[Elecraft] About the signal-to-noise adjustment method for L34

[Guy Olinger, K2AV]

A caution on using signal to noise on the weak 7 MHz birdie to adjust
L34.

When I first saw this technique mentioned, I tried it to see what
people were talking about.

First thing, the source of the signal cannot be considered as injected
prior to the controlling noise source. So...

Second thing, the adjustment quoted will always be fed a signal with
the signal-to-noise already fixed, since the RF stages of a
well-designed receiver will control S/N if the band in use is quiet
enough for the RX front-end to control S/N, which leads to...

Third, forty meters always (even mid-day) has enough noise to more
than cover the front-end noise of a receiver not nearly so good as a
K2's. Which means...

Forth, the S/N on 40 meters would have to be with no antenna present,
and the active antenna terminated with 50 ohms.

With the antennas unconnected, RF gain at max, any CW IF, the "weak" 7
MHz signal is 31 dB (!!) over the RX internal noise graph. With a 40
meter antenna attached, the same signal is WAY down in the mud, to be
expected, due to the normal high ambient noise on the lower bands.

Any adjustment to L34 results in the same exact 31 dB Signal to noise
graphic.

If one uses the SSB settings for this measurement, it may appear that
the signal to noise is changing because L34 changes the shape of the
SSB passband, a fact that would be lost on anything other than a
graphic presentation. HOWEVER...

With ANY shape to the SSB passband, if the birdie is tuned to occupy
the highest spot on the passband curve, the same 31 dB is observed as
seen on the CW settings.

The method is an urban myth.

Either tune L34 for maximum smoke on CW, or if you prefer, to optimize
the SSB IF bandpass graphic.

73, Guy.
K2AV


----- Original Message -----
From: "Don Wilhelm" <w3fpr@peoplepc.com>
To: "Eddy Avila" <k6sdw@hotmail.com>; <elecraft@mailman.qth.net>
Sent: Friday, April 11, 2003 9:40 PM
Subject: Re: [Elecraft] L34 and headphone jack question.....


> Ed,
>
> When you evaluate the L34 adjustment - look not for just the peak,
but
> instead look for the largest Signal to Noise ratio.
> If you are observing it with Spectrogram - use the 7.000 birdie
rather than
> a wideband noise source and adjust L34 for the biggest DIFFERENCE
between
> the peak of the signal (top of the display) and the noise floor
(bottom of
> the display).  If your K2 is anything like most of them, you will
find the
> correct adjustment is with the slug close to the top of the can.
>
> The headphone jack has no nut on it and on my K2 is actually
recessed a
> small bit behind the front panel.  So if your particular headphones
are
> causing a strain on the jack, you might want to implement an adapter
cable
> having a small stereo plug at the K2 end and a jack to match your
headphones
> at the other end - that way the weight of your headphone plug will
lay on
> your desk and place no strain on your K2 jack.
>
> 73,
> Don W3FPR
>
> ----- Original Message -----
> >
> > Anyways...two more minor questions: using the Spectrograph (great
great
> > product!!) I found L34 rising towards the bottom end of the the
coil and
> > wonder if I should bother to change a component to bring L34 back
into its
> > mid-range? Is so, which component? Or, is this worth the bother?
My
> receiver
> > sensitivity seems to be the same as my ICOM 751A but with a bit
less
> noise.
> >
> > Second question....the headphone jack on the front panel doesn't
protrude
> > enough to put a nut on it, is this by design? the reason I ask if
this is
> > normal cuz when using my big heavy-cabled headphones the plug flex
some
> > which can't be good!
> >
>
>
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