Message-ID: <5501.200102051521@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 5 Feb 2001 15:25:37 +0000
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Subject: LF: PLL / Loop antennas
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<?color><?param 0100,0100,0100>Dear LF Group,<br><br>I Think my E-mail account has started working again - about 80 E- mails turned up this morning. Glad to see some more signals have  made it across the Atlantic. I will wade through all the stuff -   eventually!<br><br>Re: Phase locked loops; Andre wrote- <br><br>&gt;<?/color>"...the implementation of figure 8.6 has a resolution of 0.35  <br>&gt; degrees.<br><br><?color><?param 0100,0100,0100>A fixed error in frequency means a constantly increasing error in  phase. Suppose a phase locked loop has a reference frequency of  1MHz exactly, and is trying to lock a VCO which is running at  1.000001MHz. If both reference and VCO have the same phase at  one particular moment, one cycle later the VCO will be leading in  phase by 1e-6 cycles, or 0.00036 degrees, which may well be lost  in the noise and other errors at the phase detector output. But after  2 cycles it will be 0.00072 degrees, after about 972 cycles, the  accumulated ph
ase error will be 0.35 degrees, and after 1000000  cycles it will be 360 degrees. However small the frequency error,  over a long enough time the phase error will increase to the point  where it becomes the dominant signal in the feedback loop -  probably a rather small fraction of a second in the above example. <br><br>Re - loop antennas - Marcus wrote:<br><br>&gt;<?/color>I'll repeat those readings on a 4m diameter aluminum loop, 17mm  &gt;tube, assoon as I finish my remotely controlled capacitor decade  &gt;circuit (1nf steps from 0 to 350nf). Seems that the 1 turn solution  &gt;will perform well -and the best part - no pre-amp, but the decade C  &gt;:o(<br><br><?color><?param 0100,0100,0100>In theory and to a reasonable approximation, the SNR of a loop  depends only upon it's effective area and Q, and not on the number  of turns. This assumes a preamp or receiver input which does not  add significantly to the noise generated by the loop itself, which is  usually possible.
 So a 1 turn loop is quite reasonable, with obvious  mechanical advantages. But there are also difficulties. Tuning  capacitance has to be very large at LF. Impedances are very low,  so resistances must also be kept very low in order to avoid  severely reducing Q. At U of H, we found wide range, switched  tuning capacitors difficult to implement with a single turn loop,  mostly due to trouble finding low-resistance switches. In the LF  range, and loop inductances of the order of 10s of uH, 0.1 ohm will  be significant. Bearing in mind multiple switches will probably be  needed, this can be a real problem. We found relays were really  the only option for remote switching, and even then you have to be  careful - reed relays had losses around 0.1 - 0.5 ohms, much  higher than their DC resistance.  The best bet are relays intended  for UHF antenna changeover duty.<br><br>However, problems can be greatly eased by going to larger loop  sizes - with the increased signal level, and t
he high noise levels on  LF, optimum Q is not then required, and tuning arrangements are  less critical. I am currently using 2 square, single turn loops with  3m sides, made from 1.5mm^2 wire, mounted on the same pole,  <?/color>roughly at right angles<?color><?param 0100,0100,0100>. One of the two loops is selected by a relay,  giving 360 degree coverage. This is connected to a tuner in the  shack by about 25m of RG58 coax. the tuner has (for 136kHz), a  700uH inductor in series with the loop, tuned by about 2nF to  ground. A high impedance preamp input is connected to the hot end  of the tuning C. Although this is far from optimum from the noise  viewpoint, external noise appears to be the limit on sensitivity. This  antenna has worked well for receiving the various transatlantic  signals. It can be tuned over a wide range by switching in different  tuning capacitors and inductors, and has also worked well for  73kHz.<br><br>Cheers, Jim Moritz<br>73 de M0BMU<br><br><br><br
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