Posted by
Don Wilhelm-4 on
Sep 13, 2007; 11:33pm
URL: http://elecraft.85.s1.nabble.com/K2-PLL-Temperature-Compensation-Adjustment-tp452867p452868.html
Dick,
You did not mention that you ran CAL PLL after each trial value of Ra.
If you did not do that, the frequency indicated in your tests may not be
correct.
Also keep in mind that the compensation is applied to the PLL only.
Temperature variations in the BFO will also change the frequency and
further complicate matters - there may not be a direct relationship
between the PLL compensation and the overall drift due to the addition
of the BFO drift into the equation - in other words, your expectations
may have been correct, but the actual frequency is being modified by the
BFO drift causing the overall drift to be something other than expected.
Every K2 will be a bit different, so your experience may not be universal.
73,
Don W3FPR
Lamb, Dick & Judy wrote:
> I'm trying without success to optimize resistor RA on the K2
> thermistor board.
>
> I should first say that the temperature compensation with the provided
> 33K resistor is quite good--I just wanted to optimize it. With RA at
> 33K, I'm measuring drift on the 40 and 80 meter bands after
> transmitting to be about 40 Hz. I'm measuring this by zero beating my
> K2 warmed up on receive only against a standard, then transmitting
> continuously at 20 W for about 8 min. (resulting in a heat sink that's
> quite warm), then zeroing again against the standard. For example,
> the measurement before transmitting (as displayed on the K2 readout)
> might be 4000.00, and the measurement after transmitting and zero
> beating again might be 3999.96.
>
> I expected that by adjusting the value of RA between the limits of 15K
> and 56K noted in the "K2 Temperature-Compensated PLL Reference
> Upgrade" instructions, I could move from a point of under compensation
> to over compensation. For example, given the same test conditions,
> one extreme might be 4000.00 from receive only to 3999.90 after
> transmit, and the other extreme might be 4000.00 to 4000.10.
> Someplace between would be optimum compensation with almost no change
> during warm up due to transmitting.
>
> I realize, of course, my experiment isn't precisely repeatable, and
> there will always be some random drift, and that different components
> will affect the drift at different points in the cycle. However, I
> still expected to find that changing RA made a noticeable difference
> between the extremes, and I'm not seeing that. At neither extreme can
> I get the zero beat display on the K2 to be higher than it was before
> transmitting. At most, the extreme resistance values seem to change
> the outcome about 20 Hz. I am running CAL PLL after each change in RA.
>
> Squirting component cooler on the thermistor does cause a rapid and
> extreme change in frequency, so it appears the thermistor circuit is
> at least doing something.
>
> Is my experience typical? Might there be something wrong in my
> circuit? Would changing values other than RA help?
>
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