Low receiver sensitivity

[info4mjs]

Hi,

I recently assembled and XG1 calibrator and began checking some of my equipment.  4 out of 5 radios that I own indicated an S9 signal, however, my K2 did not.  It registered an S5-S6.  A friend of mine who borrowed my K2 indicated the receiver seemed to be a bit weak.

I tweaked L34 (which was set properly) and tried peaking L1 and L2, which were also set properly.

I'm wondering if I'm missing something simple before I begin to check some other items.  The receiver appears to work properly in all other respects and to be truthful, I hadn't noticed it being a bit weak, but upon comparing it with an A/B switch, it lacks some sensitivity.

Thanks in advance for any pointers.

Mike, WA1SEO
K2 #4788
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[Ron D'Eau Claire-2]

Mike WA1SEO wrote:
I recently assembled and XG1 calibrator and began checking some of my
equipment.  4 out of 5 radios that I own indicated an S9 signal, however, my
K2 did not.  It registered an S5-S6.  A friend of mine who borrowed my K2
indicated the receiver seemed to be a bit weak.

I tweaked L34 (which was set properly) and tried peaking L1 and L2, which
were also set properly.

I'm wondering if I'm missing something simple before I begin to check some
other items.  The receiver appears to work properly in all other respects
and to be truthful, I hadn't noticed it being a bit weak, but upon comparing
it with an A/B switch, it lacks some sensitivity.

Thanks in advance for any pointers.

-----------------------------------------

You can't get an idea of the absolute sensitivity of your K2 looking at the
S-meter, Mike. You can adjust the S-meter to indicate anything you want from
any signal you want. A blockbuster signal can be S1, our you can adjust it
to make a signal down in the noise S9!

Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
"Signal-to-Noise and MDS Calculations (page 5) for instructions on how to
use the XG1 to measure receiver sensitivity. If you don't have the manual,
you can download it from www.elecraft.com

That procedure will allow you to determine the signal-to-noise ratio and the
minimum discernable signal (MDS) of your K2. You can use the same procedure
to evaluate other receivers for comparison.

Ron AC7AC


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[Earl W Cunningham]

Mike, WA1ESO wrote:

"I recently assembled and XG1 calibrator and began checking some of my
equipment.  4 out of 5 radios that I own indicated an S9 signal, however,
my K2 did not.  It registered an S5-S6."
==========
AGC adjustment R1 on the control board might be a bit off.  Try turning
it to decrease it's resistance a bit, which will increase the S-meter
sensitivity.

After adjusting R1, you'll have to go into the menu and recalibrate the
S-meter hi and lo settings.

73, de Earl, K6SE
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[PA3GYU]

Ron,

At 04:01 11-05-05 -0400, you wrote:
>You can't get an idea of the absolute sensitivity of your K2 looking at the
>S-meter, Mike.
[SNIP]

>Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
>"Signal-to-Noise and MDS Calculations (page 5) for instructions on how to
>use the XG1 to measure receiver sensitivity.
I did, and found this procedure and example:

>A. Divide S+N by N; call the resulting ratio R.
>B. Take the base-10 logarithm of R ("log" key on most calculators).
>C. Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in dB.
>D. If the S+N/N is greater than 10 dB, then the MDS is approximately
>equal to the result from (C) subtracted from -107 dBm.
>
>Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
>
>A. R = 1.0/.03 = 33
>B. log(30) = 1.52
>C. 20 x 1.48 = about 30 dB (this meets the requirement for step D)
>D. MDS = -107 dBm - 30 dB = -137 dBm

Two remarks:
1. I think the second step in the example should read: "B. log(10) = 1.52".
2. I may be missing something here, but if the outcome of B) equals 1.52,
why is "1.48" used in C)? (Not that it changes much, only 0.8 dB, but hey:
it might confuse some people!)

