[Elecraft] S-Meter Sensitivity

[Don Brown]

Hi

Thanks to all who corrected my mistake (I think it was the only one yeste=
rday) Yes the db attenuation is 20 X the log of the voltage ratio. Howeve=
r I set the 191 with a HP 3400C true RMS voltmeter. This meter is accurat=
e to 10 mhz so using 7.100 mhz as the test frequency it works out fine (I=
 have cross checked it with a scope). I set the 191 to 7.100 mhz and 5000=
 uV out with the 3400C terminated with 50 ohms. Then removing the termina=
tor connect to the step attenuator set to 40 db and then out to the K2. I=
 adjusted the CAL S LO per the manual and adjust the CAL S HI to get S-9 =
on the S-meter. All settings made with the K2 attenuator and preamp off.

I just checked four K2 radios I have in my lab and the CAL S HI needs to =
be reduced to around 10 to 12 to get a reading of S9 with the corrected s=
etup above. This means the S-meter does read a little low on these radios=
 with the manual calibration.

Don Brown
KD5NDB

----- Original Message -----
From: k6se@juno.com
Sent: Monday, June 03, 2002 11:27 PM
To: DolfinDon@msn.com; elecraft@mailman.qth.net
Subject: Re: [Elecraft] S-Meter Sensitivity

Don, KD5NDB wrote:

"I have a Tektronix 191 leveled RF signal generator and a HP step
attenuator that goes down to 100 db. I set the RF level to 0.5 Volts and
the attenuator to 40 db giving 50 UV. With this connected to the K2 I
adjust the Cal S HI so the meter reads S9. This will usually come out
+or- a little from the manual set point."
=3D=3D=3D=3D=3D=3D=3D=3D
It sounds like Don was using the power ratio for dB, where a reduction of
10 dB is a reduction in power to 1/10, therefore a reduction of 40 dB is
a reduction in power of 1/10000.  However, it takes 20 dB of attenuation
to reduce the voltage by a factor of 10, therefore to reduce 500,000 uV
to 50 uV would require 80 dB of attenuation.  His 40 dB only reduced the
signal into the receiver to 5000 uV.

Someone else commented that the Tek 191 is calibrated in peak-to-peak
voltage, while S-meter sensitivity measurments are in RMS voltage.

So I guess Don should redo his S-meter calibration by setting his Tek 191
output to 14,142.27125 uV P-P (which is 5,000 uV RMS).  Then 40 dB of
attenuation would reduce the signal generator output to 50 uV.

If it took 5000 uV to make his S-meter read S9, his S-meter is stingier
than mine was, by far.

73, de Earl, K6SE
66 countries on K2 #2622 and counting.....


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