[Elecraft] AL7FS K2 building progress success.

[Lyle Johnson]

Hello Rob!

> Can anyone briefly explain the steps required to measure my K2 receive
> sensitivity? I have a calibrated HP 8460B, a Tek TDS 220 and a voltmeter.

The ARRL web site (www.arrl.org) has an explanation of how they do all the
tests, I believe.

You can do it in a couple of ways.

One is to hook up your sig gen to the K2, tune it in, tighten up the receive
filter to the one you want to listen through, then crank down the sig gen
until you can't easily discern the signal is there and call it the
sensitivity.

Or, you can put an attenuator in line from your sig gen to the K2 - I put
the attenuator at the K2 end - this ensures the generator sees a 50-ohm
load.  I usually use a 10dB attenuator, but use whatever you have that's 6
dB or more, or make one.  This is fairly important, since few rigs are
really 50-ohm loads and the sig gen calibration can be off a fair amount -
as much as 6dB comes to mind - if it isn't seeing 50 ohms.

Then, set the bandwidth to 500 Hz or so - that seems to be the default the
ARRL tries to use.

Kill the AGC if you can and set the volume to some level well below maximum
so you ensure the audio stages are linear, not near their maximum output.

Measure the audio signal level using an RMS voltmeter.  Stare at it awhile
until you are satisfied you have a reading that is the average.  Since you
are hearing noise, the meter will not be steady.  Write this value down.  IF
you have a 4 to 8 ohm resistor you can plug into the external speaker jack
and read the voltage across it, that's even better, since a speaker is not
necessarily a constant load across its frequency range - but that depends on
how much of a purist you are. The speaker itself ought to be just fine.

Now tune the sig gen to the frequency you are tuned to and get a reasonable
pitch tone, somewhere in the middle of the filter passband.

Then turn the signal generator level down until the voltmeter is reading
twice the power (ah! that's 70% more voltage, remember the relationship
between volts and watts).  Note the signal generator reading and don't
forget to take into account the attenuator.

This level is referred to as the Minimum Discernible Signal, or MDS.  Yeah,
you can hear a signal weaker than this, but this makes it measurable for
comparison purposes.

That's probably a longer answer than you wanted!

Enjoy!

Lyle KK7P