Re: a simple spur fix

Jerry Gaffke


I'm not 100% sure what his LC traps are doing, but they may not have to move around.
My best guess is that the one at TP13 (tuned to around 90mhz)
just removes the 2'd harmonic of the 45mhz IF that gets created by those 2n3904's
running flat out.  It doesn't have to move.

Example:  Transmitting at 21mhz, clk2 is at 45+21=66mhz.
If any 2*45=90mhz energy is present, it mixes with the 66mhz clk1 at D1,D2 to create a 90-66 = 24mhz spur.

Farhan suggests the second LC trap is to suppress the 12mhz BFO energy due to crosstalk between clk0 and clk2.
Though we have a report that this also sucks out 10mhz reception, so either that particular example is tuned
differently than Farhan's, or that's an awfully wide trap, or I don't know what's going on there.  

If these mods are easily reproducible for acceptable results, good enough by me.
We'll see.


On Fri, Nov 9, 2018 at 02:45 PM, ajparent1/KB1GMX wrote:
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.


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