Hi all,
Bob said "Most ham radios are reasonably accurate but resolving less than
100 Hz is based on many variables and assumptions. Even the number of
digits in the software calculations are truncated."
I am pretty sure truncation of the numbers used on the PLL divide by N is
the reason for non perfect frequency calibration, differing on every band.
Other radios have it exact, such as my Kenwood TS-440, but that radio has
terrible phase noise compared to the Omni VI. So there are tradeoffs made
in PLL design in order to have a really quiet oscillator that also tunes in
small steps, quickly locking to the next setting and feels like a
continuous tuning as you turn the VFO knob.
DE N6KB
On Sat, Nov 2, 2019 at 8:56 AM Bob McGraw K4TAX <rmcgraw@blomand.net> wrote:
> The digital displays are merely indicated values and ARE NOT frequency
> measurements.
>
> Most ham radios are reasonably accurate but resolving less than 100 Hz is
> based on many variables and assumptions. Even the number of digits in the
> software calculations are truncated.
>
> If you need to know the exact frequency you will need an external
> frequency counter with an external GPSDO reference.
>
> The ARRL Frequency Measuring test is this month. Try your hand, if you
> believe your radio is really accurate.
>
> Bob, K4TAX
>
>
> Sent from my iPhone
>
> > On Nov 2, 2019, at 1:31 PM, Ken Brown <kenradiobrown@gmail.com> wrote:
> >
> > Hi Dale,
> >
> > The Omni VI frequency calibration is dependent on a 20 MHz reference
> > oscillator and the BFO oscillators.
> >
> > The BFO operates at several different frequencies depending on mode of
> > operation and whether in transmit or receive. The manual explains how to
> > adjust the BFO frequencies in a specific order. There is some interaction
> > between the BFO frequency adjustments and it is necessary to repeat the
> > procedure in order to get them all correct. The BFO operates
> > independently of any PLL system. It is not locked to the 20 MHz reference
> > or to anything else.
> >
> > The band select crystal oscillators should not need adjustment since they
> > are supposed to lock to the 20 MHz reference. If they have been
> improperly
> > adjusted or if there is a component fault (aged crystal perhaps) it may
> be
> > necessary to adjust the slug tuned inductors to get the band select
> crystal
> > oscillators to lock to the 20 MHz reference. This is somewhat tricky. I
> > don't recommend going there unless you are really certain it needs to be
> > done. In that case you should have a good oscilloscope to monitor the PLL
> > for the adjustment. I have damaged the ferrite slug in at least one of
> mine.
> >
> > The VFO is locked to the 20 MHz reference and therefore it's accuracy is
> > directly dependent on the 20 MHz reference accuracy.
> >
> > I suggest you check your frequency calibration error on multiple bands. I
> > suspect you will find it is different on every band.
> >
> > My experience with the Omni VI is that even after very carefully
> adjusting
> > the BFO frequencies and the 20 MHz reference the final digital readout
> > versus actual zero beat receive frequency is off by several Hertz to tens
> > of Hertz depending on the band in use. It does meet the manufacturers
> > specification, but it is not exact and not as near zero beat as the
> > oscillator adjustments have been set to. In other words if I have the BFO
> > set to less than 1 Hz error from correct and the 20 MHz reference to less
> > than 1 Hz error the final receive frequency readout will be off my many
> > Hertz and it is not the same on every band, nor is the error proportional
> > to the operating frequency.
> >
> > Cheers DE N6KB
> >
> > On Sat, Nov 2, 2019 at 3:57 AM dale via TenTec <tentec@contesting.com>
> > wrote:
> >
> >>
> >> My VI+ is of frequency by .22, Several guys got back and the answers
> >> for calibration.of the rig were from easy to do to look in the
> manual. My
> >> manual says nothing about the problem, that I can find. or how to
> correct
> >> it. I was told the rig had a txco and all was adjustable with a single
> >> adjustment. Can anyone tell me the correct
> >> oprocedure is to correct the problem. Have a very limited bench
> anymore,
> >> if that is required.
> >> thanks for any help
> >> 73
> >>
> >> dale wt4t
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>
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