Hi Henning,
I did try a filter but did not help much, I guess this issue needed a
filter with greater end stop.
Allison said she tried a filter and found improvement.
I originally visualized a pad with LC matching with xtal conditioning
filter. The pad would keep
the mixer peaceful and the BiDi will be happy with LC. Have to try this
in the next days.
What I felt is that the BiDi was amping the TX signal that it picked up
and sending to the
first mixer. When I lowered th power the spurs came down much more..
The spurs were moving together and apart in 3x 4x and 5x of the change in
VFO freq.
They came together every 2.5 Mhz and were worst behaved at 15MHz and
22.5MHz
Raj
At 05-09-18, you wrote:
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Raj,
Your mod verifies my suspicion: apart form the mixing products within the
mixer harmonics of the TX Bidi amp in front of the mixer also contribute
to the problem, i.e. the first harmonic of the BiDI amp (2*45 MHz = 90
MHz!!!).
Placing an xtal filter directly into the path without any impedance
transformation is a bad idea, the filter itself wants to see a higher
impedance than 50 ohms. If this filter is not terminated correctly its
insertion loss and passband ripple will be "ugly".
In addition: the connection from R27 to the transformer T2 will produce a
source impedance close to 50 ohms. The output of the emitter follower Q21
is low impedance. PLUS: any complex load (especially capacitive) can
cause Q21 to oscillate! A coure would then be a series resistor ( e.g.
100 ohms or so) to the base of Q21 .???
As the bidiamp produces harmoncis it would be easier to place a low pass
filter (corner freq. a bit larger than 45 Mhz but "good"???
attenaution at 90 MHz). A small pcb with good grounding should do--???
please test this small pcb for real attenuation @90 MHz or if available a
"commercial low pass filter e.g. from Mini Circuits could also do
the trick for just a test.
A" fast design" using RFSIM99 for a 50 MHz five pole low pass
filter (chebychev, 0.1 dB ripple, 50 ohm impedance)??? C-L-C-L-C gives
the following component values: 73 pF 218 nH 125 pF 218 nH 73 pf. The
simulated attenuation at 90 MHz is 30 dB.???
73
Henning Weddig
DK5LV???
Am 05.09.2018 um 10:46 schrieb Raj vu2zap:
This fix reduced the spurs by up to 10 db??? and requires ONLY ONE part
to be added.
There is big change above 10MHz in the board. There is some improvement
below also.
The way I did it, CW may not work anymore will
need some more mods for CW:
1. T2 - desolder the transformer wires that go to pin 3 and 5. Pin 1
has a square pad.
2. Bring out the two wires above board and join them together and
solder.
3. Take a 45Mhz filter- 45M15 or??? similar 2 pole - one xtal only.
Solder one end of filter
to the wires of T2 pulled out. The center filter wire to ground at one
end of R26. You will
see a ground via there.
4. Solder the third wire of filter to C10/R27 junction.
Thats it! This prevents the leaked TX signal that gets amplified by the
1st BiDi from getting into
the first mixer and creating havoc.
Farhan method of the same..much simpler and CW will work.
1. Remove R27
2. Solder the 45Mhz filter two extreme ends to the pads of the
resistor.
3. Solder the center lead of the filter to the nearest ground. R13 is
very near with a ground via.
The first method the extra filter will work in RX mode also and may help!
In the second the
filter is only in the TX path..
Folks with DSA815 or better please share your feed back. The filter may
work better properly
terminated etc.
Have fun!
--
Raj, vu2zap
Bengaluru, South India.
Raj vu2zap