Re: Test for solving Spurs #ubitx

Ian Lee


Thank you for your feedback and test results. I am not an RF expert like you. My major is not RF.
I just wondered how Harmonics and Spurs can be easily tailored to the level that users want.
Looking at the circuit and the source code, I figured out a couple of things to solve, but it seems like it was my mistake.
I have one or two experiments left for this issue, but I knew my experiment was wrong, so I do not think I need to go any further.
Once this problem is solved by experts, I can expect to learn about it.

Thank you again for testing.


2018년 8월 12일 (일) 오전 12:29, ajparent1/KB1GMX <kb1gmx@...>님이 작성:

On Sat, Aug 11, 2018 at 06:54 AM, Ian Lee wrote:
learned Spurs while reading the group 's mail today.
It seems to be difficult to control with LPF or BPF because it occurs near the main transmission frequency.
And I understood that it was caused by the frequency generated to pass the 45Mhz Filter.
The spurs development is a side effect of DBM (MIXERs) operation.  The 45mhz I refer to is the IF output 
that feeds the last DBM the other signal is the local oscillator (raduino osc-2).   

The simple explanation is any time you mix two signals you get two more and those mixed with the first two give you 8 more....
IF you add the fact that those first two signals have harmonics each possible pair do as above creating more pairs of many pairs...
Some of those will be desired, others are a side effect of the process and those will have varying strength according to the 
their harmonic order. 

The specific one that is most troublesome is the 2IF-LO (the 45mhz if *2) and the LO is clk-2..  So at 28mhz
that is a LO nominally a LO of 73mhz to convert 45mhz to 28mhz.  It happens that 90mhz (2IF)- LO is 17mhz.
( you get the same this of you say 45(if)-dial frequency of 28mhz =17mhz).   It is a problem because the radio
uses very few filters to eliminate undesired results so the 17mhz signal is amplified in the power amp and
escapes along with the desired 28mhz.

The presence is not the problem, it is the intensity of it (strong).  Because the dial frequency changes 
so does the spur frequency.

There is no way to control that in software as ubitx is an analog radio (in SDR its possible) so the
analog solution is add the missing filters.

**** for those looking to pick and ding I used round numbers to keep the concept clear.  It makes zero
      difference if I go to the nearest 1 Hz as a 16.983... hz, the spur is as unacceptable.  We do not
     need to be be precise to say its not allowed as its outside any Amateur Radio band.

I read that the Spurs were slightly beyond specification.
I have experience certifying commercial electronic devices, but I am not familiar with FCC amateur radio test specifications.
The US spec is -43dbc for all harmonics and spurs.   FCC years back did this to make USA in line with the ITU standards.
In some cases its an improvement and other cases it was a relaxed number.  The one I'm not certain on is if the FCC
PART-15 standards below 30mhz  also apply for out of amateur band signals as they do for most every other device. 
If so that is stricter as its a radiated signal standard.

I also have worked in EMI/RFI engineering prior to entering the antenna engineering and development field about 9 years ago.


Best 73
KD8CEC / Ph.D ian lee
kd8cec@... (my blog)

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