[ARC5] Receiver sensitivity

[Richard Knoppow]

    What you describe is what I was referring to as the standard method. 
Many receivers specify this method although sometimes with 6db ratio 
instead of 10db. Its a good check to see if everything is working right. 
  It is affected by the bandwidth so you can get different results with 
receivers with similar actual front end noise but different bandwidth. 
There is a variation using the BFO for measuring CW sensitivity. 
Usually, the CW sensitivity will be considerably greater, usually less 
than a microvolt.
    Some very old receivers specified the RF level needed to produce a 
given audio output level. This may be useful for trouble shooting but is 
meaningless as far as noise performance goes.
    Unfortunately there are few short wave broadcasters left. I read 
that Radio Australia is going to discontinue short wave at the end of 
the year. I can always hear them regardless of how awful radio 
conditions are. I hope Radio New Zealand continues.

On 12/10/2016 9:16 PM, Scott Robinson wrote:
> Folks,
>
> In wood German table radios, I have a very interesting exception:
>
> The usual German radio circuit uses an ECH81 (triode-hexode) converter
> with separate oscillator. The Grundig model 4040, unique as far as I
> know, uses a dual triode mixer. Neither circuit uses an RF amp. I should
> go measure its noise floor, some day.
>
> Now, about receiver noise measurement: I follow the usual commercial
> method. What you do is feed in 30% 400 Hz modulated RF and turn the RF
> level down until the output audio is 10 dB more with the modulation
> turned on than with it off. You do NOT turn the carrier off! If your
> receiver has noticeable hum, you might want to high pass the audio at
> 200 Hz or so.
>
> Measured this way, good SW receivers (My SX-28A and R-390A, for
> instance) achieve 2-3 microvolts generally.
>
> I'll also suggest a test to do with any such receiver: connect an
> ohmmeter from antenna to ground and run the bandswitch through all the
> bands. This finds the band whose antenna coil primary got zapped by a
> lightning storm before you waste time wondering why one band is so deaf.
> Don't ask me why I know this...
>
> Peace, good listening, and fond remembering of times when teh SWBC bands
> had all sorts of good stuff to hear.
>
> /scott
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL