[Elecraft] Calibration of BFO - FWD from KI6WX

Wed Feb 6 03:00:01 2002

----- Original Message -----
From: "Ron D' Eau Claire" <[email protected]>
To: "Elecraft Mailing List" <[email protected]>
Sent: Tuesday, February 05, 2002 7:25 AM
Subject: RE: [Elecraft] Calibration of BFO - FWD from KI6WX

> John, KI6WX, wrote:
>
> > The PLL reference that controls the VFO frequency is tuned by a
> > 12 bit DAC,
> > which has about 3000 steps that are useful.  For the worst case
> > tuning on 10
> > meters, this will yield a frequency resolution of about 8 Hz.  It is
much
> > better on the lower bands, getting down to about 2 Hz on 80
> > meters.
>
> I looked up what Elecraft had to say about the resolution. According to
the
> K2 Manual, Theory of Operation, "Signal Flow" section (Page 102 in my Rev.
> C, April 19, 2000 manual) it says, "... 10 Hz increments are provided by a
> 12-bit DAC driving an 11 MHz VCXO (PLL reference oscillator)."
>
> Since this oscillator is operating over the same frequency range
regardless
> of the band in use, so I don't see how the band selected affects the
> resolution. I would expect 10 Hz on any band.
>
The K2 manual only gives an approximation of the operation of the VFO
circuit.  The exact operation is considerably more complicated.  I will run
through exactly how you determine the VFO step size due to the DAC
granularity by band; please refer to the K2's schematic to see the parts of
the circuit to which I refer.

The basic frequency determining element of the VFO is the PLL reference
oscillator.  This oscillator tunes roughly (exact frequencies  may vary by
1-2 kHz) from 12081 kHz to 12103 kHz.  Only the top 12.5 kHz of the PLL
reference is used as a reference frequency for the VFO.  This maximum tuning
range of the PLL reference is determine by the lowest potential frequency of
operation, which is 0 Hz (VFO=4.915 MHz; Wayne was thinking of future
possibilities to make sure that the radio oscillator circuit could support
any future bands below 160 meters).  The top part of the tuning range is
picked because this will minimize phase noise, improve frequency stability
and decrease the step size.  The upper 12.5 kHz of this range is calibrated
in the CAL PLL setup in version 2 of the firmware.

The PLL reference frequency is tuned by a 12 bit DAC (U5) on the RF board.
Only the upper ~2800 steps of the DAC are used to tune this 12.5 kHz range.
At the lowest frequency (12090 kHz), each step size corresponds to a 6.8 Hz
step, while at the highest frequency of 12103 kHz, the step size is 2.9 Hz.
The output of the PLL reference goes to the PLL Synthesizer which generates
the VFO frequency in a classic phase lock loop with dividers on both the
input reference and the output oscillator.  The divisor ratios are only
changed for every 5 kHz increment in the VFO output, and the PLL reference
frequency is changed for the smaller step sizes.  This is an ingenious
circuit to get a small step size in a single loop PLL.

When the radio is tuned to 1.8 MHz, the VFO is tuned to 6.715 MHz.  This
requires that the PLL reference tune over the upper 9 kHz, or a frequency
range of 12094 kHz to 12103 kHz.  At 12094 kHz, the PLL reference step size
is about 5.7 Hz which translates into a worst case 2.3 Hz step in the VFO.
When the PLL reference is at 12103 kHz, the step size will be 1.2 Hz.  The
firmware chooses the step that is closest the desired 10 Hz increment.

When the radio is tuned to 3.5 MHz, the VFO is tuned to 8.415 MHz.  This
frequency requires that the PLL reference tune over a 7.2 kHz range.  As the
tuning range of the PLL reference narrows, the firmware requires that the
lowest frequency of the PLL reference to not exceed 12095 kHz, so in this
case the PLL reference will tune from 12095 kHz to 12102.2 kHz.  The worst
case VFO step size is now 3.8 Hz.

Rather than repeating this for every band, I will now jump up to 10 meters.
At 30 MHz, the VFO is tuned to 25.085 MHz and the PLL reference tunes from
12095 kHz to 12097.4 kHz.  The worst case VFO step size is 11.4 Hz.

It is important to remember that the radio firmware tunes the VFO in nominal
10 Hz steps.  The firmware selects a PLL reference frequency and PLL
Synthesizer divisor ratios to tune it to the nearest 10 Hz.  Because of the
granularity of the PLL reference DAC, you can not achieve precise 10 Hz
steps.  The radio will track closer to 10 Hz steps on the lower frequency
bands because the VFO can be tuned in smaller increments by the firmware.
At 30 MHz, you will get a number of 11 Hz steps followed by a 0 Hz step.  At
the lower end of 10 meters you will get a number of 9 Hz steps followed by a
20 Hz step.  You may also get a odd step size every 5 kHz (generally between
0 to 20 Hz depending on how much your oscillators have drifted from your
last CAL PLL).  I think these are very minor penalties to pay for the much
greater advantage of the single loop PLL circuit and its lack of off
frequency spurs.

-John
 KI6WX