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Title: Are diodes noisy? Post by: K6JEK on July 20, 2008, 04:18:58 PM One of the receive antennas I'm experimenting with is the Wellbrook loop, ALA-1530: http://www.wellbrook.uk.com/products.html#ALA1530 . It has a pre-amp at the antenna. The instructions are adamant about regulated 12V not 13.8V which, of course, is what I have floating around the shack.
I've been dropping the regulated 13.8 down to 12 with a string of diodes. Seems simple and effective. Is there anything wrong with that? Are diodes noisy? Title: Re: Are diodes noisy? Post by: Tom WA3KLR on July 20, 2008, 05:19:25 PM Diodes can be noisy. I've seen bad switching diodes that were quite noisy while conducting. Zener diodes are the worst, a good Zener does this while conducting. They are used for noise sources at times, such as in the antenna noise bridges.
Resistors are noise generators too, but is calculatable. Your pre-amp should be a constant current load, so a series resistor could be tried instead of the diodes. This gives a series impedance for additional filter capacitors to work against. Add an array of capacitors to common - large and small film and some r.f. ceramic values. Then add a series r.f. choke (the value appropriate for your frequency) and another r.f. shunt cap. ahead of your final feed choke. Title: Re: Are diodes noisy? Post by: K6JEK on July 20, 2008, 06:47:01 PM Diodes can be noisey. I've seen bad switching diodes that were quite noisey while conducting. Zener diodes are the worst, a good Zener does this while conducting. They are used for noise sources at times, such as in the antenna noise bridges. Yes, I think you're right. I'm going with the resistor, caps, etc.Resistors are noise generators too, but is calculatable. Your pre-amp should be a constant current load, so a series resistor could be tried instead of the diodes. This gives a series impedance for additional filter capacitors to work against. Add an array of capacitors to common - large and small film and some r.f. ceramic values. Then add a series r.f. choke (the value appropriate for your frequency) and another r.f. shunt cap. ahead of your final feed choke. Thanks, Jon Title: Re: Are diodes noisy? Post by: Rob K2CU on July 21, 2008, 07:47:14 AM You might consider a LM2937-12 low dropout linear three terminal voltage regulator. There are other similar parts and ones that require two resistors to set the output voltage. But the low drop out bit means that you can have the input voltage be only a half a volt or so higher than the input. Old style linear regulators, like the 7812 require 2.7 Volts higher input than output. Since your input is 13.8 VDC, a low dropout part would do the trick. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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