Self Inflicted Wound

I was attempting to peak up a double tuned LC bandpass filter for the 10/11 meter bands when I looked at my scope:

The reading isn't 30 MHz as the DDS display says, it is 20 MHz.  That's not going to work, Chuck.  

I popped the cover off the DDS and bingo, there is the reason:

see the square can marked: 50.000MHz ?

This is an AD9850 DDS design by Curtis Preuss, WB2V, in QEX of July 1997.  The firmware was modified by me but only in superficial ways.  The problem is that the clock to the AD9850 DDS is only 50 MHz.  A DDS can be expected to output a decent signal but only up to less than 1/2 the clock frequency.  If an attempt is made to specify a higher frequency than 25 MHz in this case either the firmware should flag that OR ??  See the PIC code on the projects page:

http://wb9kzy.com/projects.htm 

 

The PIC asm code in a zip:  http://wb9kzy.com/siggen3b.zip  

Back to the drawing board, I suppose I could change the firmware and use a higher clock frequency (think I have some 100 MHz oscillators) or maybe I can use a VXO to generate a signal ?  We'll see. 

Anyway at least the LT1252 amp in a tin (mentioned recently) does a decent job of amplifying the DDS output.

https://wb9kzy.blogspot.com/2025/08/lt1252-as-preamp.html 

Update: I looked at the asm file, it does have a limit frequency provision which the file suggests should be 1/3 of the clock frequency or 16.7 MHz in this case, whoops !

 

Also, per this doc from Analog Devices:

https://www.analog.com/media/en/training-seminars/tutorials/MT-085.pdf  

 there will be a lower alias frequency which appears if the clock / 2 frequency is exceeded.  This lower alias frequency is equal to the clock minus the set frequency or in this case: 50-30=20 MHz which is what was being seen.  There is also an upper alias frequency but it must be filtered out by the low pass filter ?

 

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm