Thoughts about K2 freq shift & FINE RIT...

[Steve Kallal]

I've really loved the K2 since getting one second hand back in July. I've
added the KPA100, KSB2 and the K160RX options.

But there is one really sore point with me. That is the small frequency
shift when changing filters. It is not a big issue when using CW or SSB. I
can live with that. But it is a real sore point with the digital modes. To
be fair, I was aware of this before building the KSB2 option. I didn't
except to get back into the digital modes again, and have mostly been on CW
the past few months. I have other rigs for digital use, but the K2 receiver
is really fantastic.

I understand there is a FINE RIT feature that can correct this frequency
shift. I have the RTTY mode filters set up as:

FL1 OP1
FL2 1000 Hz
FL3 500 Hz
FL4 250 Hz

These cover most of my digital needs, depending on which mode.

The common fix for this problem is to use the tx frequency lock feature in
most of the digital comm software. Can I assume that the tx and rx
frequencies will match if I use the same filter for tx and rx? In my case,
this would be the OP1 filter. I could tune to the other station's frequency
in FL1 (OP1), before locking the tx frequency in the software. Once I lock
the tx frequency, I should be free to choose a narrower filter for rx.

Again, can I assume the tx and rx frequencies will be matched if I use the
tx filter?

Next I'd like to ask about FINE RIT. A reading of the manual and this
reflector tells me there are a number of variables affecting FINE RIT. It
has a finer frequency correction on the lower bands. And the shift amount
could change even within the same band. Also the FINE RIT correction needed
may change from filter to filter.

How can I determine the correct FINE RIT value needed for each band / filter
width combination?

It would help if I had a stable frequency source for each band and frequency
in question. The XG1 only works at 7.040 MHz, plus harmonics. 7.070 is a
popular digital frequency. I don't know if the frequency shift will vary
much on 40 meters when 30 kHz apart. The other bands are 80, 30, and 20
meters right now.

My goal is too make up a little chart telling me what FINE RIT value to use
with each band / filter width combination. It would still be a hassle, but I
could live with it, if it corrects my shift when using narrower filters.

The other digital gotcha with the K2/100 is the heat sink. I've been able to
get a large 4 inch muffin fan on top of the KPA100 heat sink and easily run
50 watts without excessive heating. It also helps some to keep in the
internal fan set to HI, but then I have to set it back to NOR when I am not
transmitting digital signals. It even gets warm when running CW at full
power, but not too bad.

Thanks in advance for any suggestions or comments!

73,

Steve N6VL
K2/100 #2289

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[David Woolley (E.L)]

Steve Kallal wrote:
>
> I understand there is a FINE RIT feature that can correct this frequency
> shift. I have the RTTY mode filters set up as:

I would say the RIT steps were part of the problem, rather than part of
cure, but basically you will need frequency measuring equipment with 1Hz
or better resolution.

> Next I'd like to ask about FINE RIT. A reading of the manual and this
> reflector tells me there are a number of variables affecting FINE RIT. It
> has a finer frequency correction on the lower bands. And the shift amount

If I understand how it works, this should read bands with lower local
oscillator frequencies, rather than lower bands.  I'm not sure that the
two are the same.

> could change even within the same band. Also the FINE RIT correction needed
> may change from filter to filter.

It will cycle over about every 5kHz (need to check the manual to find
the actual value), with an overall drift over a wider frequency range.
What happens is that a stepped voltage is applied to the PLL frequency
reference to interpolate between the 5kHz (?) steps of the PLL.  The
actual step in the reference frequency will vary because the varactor
control voltage to frequency curve is non-linear, so different steps
will occur at different points in the 5kHz (?) tweaking range.

Each step will be multiplied by the ratio between the local oscillator
frequency and the reference frequency, so higher local oscillator
frequencies will result in larger steps.  On the other hand, a smaller
control voltage range will be needed, so the varactor characteristics
will be more linear and one may be able to select a part of the range
that supports relatively small steps, whereas, for low local oscillator
frequencies one would need a larger range, so might end up with some
quite small steps at one end of the range.

There will also be a variability because the the actual BFO frequencies
are not exact round frequencies, but depend on the result of applying
different control voltages to the varactors.  That means there will be
an additional error due to the resolution of the frequency counter, and,
I think to any change in the 4MHz frequency since the CAL PLL was run.
I think that is 10kHz.

>
> How can I determine the correct FINE RIT value needed for each band / filter
> width combination?

