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IZ6MAF
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UHSDR TX Carrier shift instead Audio Shift
« on: 26. May 2018, 08:25:44 »

The UHSDR firmware now has a good TX image rejection setting, but it can not improve carrier suppression that depends on the hardware and is of about -30/-35 db.
Of course we could add a hardware balance as it is in the Eagle TRX, but its calibration is valid for one frequency only and does not solve the problem
To avoid transmitting unwanted signals in the band away from the carrier frequency, it is possible to adapt the UHSDR firmware to switch from a TX  audio shift approach to a carrier shift approach as better described in the HB9DRI article (http://www.linkrf.ch/IQ+XT.html)?
’73 Paolo IZ6MAF

EDIT:
corrected link
« Last Edit: 26. May 2018, 09:05:20 by DF8OE » Logged
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #1 on: 26. May 2018, 09:09:27 »

I do not see there the way carrier (LO) suppression is made. It is impossible that it is only an oscillator with less phase noise - phase noise is not the reason. It is the isolation of TX mixers.

Additional, we already have "carrier shift". When we change frequency not the audio signal is tuned to new frequency, but the LO as it is described in the pereliminary paper.

What I am thinking about is to switch from "Xlate TXing" to "non-translated TXing" in all modes. The result would be that the unwanted LO signal is very near to your TX signal (in fact it is same as in conventional analog TRX). Possible impact: suppression of unwanted sideband is decreasing.

At OVI40 RF I go a complete other way for CW: CW is not generated by mixing but by "switching LO signal directly". I have developed a schematic that suppresses output in not-keyed state > 80dB and does not have any spurs (except hamonics of 3rd, 5th, 7th and so on order).

vy 73
Andreas
« Last Edit: 26. May 2018, 09:24:08 by DF8OE » Logged

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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #2 on: 13. June 2018, 20:28:25 »

Hello Andreas, I am attaching a brief summary of the tests I have done to improve the TX Carrier suppression on my mcHF/Eagle and what I hope you can do even better by optimizing the firmware
vy 73 Paolo IZ6MAF
 Carrier_suppression_improvements.pdf
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #3 on: 14. June 2018, 06:59:28 »

Hi Paolo,

many thanks for your detailed description. Yes, all you wrote is right and much of this is already implemented in OVI40 TX mixer. This is not very astonishing because all this "are not secrets" and you can find hints scattered (!!) all over the web. My prototype is running well (and that is the only difference to your proposal it does not use high precise opamp). Swapping the opamp does not result in measureable improvements. But the rest does (and concept gets simpler and better reproduceable by leaving away components)...

I am already thinking about txing without any shift but because that is not hardware and can be done at any time I have not completed my rhoughts (I am very hard working at the OVI40 RF hardware).

So let us discuss about changing to non-xlate txing!

In theory this results in a complete loss of spurs in 2 * xlate difference to your TX signal but in increasing txing "the wrong sideband". I have not done any investigations but may this be problematic?

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #4 on: 14. June 2018, 08:28:51 »

Hi,
first of all, thank you Paolo for your work! All the mods you've done are pushing old mcHF project much forward.
We can do option in menu for test (Debug menu) and have some tests against problems. Will look into the code.
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #5 on: 14. June 2018, 09:02:36 »

Hi Slawek,

it is easy I think. You must look for code which does decision in TX to send xlated or not and if you set option "TX not xlated" it must always result in "no translation". We already Tx in CW non-translated always so this code parts can be the anchors for looking.

I have tested it by manually disabled xlate in RX and TX and found no significant difference in spurs at TX. But the hardware improvements are mandatory (some of them are already in the papers from Joe Tayloe which are much older than mcHF...)

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #6 on: 14. June 2018, 09:10:56 »

Yes, ofcourse Paolo can test this idea with Xlate off without firmware change, and then we decide to add it or not to menu, ok?


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73 Slawek
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #7 on: 14. June 2018, 09:50:56 »

Yes - 100% my idea. Or - extended to: remove all xlate stuff for TX.

I only have done empiric tests (receiving my own signal which is produce by a mp3 sound source). I recognized complete removing of all spurs outside +/- 3KHz from TX frequency (except intermodulation spurs but they are very, very weak). But now "mirror suppression" is working as a suppression of "unwanted sideband" (of course). It is neccessary in non-translated mode, too but if we decide to roll back to non-translated mode we would only need ONE TX IQ setting and not two which we have actually (SSB and CW).

I am tending to always transmit in non-translated mode and remove all stuff  which is used for xlating in TX. But my tests are only empiric (no measurements done). Paolo: can you please measure TX behaviour if you switch to non translated and let us know the result? I think this would be a big step for mcHF. Not sure if Chris would accept this for his new release but it could be added to our modification page of mcHF immediately. For OVI40 it is not neccessary because of hardware concept includes your ideas already.

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #8 on: 14. June 2018, 09:59:23 »

I will also test this with spectrum analyzer, and publish some pictures here at evening.
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #9 on: 14. June 2018, 10:22:37 »

Very good. We need reliable results which are manifested by measurements.

You can test it without the hardware modification of Paolo. The only difference will be a lower suppression of carrier. All other spurs observations are portable.

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #10 on: 14. June 2018, 10:32:37 »

I already have almost all modifications described by Paolo, but without voltage bias decrease for TLV2464 and thus FST3252. I played some with potentiometers but as Paolo described - it works but only for one band.
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #11 on: 14. June 2018, 11:04:43 »

The reason is that the effect is produced by unsymmetrical capacitive elements on both switching channels. They are frequency dependent. One solution is to use switches with a much lower capacitive influence.

For OVI40 RF I am using GHz multiplexers from Potato. They are capable of multiplexing much higher frequencies and do have of course significant lower capacitive influence.

Disadvantage for mcHF:
They are not available in same package so no drop-in replacement is possible. If Chris would change the layout  mcHF can use old switches and Potato similar - package of Potato is availble for "standard multiplexers", too...

But Chris has ignored existence of Potato divider since years so I do not think this will happen.

EDIT:
We are discussing in the UHSDR board. So main interest is the firmware part not the hardware part... Ideas of Paolo and everyone are of course highly welcome and may result in an integration to OVI40 - now or later. OVI40 will live because of such contributions

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #12 on: 14. June 2018, 11:19:15 »

I quickly repeated the measurements at 3500 and 28500 khz with and without Xlate and as you can see from the images without Xlate there is a considerable production of spurious. What will they be due to?
'73 Paolo
 2018_06_14_Test_TX_Carrier.pdf
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #13 on: 14. June 2018, 11:27:28 »

That must be discussed.

Are you using TUNE function with TUNE frequency 750Hz? We must calculate what can be the reason for the spurs...

vy 73
Andreas
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Re:UHSDR TX Carrier shift instead Audio Shift
« Reply #14 on: 14. June 2018, 17:05:53 »

I am using tune, single tone, 750 hz.
It would be very useful if others could repeat the tests I performed to make a comparison.
vy 73
Paolo
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