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LNA power supply?

Posted: Sun May 12, 2019 11:22 pm
by Kev7274
I'm using a LNA with a 9v and was thinking about using something like a 12v 1a phone charger to power the LNA. Was wondering if that would create any noise though. Anybody tried this?

Re: LNA power supply?

Posted: Mon May 13, 2019 3:40 am
by hotpaw2
Most 12V wall-wart power supplies use switching regulated voltage converters. A consumer-grade switching voltage regulator generates tons of RF noise, possibly not even meeting FCC part 15 RF emission regulations with the most cheaply made ones.

Best bet, other than a primary battery, would be a linear regulated power supply, if you can find one. They are less efficient, and run warmer, but generate almost no RF noise.

Another option, which may or may not work, is filtering. Lots of ferrite snap-ons, or better yet, wrapping multiple turns though ferrite cores/donuts at both ends of all power cords.

Whatever is powering the computer into which your RTL-SDR is plugged can have similar issues.

Re: LNA power supply?

Posted: Tue May 14, 2019 1:37 pm
by grafter
Fleabay, and look for old CB radio power supplies. I use a couple of Bremi BRS27s because they have variable voltage output accessed from the PCB, one runs at 12V for radios, the other at 16V to power a laptop with no noise on HF or VHF.

Re: LNA power supply?

Posted: Tue May 14, 2019 4:38 pm
by snn47
linear regulated power supply (PS), .... They are less efficient, and run warmer, but generate almost no RF noise.
I disagree just having a PS with linear regulator doens't mean that the DC-Output voltage has less noise or spikes (due to lack of isolation between AC Input and DC Output,) compared to a good switching PS.

It is not just RF-Noise created by the switching part of a PS, but also noise, and spikes in all form on the supply voltage.

If you have access to 12V battery try it.

Re: LNA power supply?

Posted: Tue May 14, 2019 7:36 pm
by hotpaw2
Yes, a linear regulator can pass supply RF noise through its transformer and drop-out impedance. But it's far less likely to generate RF noise of its own, as do almost all switching power supplies.

distinctions between RF-/AF-noise/-interference -- LNA power supply?

Posted: Thu May 16, 2019 10:47 am
by snn47
@hotpaw2 and all
One cannot judge that switching PS generates more AF/RF noise/interference than any linear regulated PS design, without having tested a PS or used it to supply a LNA.

Depending on design and components used, also linear regulator circuits can generate substantial AF noise on the supply voltage. I have encountered many e.g. very noise Zehner diodes, both in form of discrete Diodes and in form of an integrated diode in a linear voltage regulator IC.

A battery placed as close as possible to the LNA which can supply the LNA alternately to the choosen PS, is imho the only choice for Kev7274 and everyone, to test/identify if their choosen PS meets their expectations or if the PS creates unacceptable noise/interference.

If you talk about RF-noise I assume you mean interference, but don't care if it is really AF-noise/-interference on the DC-output or really any RF-interference on the output. If you really mean RF interference, you cannot eliminate without meassurements that RF-interference is from the PS and was not picked up by an unshielded DC supply cable between PS and LNA? The longer the DC cable is the more likely it is that the LNA can pick strong RF sources, which can create additional interference in an LNA.

- My understanding of noise is that noise consists of an indefinite number of frequencies for a given bandwidth. Noise differs, unless flat in the amplitude distribution, in the form of the amplitudes with frequency. White noise has an identical amplitude with frequency there are also other forms like pink or Gaussian distribution used e.g. for AF noise characterization. An AF noise on the DC-voltage is imposed on the DC of the LNA supply voltage and therefore also on the amplified output of an LNA.

Anything else is in may understanding interference. A switching PS can produce interference distributed over a very large frequency range, but since it will produce only a very limited number of frequencies it is not noise. This does not mean that it will not severely interfere at many frequencies, but noise implies that any frequency will have noise and not only those frequencies which are interfered.

- Spikes are very short e.g. from short transitions generated by sparks from mechanical switches, sparking motors or lightning, distributed over the AC-lines through a PS on the DC-voltage.
- harmonics or wideband interference originate e.g. from badly, for cost purposes cheap designs or designs with only minimal filtering like in switching PS, AC regulators. Due to their large power consumption moteors e.g. from electrical trains can produce large interference which is distributes over their AC-lines.
- RF-interference is radiated and limited in bandwidth, or are harmonics.
- wideband RF-interference exists to e.g. generated by power line modems, which can exceed 100 MHz by now or by UWB transmitter which can have several GHz in bandwidth, but even though the signals are distributed over a very wide frequency range it still is not not noise.
- While there exist specialized wideband noise sources, e.g. NS301 Noise generator diode (10 Hz - 3 GHz, 8/12V, 5mA, output level 30/35dBENR -144/-139dBm/Hz) they are very low power only sufficent for receiver test purposes. Only very few really wideband higher power RF-noise transmitter are available and it is therfore highly unlikely any one would be subjected to RF-noise.

Re: LNA power supply?

Posted: Thu May 16, 2019 6:24 pm
by hotpaw2
Interesting. I've always assumed that unwanted interference was a subset of noise, not a disjoint set.