[R-390] Sensitivity Measurement Question

[Roger Ruszkowski]

Dennis,

You just are not living in a screened and well grounded noise free shack.
Aggravation is going to exceed return on investment.

I have not read Chucks procedure lately.
Are you using a 600 ohm load on the line output to give you
proper readings for a loaded circuit?

You are right the cable acts as an antenna.
First choice is the screened and grounded shack.
Second choice is real good short grounds to the test bench.
Third choice is balanced feed to cancel as much noise as possible but not practical
between the generator and receiver and knowing losses through the circuit.
Plus you still have everything hanging in free space acting as an antenna. 

At the line level meter you are now using the receiver as a frequency selective 
meter. You can try for a quiet spot between 2.000 and 3.999.

Just align the receiver as best you can and do not worry about these absolute test values.
The receiver uses the same tubes from .5 to 7.999.
If you have good signal to noise any where in that range the receiver will be OK.
For sure you want each octave aligned as best you can get it. 
But do not worry about the exact numbers.

Use the CW to modulated procedure outlined in the TM for signal to noise ratio.

Put a key in the cable between the generator and the receiver and then with the
receiver setup for CW and your head phones find the minimum discernible   
signal. How far can you dial the generator output level down and still get a CW
tone you can copy above the noise floor. 

Connect the generator as Chuck says. Dial the receiver off the generator frequency.
and set the line level to give you the -10. Dial the receiver on to the generator frequency
and reduce the generator output level t get the 0 reading you seek.
See how these numbers work out.
You should get the same range of values on all frequencies. 
They will vary but it will give you the confidence the receiver is working well
and every thing is well aligned.

Roger Ruszkowski AI4NI




 
 
 
-----Original Message-----
From: Dennis Wade <sacramento.cyclist at gmail.com>
To: R-390 HF Receiver List <r-390 at mailman.qth.net>
Sent: Mon, May 18, 2015 1:17 am
Subject: [R-390] Sensitivity Measurement Question


Good evening,

         After securing another Z202 from Fair Radio and
installing it into
the RF deck and re-aligning that octave, I set about to
measure sensitivity
according to Chuck's method as published in the Pearls. 
The instructions
specify to set the noise level on the line level meter to -10
db without
the generator connected, then connecting the generator and finding
the
level that yields a 0 db reading on the Line Level meter for a
measurement
of the 10db S/N + N sensitivity.

           Following this
method on the 0 and 1 mhz bands went well,
yielding about 0.3 - 0.35 uv
readings.

            The problem I run into on the higher bands (2 and
3
specifically) is this:  I can set the -10 db level just fine, but when
I
attach the cable to the generator, it acts as an antenna and the noise
level
rises dramatically.  In other words, with the cable connected, and
the RF
turned off, the line level is well above the 0 db mark.

            The
generator is an HP 8657B.  Feeding the output into a Tek
DCA 602 scope doesn't
reveal any added noise over the baseline that I can
discern with just the cable
connected with no RF out.

            What am I doing wrong?

 Thanks!

 
Dennis

-- 
"If they trust you, it is an extraordinary privilege, and you
simply can't
abuse it."
      - A. Alan Post  1914-2011.  California
Legislative Analyst 1949-1977.

-------------------------
Dennis L.
Wade
KG6ZI
Carmichael,
CA
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