Best, BartW de PA3GYU


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[Ron D'Eau Claire-2]

Bart, PA3GYU wrote:

>Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
>"Signal-to-Noise and MDS Calculations (page 5) for instructions on how
>to use the XG1 to measure receiver sensitivity.
I did, and found this procedure and example:

>A. Divide S+N by N; call the resulting ratio R.
>B. Take the base-10 logarithm of R ("log" key on most calculators). C.
>Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in
>dB. D. If the S+N/N is greater than 10 dB, then the MDS is
>approximately equal to the result from (C) subtracted from -107 dBm.
>
>Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
>
>A. R = 1.0/.03 = 33
>B. log(30) = 1.52
>C. 20 x 1.48 = about 30 dB (this meets the requirement for step D) D.
>MDS = -107 dBm - 30 dB = -137 dBm

Two remarks:
1. I think the second step in the example should read: "B. log(10) = 1.52".
2. I may be missing something here, but if the outcome of B) equals 1.52,
why is "1.48" used in C)? (Not that it changes much, only 0.8 dB, but hey:
it might confuse some people!)

----------------------------------

Sorry for the delay responding Bart! I've been away all day and I see that
none of our other sharp-eyed contributors have answered your question!

Yes, you are right. B. should read log(33) = 1.52 (the base 10 is
"understood" unless a different base is shown and, if so, the base is shown
as a subscript following log, not in parenthesis). The procedure is to take
the result of A. and find the logarithm of that.

I'm really confused about where the 1.48 came from!! That should be 1.52 so
20 x 1.52 = 30 dB.

That instruction sheet was not one if my projects, so I never looked at that
example closely. I'll pass this info along to Wayne.

Ron AC7AC



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[Don Wilhelm-3]

My guess is that somewhere in writing that example the numbers changed
(likely because the author wanted the result to be 30 dB instead of dealing
with 29.6 dB and explaining away the fact that the tenths digit is
insignificant so should be rounded up).  1.48 *is* the log of 30, so
apparently the quantities on the right of the equal sign were changed to
reflect the log of 33, but the quantities on the left were unchanged.  End
result --  'unequal equal signs' <G>.

73,
Don W3FPR

> -----Original Message-----
>
> Bart, PA3GYU wrote:
>
> >Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
> >"Signal-to-Noise and MDS Calculations (page 5) for instructions on how
> >to use the XG1 to measure receiver sensitivity.
> I did, and found this procedure and example:
>
> >A. Divide S+N by N; call the resulting ratio R.
> >B. Take the base-10 logarithm of R ("log" key on most calculators). C.
> >Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in
> >dB. D. If the S+N/N is greater than 10 dB, then the MDS is
> >approximately equal to the result from (C) subtracted from -107 dBm.
> >
> >Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
> >
> >A. R = 1.0/.03 = 33
> >B. log(30) = 1.52
> >C. 20 x 1.48 = about 30 dB (this meets the requirement for step D) D.
> >MDS = -107 dBm - 30 dB = -137 dBm
>
> Two remarks:
> 1. I think the second step in the example should read: "B.
> log(10) = 1.52".
> 2. I may be missing something here, but if the outcome of B) equals 1.52,
> why is "1.48" used in C)? (Not that it changes much, only 0.8 dB,
> but hey:
> it might confuse some people!)
>
> ----------------------------------
>
> Sorry for the delay responding Bart! I've been away all day and I see that
> none of our other sharp-eyed contributors have answered your question!
>
> Yes, you are right. B. should read log(33) = 1.52 (the base 10 is
> "understood" unless a different base is shown and, if so, the
> base is shown
> as a subscript following log, not in parenthesis). The procedure
> is to take
> the result of A. and find the logarithm of that.
>
> I'm really confused about where the 1.48 came from!! That should
> be 1.52 so
> 20 x 1.52 = 30 dB.
>
> That instruction sheet was not one if my projects, so I never
> looked at that
> example closely. I'll pass this info along to Wayne.
>
> Ron AC7AC
>
>

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[Bob - W5BIG]

Line B. of the example should read :  log(33)=1.52

Apparently there was a typo in line B that propagated into line C. The log
of 30 is 1.48, that's where the value of 1.48 in line C came from.

