Guys,
There has been quite a bit of discussion about use of very wide filters
for HF narrow signal mode operation here recently. If QRM gets to be a
problem the fallback position is to narrow the filter.
Keep in mind the following. The AF pitch you will get for a fixed
frequency signal is a function of the filter. The amount of pitch shift
between filters can be 3 Hz at times. How the decoder handles such a
step function change in pitch, I don't know.
Here is some data (CW mode, pitch =500):
filter width OFFSET IF FREQ pitch change
400 Hz 0 8.215 MHz defined to be zero
200 -.83 8.214170 -3.6 Hz
250 -.02 8.214980 -2.5
1800 0 8.214450 -0.9
2.7 -.85 8.213200 -1.2
The IF frequencies are nominal. They reflect the filter offset and other
considerations. In reality, actual frequency produced by the synthesizer
board and its AD9834 synthesizer IC and may differ. One reason may be
the programming 32 bit word is inaccurate. I ran into this with another
synthesizer where it was impossible to calculate an precise 32 bit
programming word with 32 bit IEEE floating point math. One gets about
25-26 bits of accuracy in the floating point operations results. There
are certain lucky numbers which produce exact results. Like here, the
observed differences were in the 0-5 Hz range. In that case, I was able
to augment the accuracy through error look up tables and reduce the
error to under 0.5 Hz. The math coprocessor used did not permit 64 bit
math. Numerical studies showed that 64 bit math would have completely
solved the problem.
To see if the errors were similar to the numerical experiments, I used
the 250 Hz filter and varied the filter offset between -0.050 and +0.050
in 0.01 steps. The result was a sawtooth pitch error curve ranging from
-2.5 Hz to zero. (Numerical studies showed a sawtooth error form). There
are also certain "magic" numbers which produce zero error. Based on
this, I'm guessing the K3 math used for the programming word calculation
is 32 bit floating point. The resultant synthesizer 32 bit programming
word error is most likely producing the observed effect.
The problem is the amount of the error is a function of your filter
offset. One has no idea how much difference there may be as filters are
switched. If this is a problem for JT9 users, there is a solution for
the wider filters-- simply tweak the offset in 0.01 steps till the pitch
difference between filters disappears.
I then looked at the linearity of the 1Hz tuning steps suspecting the
same sawtooth effect to show up. It does. 2-3 Hz departures from
linearity exist.
I suspect the small band to band differences in accuracy (calibrated at
one frequency and off at another) are also due in part to synthesizer
output frequencies.
All of this is irrelevant to most users. I'm not sure if JT9 users are
impacted or not.
How many other amateur rigs can say they do this well? I am always
amazed that the almost lab-grade performance of the K3.
Keep in mind of don't have super lab grade equipment here. However the
HP8675B generator is locked to Rb and has a very clean stable signal.
SpectrumLab, as employed, is able to resolved pitch differences of
<<.1Hz. The K3 is using the high stability oscillator. YMMV
I don't claim to be an expert in all of this stuff. If the conclusions
are off base, please correct me.
73 de Brian/K3KO
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