[R-390] Sensitivity Measurement Question

[Dennis Wade]

Thank you Roger, some great suggestions.  The lead from the generator to
the radio is rather long so I'm sure shortening is will help.  I'll try the
modified method (moving the receiver off the signal instead of
disconnection).  That has promise.  My purpose is a relative record so I
can identify bands/octaves that are beginning to deteriorate over time.

                 Dennis

On Mon, May 18, 2015 at 12:33 PM, Roger Ruszkowski <
flowertime01 at wmconnect.com> wrote:

>
> 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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-- 
"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