[Elecraft] RE: K2 Frequency Alignment

[Don Wilhelm]

Folks,
Let me add to everything Ron said by adding that the K2 PLL (VFO) is
designed to be most accurate within the ham bands.  When we tune outside the
ham bands, we are moving the VFO frequency further and further away from the
intended "Best calibration" points.  So several factors will influence the
displayed frequency - the biggest is - just how closely do the voltage
variable diodes actually installed in your particular K2 track in
characteristics for capacity/voltage change - that is not a linear
relationship and there are manufacturing tolerance ranges to deal with.  So,
yes, the K2 dial readings are compromised by several factors, and I believe
this one contributes to differences in dial readings from one K2 to another.

Add also that the 'frequency standard' transmissions at HF are not reliable
for extreme frequency accuracy due to frequency shift caused by propogation
characteristics - I don't know how much that is, but we may be approaching
the limits here when splitting 10 Hz.  Recall that WWVB was created as an
answer to this problem.

Now, having said that - In my opinion, the best frequency standard to use in
judging the accuracy of your results is the one closest to the center of a
ham band - 10 MHz WWV or CHU rather than the 15 or 20 MHz standard signals
since these are about 1 MHz away from the nearest ham band. (i.e. the dial
reading error can become greater the more you move away from the design
center - it all depends on your diodes <G>).  Apologies to those outside
North America where these standard frequencies are not available (or if
available are subject to more shift due to propogation).

For those still wanting greater dial accuracy, there is always the solution
of setting things up, then checking to see what the deviation from 'dead on'
really is and compensating by shifting the 4MHz oscillator to get it
closer --- warning, this could take several interations!!!

73,
Don W3FPR

----- Original Message -----
From: "Ron D'Eau Claire" <rondec@easystreet.com>
To: "'Elecraft Reflector'" <elecraft@mailman.qth.net>
Sent: Sunday, July 27, 2003 10:42 AM
Subject: RE: [Elecraft] RE: K2 Frequency Alignment


The 30 Hz error is common. The 80 Hz error may be normal too. I don't recall
the readout accuracy at 20 MHz, but it is not as good outside the Ham bands
as it is inside the bands. It's gotten a lot better with newer versions of
the firmware, so I don't recall offhand just how good it is and I haven't
heard WWV on 20 MHz in a while here.

Keep in mind that the K2 frequency display is NOT reading the actual K2
frequency. What you see on the LCD display is what the firmware calculated
the frequency should be when you last ran CAL PLL, not what the frequency
really is at the moment. There are some inherent errors in that system.

When you run CAL PLL, the actual frequency is measured by the internal
frequency counter. That's when it's important for the 4 MHz oscillator to be
right on 4 MHz, so the frequency counter will be as accurate as possible.
Any error in the 4 MHz oscillator while CAL PLL is being run will produce an
error in the dial calibration.

CAL PLL checks the K2's actual frequency at a number of points and creates a
look-up table that is used in normal operation. It's an electronic analog of
an mechanical dial using a pointer and paper scale with frequencies written
on it. To calibrate the "dial", the frequencies at certain points over the
range are actually measured and noted. The frequencies in between those
points are calculated based on assumptions about the tuning characteristic
of the oscillators. Any errors in the calculations will show up as readout
errors in normal operation.

In normal operation, the LCD frequency display can be "off" due to several
reasons:

1 - The accuracy by which the frequency was measured during CAL PLL. This is
affected by two things: The accuracy of the 4.000 MHz oscillator and the
resolution of the digital-to-analogue converters (DACs) used in the K2
logic. I believe the DAC's have a potential error of about 30 Hz, so any
dial reading more accurate than 30 Hz is a matter of "luck".

2 - Any "drift" in the VFO circuits after CAL PLL was run. CAL PLL builds a
table of tuning voltages that is stored in memory for future use. When you
tune the K2 in normal operation, the system assumes that if a certain
control voltage is sent to the oscillator, it will produce exactly the same
frequency in response that it did during CAL PLL. Any drift in the
oscillator will produce errors in the actual frequency.

Ron AC7AC
K2 # 1289