Re: RF power chain mods and improvements..


Howard Fidel
 

I just looked for spurs on the frequencies you mention, and all the spurs I see are nearly -60db from the carrier level. Is that a problem?

Howard


On 6/3/2018 12:27 PM, ajparent1/KB1GMX wrote:
The spur of interest is predictable;   Fspur= 45mhz - output frequency(mhz)

For    Spur at
29        16
28        17
24.9     20.1
21.45   23.55
None above are filterable with simple low pass filter for the band of inerest its a band pass issue.

18.1     26.9   (happens to be higher than the 20m to 17M low pass filter. and not as big a problem

It occurs at 45mhz minus the output frequency so for 28mhz the spur will be at about 17mhz during transmit.
Its level ican be as much as -50dbc or if the audio drive is turned up to near overdrive -25dbC
as measured before the power amp.  Since the amp is has more gain at lower frequencies 
the spur may be 3-6 db stronger.

It can only happen for the band that use the 30mhz low pass filter (20mhz and up).
All the other bands lower the inboard 9 element low pass filter at 33-35mhz stops it
and the band selected filters with cutoff lower than that (80, 60, 40 30, 20, 17M)
are better but try to not over drive.

This is true for all modified or not.  None of the mods (anyones) are the case its
generated by the base radio and is only there if running SSB.  The CW
generation scheme does not create or use the 45mhz IF.

The only fix is a before the amp bandpass filters for 15, 12 and 10M and they have to be 
fairly narrow.

I'm still looking at other ways to deal with this but its there.  I have stopped with amp
work for the moment as a flat amp helps and also makes it worse.

Allison


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