<?color><?param0100,0100,0100>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>><?/color>"...the implementation of figure 8.6 has a resolution of 0.35 <br>> degrees.<br><br><?color><?param0100,0100,0100>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>><?/color>I'll repeat those readings on a 4m diameter aluminum loop, 17mm >tube, assoon as I finish my remotely controlled capacitor decade >circuit (1nf steps from 0 to 350nf). Seems that the 1 turn solution >will perform well -and the best part - no pre-amp, but the decade C >:o(<br><br><?color><?param0100,0100,0100>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><?param0100,0100,0100>. 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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