si5351 crosstalk #radiuno


Another stone turned.

While testing VFO/BFO replacement for an Atlas210 I came across a spur at the BFO frequency 
while looking at the VFO signal quality.  I first figured it was layout or some error.  Sadly it was not.
As the Atlas210 was a high performance radio for ts day this would compromise it.  For that the
crystal oscillator BFO goes back in.

So I went to the Raduino and did some research and built two other commercial SI5351 breakout
boards to test for CROSSTALK.   Indeed if you load (resistive load) a oscillator port you will see
it as a weak spur on the other operating port.  The amount of the spur is proportional to the
load current for that oscillator.  Its chip level not board level.

Two oscillators 0 and 2 operating with 0 set for 4.0000mhz and 2 set for 5.520mhz.

The output hooked to SA via 3db pad.  Its output is measured as 8.94 DBM, I expected 13!
Not a big problem and clean.  Add a 50 ohm laod to osc-2 and there it is at 5.520mhz, a
signal exactly 58dbc.  Move SA to osc-2 and big as life and 8.94 DBM again (where is the 13dbm?)
and sure enough a -58dbc spur at 4.0000mhz.  Take the load off the 4mhz osc-0 and
the spur drops into the noise about -90dbc down.

Seems running multiple oscillator lowers the output power of any one too.

Make the load resistor 25ohms and it gets about 6db worse and at 240ohms its about -70dbc.

These do not effectively cause spurs of consequence for TX output. 

However this does help explain why an external crystal osc was needed to get
carrier leakage to a expected level.  Also where the receiver birdies are coming
from.  Its not the Raduino its the 5351 chip.   This makes it a poor LO replacement
unless its the sole operating oscillator as even the lowly AD9850 DDS 
has about the same SFDR (spurious free dynamic range).  IF only one oscillator is
in use the signal has no spurs only harmonics.

In short for higher performance radios only use one oscillator in the 5351 and
use multiple 5351s is you need more oscillators.


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