P3 Question

[Craig Smith]

Here's a simple (hopefully) question that I'm sure one or more of you RF
gurus can easily handle.  But I'm having difficulty with it, at least today.

I was wondering what baseline level I should be seeing on my P3 with the
input terminated in 50 ohms (no signal in).  This isn't given in the
specifications.  But the noise figure is given (less than 10 dB) so I
figured it would be easy to calculate with the usual sensitivity formula as
a function of noise figure.  Of course you need Boltzman's constant (no
problem), temperature (no problem), and desired S/N ratio (assume 5 dB).
But what should I use for bandwidth, since with the P3 there isn't a normal
bandwidth filter in the circuit such as with the K3.  Do I use the full 200
KHz maximum span?  Something else?  

My P3 shows about -132 dBm with its input terminated.  And about -130 dBm
when connected to the K3 with the K3 antenna inputs terminated.  Is this in
the expected range?

73   Craig   AC0DS  



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[Alan Bloom-2]

> Here's a simple (hopefully) question ...

The answer is a little complicated.  :=)

First, the 10 dB noise figure is for the P3 itself.  The total noise on
the display includes the contribution from the K3.  Assuming you have
done the IF Out mod on the K3, the noise figure of the K3/P3 combination
should be within a dB or so of the NF of the K3 alone.

The effective bandwidth of the P3 is more-or-less one display pixel,
with a lower limit of a little over 7 Hz at the narrowest spans.  There
are 468 pixels, so for spans greater than 3.5 kHz, the bandwidth is
approximately span/468.

The sensitivity of the K3 depends on the state of its preamplifier and
attenuator.  With preamp on, the K3 data sheet says the sensitivity is
-136 to -138 dBm typical in a 500 Hz bandwidth.  So, assuming the total
noise of the K3/P3 combination is -136 dBm in a 500 Hz bandwidth, with
the K3 antenna connector terminated in a 50-ohm dummy load, the noise
level on the P3 display should be:

-136 - 10 * log [500 / (span/468)]

With a 50 kHz span, that works out to about -143 dBm.  And of course
higher than that with the K3 preamp off.  And of course much higher than
that with an antenna connected to the K3.

There are a number of factors, both theoretical and practical, that can
throw that number off a bit, but it should be in the ballpark.

Alan N1AL





On Tue, 2010-08-17 at 17:16 -0600, Craig D. Smith wrote:

> Here's a simple (hopefully) question that I'm sure one or more of you RF
> gurus can easily handle.  But I'm having difficulty with it, at least today.
>
> I was wondering what baseline level I should be seeing on my P3 with the
> input terminated in 50 ohms (no signal in).  This isn't given in the
> specifications.  But the noise figure is given (less than 10 dB) so I
> figured it would be easy to calculate with the usual sensitivity formula as
> a function of noise figure.  Of course you need Boltzman's constant (no
> problem), temperature (no problem), and desired S/N ratio (assume 5 dB).
> But what should I use for bandwidth, since with the P3 there isn't a normal
> bandwidth filter in the circuit such as with the K3.  Do I use the full 200
> KHz maximum span?  Something else?  
>
> My P3 shows about -132 dBm with its input terminated.  And about -130 dBm
> when connected to the K3 with the K3 antenna inputs terminated.  Is this in
> the expected range?
>
> 73   Craig   AC0DS  
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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