Example should read:
A. R=1.0/0.03 = 33
B. log(33)=1.52
C. 20*1.52=30dB
D. MDS= -107dBm - 30dB = -137dBm

Let's see where the value -107dBm came from:

1 dBm means 1 milliwatt of power.  In this case the load resistance is
assumed to be 50 ohms, so 1 mW  will produce a voltage across the 50 ohm
load that is =  SquareRoot( 0.001*50) = 0.224 volt rms.

The XG1 output is 1 microvolt into a 50 ohm load for this test, so the ratio
of it's output to the 1dBm reference signal is:
1 microvolt/0.224volt = 0.000004464.
The log(0.000004464) = -5.35
Multiplying by 20 to convert this voltage ratio to dB, we get 20*(-5.35)
= -107dB

The level of the XG1 test signal is -107dBm, that is, 107dB weaker than one
milliwatt. Since this produces an output from the receiver that is well
above the noise level, then the minimum detectable signal would be even
weaker than -107dBm. Typically for the K2 it could be around 30dB weaker (as
we found in step C above), so the minimum detectable signal is
approximately -137dBm.

73/ Bob - W5BIG

========================================================

----- Original Message -----
From: "Ron D'Eau Claire" <[hidden email]>
To: <[hidden email]>
Sent: Thursday, May 12, 2005 1:24 AM
Subject: RE: [Elecraft] Low receiver sensitivity


Bart, PA3GYU wrote:

>Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
>"Signal-to-Noise and MDS Calculations (page 5) for instructions on how
>to use the XG1 to measure receiver sensitivity.
I did, and found this procedure and example:

>A. Divide S+N by N; call the resulting ratio R.
>B. Take the base-10 logarithm of R ("log" key on most calculators). C.
>Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in
>dB. D. If the S+N/N is greater than 10 dB, then the MDS is
>approximately equal to the result from (C) subtracted from -107 dBm.
>
>Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
>
>A. R = 1.0/.03 = 33
>B. log(30) = 1.52
>C. 20 x 1.48 = about 30 dB (this meets the requirement for step D) D.
>MDS = -107 dBm - 30 dB = -137 dBm

Two remarks:
1. I think the second step in the example should read: "B. log(10) = 1.52".
2. I may be missing something here, but if the outcome of B) equals 1.52,
why is "1.48" used in C)? (Not that it changes much, only 0.8 dB, but hey:
it might confuse some people!)

----------------------------------

Sorry for the delay responding Bart! I've been away all day and I see that
none of our other sharp-eyed contributors have answered your question!

Yes, you are right. B. should read log(33) = 1.52 (the base 10 is
"understood" unless a different base is shown and, if so, the base is shown
as a subscript following log, not in parenthesis). The procedure is to take
the result of A. and find the logarithm of that.

I'm really confused about where the 1.48 came from!! That should be 1.52 so
20 x 1.52 = 30 dB.

That instruction sheet was not one if my projects, so I never looked at that
example closely. I'll pass this info along to Wayne.

Ron AC7AC



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[Bill Coleman-2]

On May 10, 2005, at 10:36 AM, [hidden email] wrote:

> I recently assembled and XG1 calibrator and began checking some of  
> my equipment.  4 out of 5 radios that I own indicated an S9 signal,  
> however, my K2 did not.  It registered an S5-S6.  A friend of mine  
> who borrowed my K2 indicated the receiver seemed to be a bit weak.

Is the receiver provably weak, or is the S-meter just stingy? The K2  
receiver may be working just fine, you're S-meter may just be  
adjusted so that it is very stingy.

I had troubles with my AGC and S-meter on my K2. My notes are in the  
archives.
<http://www.ac6rm.net/mailarchive/html/elecraft-list/2002-03/ 
msg00740.html>

I eventually figured this out. I changed R1 to 30.1 k (newer K2s are  
adjustable here) and picked D1 to be a diode with a higher Vf. These  
measures helped a little, after I re-adjusted S-HI and S-LO.

The real problem was that Q12 was bad. Since I didn't have a PN2222A,  
I substituted a 2N2222A (metal TO-18 package). It's been working  
great for a couple of years now.

Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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