You need the output from CAL PLL, unfortunately that's not available
externally.  You also need to measure the BFO frequency more accurately
than the RIT step size.  That probably means you need to use an external
frequency counter, with 1Hz, or better, resolution

> popular digital frequency. I don't know if the frequency shift will vary
> much on 40 meters when 30 kHz apart. The other bands are 80, 30, and 20
> meters right now.

The variation across the band will be the result of RIT step resolution,
so not correctable by RIT, although the number of RIT steps needed to
correct the BFO frequency measurement error will vary, but will tend to
cycle over whatever is the intrinsic PLL frequency step.
--
David Woolley
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[David Woolley (E.L)]

Steve Kallal wrote:

Further to yesterday:

> The common fix for this problem is to use the tx frequency lock feature in
> most of the digital comm software. Can I assume that the tx and rx
> frequencies will match if I use the same filter for tx and rx? In my case,
> this would be the OP1 filter. I could tune to the other station's frequency
> in FL1 (OP1), before locking the tx frequency in the software. Once I lock

You must use FL1 (and the same mode and reverse setting).  Using OP1 on
a different filter number may produce an offset because:

- you deliberately chose a different position of BFO with
   respect to the passband;
- it's possible, that two different BFO control voltages,
   at one end of the range, show the same frequency to
   within 10Hz;
- the measured frequency normally has an uncertainty in the last
   digit, and, depending on when the frequency measurement is captured,
   and maybe whether there is any filtering to improve the results,
   either of two values 10Hz apart might actually get stored.

Actually, the last point means that, as well as measuring the BFO
frequency to better than 1Hz, you also need to perform some measurements
to determine which of the two candidate measured frequencies was
actually stored.  I'm assuming that the firmware doesn't use a longer
gate time when capturing the value to store, and actually try and set
the local oscillator more finely than 10Hz, in which case RIT step size
might be the primary source of error!

Some of the theory I missed yesterday:

As I understand it, when you use the Display key mode in CAL FIL, you
are stepping the BFO varactor control voltage.  The steps vary from less
than 10 Hz, to a few 10s, depending on which end of the range you are
at.  No calibration data is stored for this, but it means that the BFO
will have discrete possible offsets from the nominal 10Hz tuning points.

What also happens is that the internal frequency counter measures the
actual BFO frequency, but only to 10Hz precision and with a tolerance of
about 10Hz, although, as noted elsewhere, it could do a more precise
measurement when storing, compared with what is displayed.

When you tune to a frequency in SSB or RTTY mode, the firmware looks up
the stored BFO frequency for the current filter, adds or subtracts it
from the requested frequency and gets a 10Hz resolution desired local
oscillator frequency.  This will have an error due to:

- differing thermal environments (which will tend to affect all
   filters the same, if calibrated at the same time);
- the uncertainty in the last digit from the frequency counter;
- the offset between the actual BFO frequency step and the corresponding
   10Hz step.

Assuming that it doesn't try to be very clever, it will then set the PLL
  to the nearest 5kHz(?) and set the reference oscillator varactor
voltage to the RIT step that is closest to the target 10Hz step.

Some of the uncertainties here are:

- the calibration data is based on measurements which have a 10Hz
   uncertainty in the  measurement, although it might smooth those out;
- calilbration data isn't stored for every possible control voltage, so
   there will be some interpolation errors, although these are probably
   limited so as not to dominate other error sources;
- the RIT steps will not fall on exact 10Hz points and the offset will
   be frequency dependent.

The firmware may try to be clever and be more accurate than this, but
with closed source firmware, one can't really tell.  One way it could be
clever is by relying on the frequency reading errors being random over
large ranges and try to get a fit for the reference oscillator frequency
versus control voltage step curve that is much more accurate than any
individual reading.  It might then choose a 5kHz (?) setting that is not
the closest.  However, I'm not sure that the pulling range is large
enough to allow that on all bands (although it is the bands which would
  have the largest RIT steps that would most likely allow it).  By doing
that, it might be able to find a step that was closest to the desired
10Hz.  (I suspect it would exacerbate the popping problem though, ss the
PLL would need stepping up and down at frequent intervals.)


> the tx frequency, I should be free to choose a narrower filter for rx.
>
> Again, can I assume the tx and rx frequencies will be matched if I use the
> tx filter?

As long as you mean FL1, same mode and same reverse setting.


--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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