Re: a simple spur fix

barry halterman
 

I installed the series trap across C202 in the LPF and it reduced the 30 meter sensitivity drastically.


On Fri, Nov 9, 2018, 5:46 PM ajparent1/KB1GMX <kb1gmx@... wrote:
Farhan,

Your doing what I tried back in June/July time frame, it failed then.

Reason first series traps good enough to suck out the spur at a given frequency
are narrow enough  to be only a spot frequency fix.  The 2IF-LO spur for 28mhz
BAND is between 17 to 16 mhz for 10M alone.  So you need a series trap that
can not do a band.

The other issue is the power amp.  At 80M and 10W out the gain is about 60db
but as you go up, at 20M the gain is down to ~56db, and at 10M the gain is maybe
~52DB.   So at the spur frequency you have more gain available than the desired
output frequency and the differential gain is enough to put that spur as shown at
-45db at more like -37db below 2W.

The fix has to be more comprehensive.  Also it must be field verifiable or factory
set to allow for 1 or 5% parts.

The 45mhz section needs:
  45mhz filter inductors shielded or better that section relocated.
  Amplifier distortion clean up.
  Swap the 45mhz port and LO ports as IF ports are most sensitive to mismatch.

A thought to look into...
  Replace the TIA amp at 45mhz with a more common resistive feedback amp (typical 2n5109 amp)
  with 20db gain.  Here is the hint:  Often when that amp is shown due to the fact that changes at either
  port impact the opposing port they often have a 6db pad at the output.  However for the TX case that
  same pad can be at the input and it insures the 45mhz filter will have a good match for the network
  and the transistor with its input impedance stable will have output stability going to the mixer.

Allison




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