[Elecraft] Monitoring RF Outout

[Bob - AG5Q]

----- Original Message -----
From: "Mike Walkington" <walkingtons@attbi.com>
To: "Elecraft Reflector" <elecraft@mailman.qth.net>
Sent: Saturday, January 04, 2003 3:27 AM
Subject: [Elecraft] Monitoring RF Outout


Hi Mike,

If  the K2 is driving a 50 ohm load, the power output can be calculated
using the voltage at R66 with this formula:

Power into 50 ohms = 0.58*(VRFDET+0.3)^2

Where ^2 means "Squared"

That is:
            Power into 50 ohms =  0.58*(VRFDET + 0.3)*(VRFDET+0.3)

The 0.3 term takes into account the voltage drop across the Schottky diode
D9.

A quick approximation is:

Power = 0.5*VRFDET^2

If you have an unused connector on the back of the K2 you could bring out
the voltage at R69/C77  for easier connection. It would be a good idea to
put another resistor near R69/C77 to isolate this external line from stray
pickup inside the K2 and also protect the K2 circuit from an accidental
short on the output.  If you're using a DMM for monitoring, the isolation
resistor could be in the range of 10K to 20K without affecting the accuracy.

If you know the input impedance of the DMM (say, 10 megohms) then the
isolation resistor could be used to scale the DMM reading to make
calculating power a bit easier. If the isolation resistor is 3Megohms, then
the formula for power becomes:

  Power = (Vdmm)^2  ( this neglects the diode drop of D9)

Even if you don't want to use an external connector to make contact to this
point, you could solder one end of a resistor to the pad and leave the other
end sticking up to make a test point to hook on to.

73/ Bob - AG5Q



> Some time ago when tuning my Rev A K2 for the first time, a friend
suggested
> that in addition to using the K2s internal wattmeter, I could monitor the
> output of the RF Output Detector (R66) with the analogue display on my
Fluke
> DMM.  I did this, and found that I seemed to have a bit more sensitivity
in
> making tuning adjustments.  So, my questions are:
>
> 1)  Is there general agreement that this is another valid way of getting a
> tuning indicator?
> 2) What would be the formula to convert the voltage measured at R66 into
> Watts?
>
> 73s
>
> Mike
> AC7MZ
> VK1KCK
> K2 2599
>
>
> _______________________________________________
> Elecraft mailing list: Elecraft@mailman.qth.net
> You must be a list member to post to the list.
> Postings must be plain text (no HTML or attachments).
> See: http://mailman.qth.net/mailman/listinfo/elecraft
> Elecraft Web Page: http://www.elecraft.com
>