine for the step attenuator).

Third, those numbers are only good for Orion MKII based hardware (7000 and 8000 models). The preamps in those units have 14dB of gain. In all other models the preamp has 20dB of gain, so the numbers will be different by 6dB.
Danie
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Re: Pure Signal & external amplifier

Postby Danie » Mon Jul 01, 2019 3:08 am

Hello, I going to do my best with my writting because, I'm french from Montreal, I got my Anan MK II 7000 dle with no pc, so my question is, I'm using a coupler extern with an Amplifier Acom 2100, everything are correcting with puresignal everything is green, but I got nothing on ATT on Tx all time is 0 for correcting, so I check all pl-259 I check everything, so maybe I forgot something in setup HPSDR ?????

I brought my anan on another hamradio and it was giving an ATT correcting like 8-9-10 so I thing I missing something in setup in HPSDR I'm using v3.4.9.

Thank a lot to your help,

ve2da
Danie
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Last edited by Danie on Tue Jul 02, 2019 3:47 am, edited 1 time in total.
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w-u-2-o
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Re: Pure Signal & external amplifier

Postby w-u-2-o » Mon Jul 01, 2019 9:03 pm

Danie,

In Setup > General > Ant/Filters > Antenna you need to make sure that ATT on TX is ON, and RX BYPASS on TX is ON.

Finally, you need to have the coupled output from the coupler attached to the BYPASS input. Do not exceed +13dBm on that input.

73,

Scott

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