Author Topic: Need the insight of a RF engineer  (Read 8257 times)

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Online G0HZU

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Re: Need the insight of a RF engineer
« Reply #25 on: May 13, 2015, 07:08:13 pm »
The two tone test signal is probably going to be too confusing to work with so just look at the (single tone)  simulation and the measured plot in my previous plots.

You can see the main wanted signal but you can also see the tone just to the left of it. This is the 3RF 2LO mixer term KJDS warned you about way back in post #9.

(29.050 *3) - (116 *2 ) = 144.850MHz. So you can see that there is an unwanted (distortion) tone at this frequency in the plot :)

It is low enough to not really worry about in this case. if it was -40dBc then I guess it would become a bit naughty to transmit a spurious like this but as long as it is lower than -45dBc then there isn't a problem. In the measured plot above it is -55dBc with -5dBm drive level.
« Last Edit: May 13, 2015, 07:12:15 pm by G0HZU »
 

Offline notaroketscientistTopic starter

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Re: Need the insight of a RF engineer
« Reply #26 on: May 13, 2015, 07:17:19 pm »
So one last dumb question then I gotta hit the books:

what is the best approach to reduce those spurs?
There are sharper knives in the drawer. I am trying to get a finer edge on mine.
 

Online G0HZU

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Re: Need the insight of a RF engineer
« Reply #27 on: May 13, 2015, 07:23:20 pm »
Assuming you want to keep the same frequency plan then you can either reduce the mixer drive level and/or try running a mixer that needs a higher LO drive level. eg level 10 or level 13 (or higher).

But there really is no need if your transverter already achieves -55dBc as per my plot.

Note if you scroll down to table 2 in the review link below, you can see that the claimed spurious spec is -56dBc with -58dBc measured. I would expect that the spurious they measured is this 3RF 2LO term.

http://hamradio.online.ru/ftp/pr0006.pdf

It really isn't a problem. You may find that your Icom radio that drives the transverter will add a few spurious terms of its own and they may be even bigger than this! But it really isn't going to be an issue in the real world :)
 

Offline PA0PBZ

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Re: Need the insight of a RF engineer
« Reply #28 on: May 13, 2015, 08:02:21 pm »
You can see that the IMD3 terms are at about -40dBc as predicted by the classic equations I posted up earlier.

I fail to see anything above -55dBc in the plot, what am I doing wrong?

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Offline KJDS

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Re: Need the insight of a RF engineer
« Reply #29 on: May 13, 2015, 08:14:35 pm »
You can see that the IMD3 terms are at about -40dBc as predicted by the classic equations I posted up earlier.

I fail to see anything above -55dBc in the plot, what am I doing wrong?



The IMD3 terms are the ones close to the two wanted carriers. The ones further out at -55dBc are IMD5, they come from the 3LO-2RF, 3+2=5 so it's IMD5. The close in ones are a 2-1 product, and 2+1 is 3 so are the !MD3 products.

Hope that's a simple enough explanation.

Offline PA0PBZ

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Re: Need the insight of a RF engineer
« Reply #30 on: May 13, 2015, 08:16:14 pm »
Hope that's a simple enough explanation.

Yes, thank you, it's clear now.
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Online G0HZU

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Re: Need the insight of a RF engineer
« Reply #31 on: May 13, 2015, 10:52:01 pm »
In case anyone else is still a bit unsure then the -40dBc relationship for the IMD3 terms then it is shown in the marked up image below. This level of (distortion) performance is predicted with the classic equation in post #18 when the mixer drive tones are at -5dBm each. The mixer has 7dB conversion loss so the plot below shows the two main (wanted) test tones leaving the mixer at -12dBm each in the analyser plot below. The two (unwanted) IMD3 tones are about 40dB below this level as predicted byt the equation :)

Also, I pasted together a schematic of the tenTec 1210 transverter from a German review of this transverter. You can see the transmit signal path from the input, through the attenuator and the 28-32MHz BPF, the mixer and then to the J310 JFET Q6.

I've circled PIN diode D8 in the input attenuator and also the L4/C13 trap in the 28-32MHz BPF and also the J310 JFET Q6. 

« Last Edit: May 13, 2015, 11:05:48 pm by G0HZU »
 


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