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Date: Sat, 16 Dec 2017 23:37:23 +0100
From: DK7FC <selberdenken@posteo.de>
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In-Reply-To: <36a938b7-9db4-7417-26d6-5b4c4eac900e@abelian.org>
Subject: Re: LF: RE: Todmorden receiver
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Hi Paul,

looks good. I'm using the same waterproof box for the stuff on the tree :-)
What type of isolation transformer are you using? Datasheet? Distributor?

As soon as the decode of the 3 char message happens, there will be 
regular transmissions on 5170 Hz again. The 7th transmission has already 
started...

I'm curious about ULF! Can't wait for the > 100 km waves... Soon the 
days become longer again...

There is some progress on the 970 Hz project. I will report later...

73, Stefan


Am 16.12.2017 23:06, schrieb Paul Nicholson:
>
> New H rx under test
>
>  http://abelian.org/vlf/tmp/171216a.jpg
>
> with the box on the left injecting a calibrated current in
> parallel with the dummy antenna.  This is an end-to-end test:
> antenna to sound card, with 150m cat5 and the isolating
> transformers.
>
> Most of the testing is automatic: the VLF calibrator works
> through a time/frequency schedule and a vlfrx-tools script
> does the measurements to the same schedule.
>
> System noise measurement is a bit trickier.  The output noise
> of the rx is only a little above the sound card's noise floor
> (in fact is lower at higher frequencies).  I allow for this
> by subtracting the sound card noise power but to make it more
> accurate I set a 10dB higher rx gain than when deployed.
> That higher gain can't be used in the field because it would
> overload on the 50Hz and strong MSK signals.  This illustrates
> the tough dynamic range requirement of a VLF rx, even when
> using a high quality sound card such as the 24 bit M-Audio 192.
>
> Intermod measurement is another tricky one.  The sound card
> generates a 2-tone test at some high frequency, through a
> passive high pass filter into the rx.  Rx output through
> a passive low pass back to the sound card input to measure
> the level of the difference frequency.   Often a receiver
> will look good on this test but do badly on the antenna.
> The culprit is usually the strong 50 Hz signal distorting
> very slightly as it goes through the isolating transformers
> and modulating the whole band as it does so.   The cure is
> to tailor the rx HP response so that the 50Hz is low enough
> not to cause noticeable intermod.   What exactly 'noticeable'
> means depends on whether you're listening to whistlers and
> tweeks, or trying to dig a weak carrier out of the noise.
> In a bad case you can hear the intermod - the background noise
> has a sizzling buzz to it instead of a pure hiss.   A not so
> bad case will not be audible but will raise the noise floor a
> few dB.  The whole system: rx, transformers, and sound card,
> have to be impeccably linear to actually achieve the noise
> floor capability of the front-end device.
>
> The final thing to do is set the gain, which is fixed by
> components.  I try to get within a factor of two either way
> by bench testing.  When it's connected to the antenna I'll do
> some measurements then will probably have to bring it back
> in for final adjustment to the gain.   As it stands, the 2
> channels are matched to 0.52% at 8.3 kHz.
>
> Hopefully next week it will go outside for a real test.
> Only one pair of loops and feeder cable so can't run the old
> and new H rxs for a side-by-side comparison.  I'll be making
> good use of some regular distant amateur signals.
>
> -- 
> Paul Nicholson
> -- 
>