VCO Relationship with 4 MHz Oscillator

[Mike Walkington-3]

Hi Ron,

Thanks for this explanation.  Is there an implication here that if you had a
more accurate external 4MHz source you could switch it in when you run CAL
PLL to get a more accurate result in the look-up table?

Mike
VK1KCK




------------------------------

Message: 29
Date: Thu, 25 Nov 2004 20:03:33 -0800
From: "Ron D'Eau Claire" <[hidden email]>
Subject: RE: [Elecraft] VCO relationship with 4 Mhz ocsillator
To: 'Jean-Fran?ois M?nard' <[hidden email]>
Cc: 'Elecraft - Maling List' <[hidden email]>
Message-ID: <000001c4d36c$e8f53810$c5ddfea9@RONPORTABLE>
Content-Type: text/plain; charset="iso-8859-1"

That's not the case for the 4 MHz oscillator in the K2, Jean-Frangois.



-------------------------snip--------------------------


When you run CAL PLL, a frequency counter built into the K2 measures the PLL
frequency. The K2 applies a tuning voltage to the PLL reference oscillator
and notes the frequency measured by the counter. This information is
recorded in memory so it can be looked up later. The K2 tunes through the
whole range of frequencies the PLL can cover noting the actual frequency
measured at regular intervals, and then recording the voltage applied to the
reference that will produce the frequency.

-------------------------snip--------------------------


Notice that the accuracy of the readout depends upon how accurately the
frequencies and tuning voltages were stored when you ran CAL PLL. That's
where the 4 MHz clock and C22 come in. The Control Board 4 MHz clock is the
time base for the frequency counter! So the closer it is to 4 MHz the more
accurately the K2 will measure the K2 PLL frequency and the more accurately
the dial calibration of the K2 will be in use.


Ron AC7AC



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[Don Wilhelm-3]

Mike and all,

An external 4 MHz oscillator does not work because there is no provision to
inject an external oscillator into the MCU in place of the crystal
oscillator.  It would require a hardware mod to use it.

Setting the reference oscillator accurately is not difficult - if you have a
very accurate external counter, you can read the same frequency source with
both the internal and external counter and simply match the interal counter
reading.
The newer method of setting the reference now on the Elecraft webside
(previously known as the 'N6KR method') is even more accurate because it
automatically compensates for slight variations in the reference frequency
required - a few folks have reported that the reference must be set a few
cycles away from 4 MHz  for the most accurate results.

73,
Don W3FPR

----- Original Message -----
>
> Thanks for this explanation.  Is there an implication here that if you had
> a
> more accurate external 4MHz source you could switch it in when you run CAL
> PLL to get a more accurate result in the look-up table?
>


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[Ron D'Eau Claire-2]

In a Don, W3FPR, points to a very good reason not to try to use an external
4 MHz clock for the counter. There's an issue with the accuracy available
too.

The tuning voltages are converted to digital numbers for storage in memory,
and the digital numbers are converted back to analogue voltages as required
when tuning across the band. The conversion is done by DAC's - Digital to
Analogue Converters. During CAL PLL the DAC's sample the tuning voltages
applied to the oscillators and convert them to digital numbers for storage.
In operation the DAC's convert the digital number back to the tuning
voltage. The DAC's used by Elecraft are a trade-off between cost, complexity
and accuracy. From what I've read, they provide a tuning accuracy of about
30 Hz. Even if the K2 was able to measure the oscillator frequency with
absolute accuracy while running CAL PLL, the DAC's can't reproduce the
tuning voltage accurately enough to put that oscillator closer than about 30
Hz of the original frequency in operation.

So getting the 4 MHz clock closer than is needed for an accuracy of about 30
Hz is an exercise in futility. It can happen accidentally that the value of
the tuning voltage returned for a specific frequency is exactly on, but
that's not predictable.

Also, there is some drift in the voltage tuned oscillators even though the
PLL reference oscillator now has an active temperature compensation circuit
and the beat frequency oscillator has been improved. Of course *all*
oscillators drift somewhat, and those designed to be tuned over a range of
frequencies tend to drift more.

The way designers have held the drift of other rigs to an absolute minimum
is to not tune the PLL reference oscillator. Instead they use a very highly
stabilized design that may even include putting the crystal in a
temperature-controlled oven. That does produce a dial accuracy within a
couple of Hz in some cases, but it does it at the cost of having the
phase-locked loop tuning in very small steps of a few Hz at a time instead
of the 5 kHz steps the K2 uses. That produces more phase noise in the local
oscillator that affects the receiver performance. (To the operator, the
phase noise usually sounds just like a somewhat higher "band noise" level.)

So the oscillator design boils down to a trade-off between phase noise and
stability. Elecraft went for lower noise and a stability and accuracy of
some tens of Hz, instead of a stability and accuracy of a few Hz and more
noise.

What really impresses me about really good engineering is not that someone
does things perfectly. Everything we do is a compromise of some sort; we can
always imagine it being more 'perfect' than it is. What impresses me about
really good engineering is the ability to choose the best compromises for
the challenge. The performance of the K2 is a good example.

There is indeed a real 'art' to engineering, whether it's an airplane, or a
bridge, or a Ham rig.

Ron AC7AC

-----Original Message-----


Hi Ron,

Thanks for this explanation.  Is there an implication here that if you had a
more accurate external 4MHz source you could switch it in when you run CAL
PLL to get a more accurate result in the look-up table?

Mike
VK1KCK


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