[Elecraft] KX1 Spring Tune Up Update

[Don Wilhelm]

Jack,

First, the 10 MHz 'scope probe is your limiting factor on your 'scope 
measurements.  What that 10 MHz rating means is that at 10 MHz, the 
response of your probe is down -3dB from the actual voltage.  If the 
probe's rating is 'right on the money', the actual voltage measured at 
10 MHz will be the measured voltage *divided by* 0.71.  Using a 'rule of 
thumb' that says the measuring instrument must be rated for more than 3 
times the frequency of operation, measurement of a 14 MHz signal 
requires a 'scope probe rated for 45 MHz or greater.  Even using the 3 
times 'rule of thumb' will produce some inaccuracies at the high 
frequency end - a better rule is to use a probe (and 'scope) that is 
rated at 5 times the highest frequency that will be measured for voltage.
Note: I do not depend on the voltage readings from my Tek 465B 'scope 
(rated at 150 MHz) with 100 MHz probes at frequencies much higher than 
the 15 meter band.  With 200 MHz probes, I can depend on it to 30 MHz 
(in this case, the 'scope limits the frequency rather than the probe).

Secondly, the formula in the DL1 manual is in error.  I am not certain 
where all that 'mish-mash' of calculations come from, but it is in error 
and can certainly be simplified as follows:

The diode probe in the DL1 reads the peak RF voltage across 1/2 of the 
dummy load (the actual zero to peak RF voltage across the 50 ohm dummy 
load is 2 times the DMM reading plus the diode drop) .  The diode drop 
must be added to the DMM reading before doing any other manipulation 
with the quantities.  A real formula that will work accurately is (Vdmm 
+ Vdiode)*2/25 or if you assume 0.15 volts drop in the diode, the 
formula is  (Vdmm +0.15)*2/25.  The derivation of that formula is left 
as an exercise "for the student", but the results from your DMM readings 
are:
3.5 MHz - 7.60 volts -> 2.40 Watts
7 MHz  -  7.22 volts -> 2.17 Watts
10 MHz - 8.25 volts -> 2.82 Watts
14 MHz - 6.80 volts -> 1.93 Watts.

I agree that the formula in the manual will yield results that are "in 
the ballpark", but they are not correct.
If you want to go one step further in accuracy, measure the actual diode 
voltage drop in your DL1 and use that value instead of the 0.15 volt 
figure.  Once calibrated for the actual diode drop, the DL1 measurement 
is very accurate if all the factors are considered, even at frequencies 
well above 30 MHz..

Thirdly, the wattmeter in your KX1 is set a bit too high.  Reduce the 
setting of R1 a bit and try again.
Remember that the power reading on the KX1 is +/- 1 (or sometimes 2) 
digits.  So for an actual power of 2.5 watts, the KX1 display can show 
anywhere between 2.4 and 2.6 watts (or in the extreme case 2.3 and 2.7 
watts).

73,
Don W3FPR


Jack Regan wrote:
> I purchased an Elecraft DL1 because of its' linearity and the built in VDC
> test points. Here is the first set of test data.
>
>  
>
> Notes:
>
> The SCOPE PWR was Vp-p * .3535 = Pav
>
> The TP1 PWR was ((V * 1.414) + 0.15))2 / 50 = P(watts)
>
> BAT: 1) DVM 11.93 UNLOADED 2) DVM 11.90 KX1 RECEIVE 3) KX1 11.6 KX1 RECEIVE
> 4) sDVM 11.72 KX1 TXMT 
>
>  
>
> FREQ  SCOPE P-P   SCOPE PWR   TP1         TP1 PWR     KX1 PWR     KX1 SWR
>
>  3.55 30 VP-P     2.25 WATTS  7.60 VDC    2.424 WATTS 2.8-2.9     1.0
>
>  7.05 28          1.96        7.22        2.146       2.5-2.8     1.0
>
> 10.12 32.5        2.64        8.25        2.79        3.8-4.0     1.0
>
> 14.05 27          1.822       6.80        1.79        2.3-2.5     1.0
>
>  
>
> In every case the scope power is lower than the DL1 power and the KX1 TUNE
> power is higher than both!
>
>  
>
> The DL1 specification claims 10% Accuracy! Is that 5% plus or minus?
>
>  
>
> The scope used is a Tektronix 465. Its' self calibration shows that it is
> functioning properly.
>
> It was certified when I bought it. It uses a 10 Meg probe.
>
>  
>
> The rig is fully loaded. 4 bands, ATU, power mod and the SMD CW TUNING AID. 
>
>  
>
> Projects left to do. 
>
>  
>
> 1.    Understand why the Scope and the DL1 are not showing the same results.
>
>  
>
> 2.    Get the rig and the scope/DL1 readings to agree.
>
>  
>
> 3.    If in fact the output is lower than spec for 20 for example, correct
> that!
>
>  
>
> 4.    Explore the possibility of increasing the power output to a full 5
> watts.
>       Don has pointed out the fact that more than just the oscillator, final
> rf
>       transistor and toroids are involved in putting out more than 4 watts.
> Perhaps an
>       out board power amp using the KX1 as a VFO would be the way to go!
> QST has had several
>       articles on power amp construction lately! 
>
>  
>
> Just for the record, the KX1 is working on all bands.
>
>  
>
> I'll be going over the KX1 manual next. In the mean time, if anyone has any
> suggestions re: the data presented above, I would be glad to hear from you.
>
>  
>
> Jack AE6GC
>
>  
>