Re: RF out with RF OFF: Clock feed through


EB4APL
 

Hi,

I must say that the feedthrough issue was discovered by Camille Farrougia, F4IBA, who emailed me about this. He is also building the French language support file. As Nick says, the forthcoming version will have multilingual support, the user will be able to make his own translation.

Regards,

Ignacio, EB4APL


El 09/07/2017 a las 15:19, Nick Kennedy kennnick@... [PHSNA] escribió:
 

Got an email from Ignacio EB4APL where he talks about RF output still existing with RF OFF in effect. He says it’s the 30 MHz oscillator and/or the 180 MHz clock that results from the x6 multiplication in the AD9851 that comes through. I verified it with a couple of readings using my hardware.


I thought I’d check at two frequencies and arbitrarily chose 3.5 MHz and 30 MHz. It didn’t make much difference though.


My floor with things hooked up but no RF input is about -72 dBm. 


Here with the DDS output connected to the AD8307 box' input:

  

MHz

RF ON

dBm

RF OFF

dBm

3.5

0.1

-44.9

30

-0.1

-43.0

 

So with RF OFF, I have output that’s about 27 dB higher than my measurement floor but 44 dB lower than my RF ON output level.


Ignacio says that a spectrum analyzer shows a small pip at 30 MHz and a larger one at 180 MHz. He and I are both using DDS-60 boards.


Of course, we don’t do measurements with RF OFF, but assuming that the spurs are present when RF is ON, they could have an effect. Ignacio points out that this thing shouldn’t affect measurements of lowpass and bandpass filters or crystals, but could affect highpass filter measurements.


Now I wonder what people see with eBay boards and custom filters, both with AD9850 and AD9851 chips.  Anyone?


Regarding  the “RF OFF” function, it’s accomplished by setting the DDS frequency to zero. 


**


Otherwise, there hasn't been a lot of work on the PHSNA software on either side of the Atlantic for some time. However, there are still some nice improvements by Ignacio in the pipeline. I think we've got log scale graphs with some user options to come. Also, improved handling of baud mismatches between the Arduino and PC, plus a system that will allow changing PHSNA message and button labeling text for non-English users.


Hope to have it out in a week or so.


73-


Nick, WA5BDU


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