Message-ID: <001e01cad658$85771340$0101a8c0@amd4800>
From: "George Vastianos" <george@vastianos.com>
To: <rsgb_lf_group@blacksheep.org>
References: <MFEJIKCLCMPDFHOKMANAGEIPOFAA.paulc@snet.net> <001c01cad439$aa7aa830$0101a8c0@amd4800> <38A51B74B884D74083D7950AD0DD85E82A1BD5@File-Server-HST.hst.e-technik.tu-darmstadt.de> <003801cad57e$a13410e0$0101a8c0@amd4800> <8585FA1C733D9A42AEA98C724D8FB811C05393@FBCMST18V01.fbc.local> <002301cad597$6b32cdb0$0101a8c0@amd4800> <8585FA1C733D9A42AEA98C724D8FB811C05394@FBCMST18V01.fbc.local> <001901cad61c$5ffbdb50$0101a8c0@amd4800> <4BBC3FC1.5070806@iup.uni-heidelberg.de> <000901cad63c$cdc63020$0101a8c0@amd4800> <3C4FBB8411D9476686BB6F23CE272AC9@JimPC>
Date: Wed, 7 Apr 2010 16:45:01 +0300
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Subject: Re: LF: About 9khz loop
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thanks for all info!

i just returned from the market...and have all the electronic parts on my 
bench. The construction is very close

I hope to be ready on Stefan's next try on VLF

73

sv8gxc
----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, April 07, 2010 3:02 PM
Subject: Re: LF: About 9khz loop


> Dear George, LF Group,
>
> I have attached the 9kHz loop/preamp/filter schematic again, as it would
> seem some explanation would be useful - OK, here goes...;-)
>
> There are basically 3 frequency response determining elements in the 
> design.
> The first is the loop inductance, the shunt input capacitance C8, and the
> input resistance of the preamp (about 300R, determined mostly by resistors
> R1, R2 and R4, which also set the gain of the input stage). These together
> form a singly-terminated, second-order low pass filter with a roughly
> Butterworth response and cut-off frequency something over 20kHz. The 
> preamp
> stage gives about 30dB gain, and a reasonably low noise level - evidently
> below the external band noise. This was arrived at after some
> back-of-envelope calculations and tests of likely noise levels, induced 
> EMF
> in the loop, and so on.
>
> The second element is the op-amp gain stage IC1A. This is configured as a
> "lossy integrator", with a gain that rolls off at 20dB/decade in the
> operating frequency range. This compensates for the loop output, which 
> rises
> at 20dB/decade below its cut off frequency, due to the loop EMF being
> proportional to d(phi)/dt. It isn't essential to do this, but I felt this
> would be useful in comparing noise levels, etc., and reduce the likelihood
> of overloading by VLF utilities. At 9kHz, the gain of this stage is about
> 30dB also; the overall 60dB gain gave a good signal level for the audio 
> line
> input socket on my lap-top, determined by trial and error.
>
> The final element is the bandpass filter. This was a straightforward 
> ladder
> filter designed with a n=3 Butterworth response, with the component values
> fiddled a bit to suit off-the-shelf inductors. I aimed for an impedance
> level that could easily be driven by normal op-amps. The filter is
> terminated by 1200ohm resistors R10, R11, which are necessary to give the
> correct response (actually, the initial design values were about 900ohm or
> something, but I increased the value to "absorb" the loss resistance of 
> the
> rather low-Q 4.7mH inductors). The IC1A stage has a high input impedance
> that does not load the filter significantly, but the input preamp stage 
> has
> a fairly high output impedance, and its gain would also be affected by
> connecting the filter as a load. So the IC1B buffer stage was included,
> which also provides 9dB gain to make up for the 6dB loss between the
> terminations, plus approximately 3dB insertion loss of the filter.
>
> So all the parts of this circuit were designed to work together - if you
> wish to build a circuit with a different antenna, or use the filter in a
> different system, you need to take similar considerations into account -
> there are a near-infinite number of feasible solutions that will perform 
> as
> well or better, but all will have differences.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>


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