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LNB Power insertion

Posted: Thu Jan 11, 2018 9:19 pm
New forum user here,

I have plenty of clean sources of dc in the form of 5-12-13.8 regulated power supplies I've acquired over the years. I have an Avenger dual LNBF (PLL322S-2) and the dongle v.3 I read that there is a bias t inside but it only can output 5v. Now realistically, there has to be a way with this hardware, a soldering iron, and some patience to get 12-13v to the LNB without buying anything else. I have loads of coax and rg6, crimper, f connectors, a sup-2400 (probably irrelevant). Bottom line, bare minimum instructions to get this working or a link to a CHINESE bias tee. I'm not interested in buying any of these purpose built units out of convenience, I prefer hacking together what I have. I don't need any "you could break this if you don't do it the right way" type of responses. The LNB was 5 dollars and the dongle was 20. I already have an exact duplicate v3 and a fc0012 lying around so I'm open to experimentation. I figure it's resistors and diodes since diodes only allow power to flow one way. ALSO I read that a 22khz signal is required to tell the LNBF what mixer to use or else it defaults to 9.57Ghz?
I've been playing with SDRs for a few months now (long time ham) and have successfully decoded a multitude of digital signals in both Linux and windows environments. I have a dedicated pager decoder running on a raspberry pi. I would not consider myself a "new" user as I have achieved excellent quality weather fax from NOAA sats on my home made pvc QFH (was a hell of an experience building that). I'm ready to make the jump from 854 km to 36000 km. And yes, I realize HRPT from GOES and NOAA polar orbiters (15-18-19) is L band 1698Mhz about, that also is not what I'm asking, just how to hack power into an LNB with scavenged hardware , please advise. Thank you in advance for your on-topic, prompt, courteous response.

Re: LNB Power insertion

Posted: Thu Jan 11, 2018 10:12 pm
by rtlsdrblog
If you're okay with building something then why not build an external bias tee with 12v input yourself? A simple bias tee only consists of a capacitor and inductor.

There are also several cheap external bias tees available on eBay that you could buy or copy the circuit of ... e&_sacat=0.

Just be careful and double check that the 12V is definitely not feeding back into the dongle.

But about the 22 kHz signal, i'm not sure about that. You'd need to research the LNB and find exactly what type of control signal it needs. Then you could look at a way to generate and inject that signal. Probably there would also be a way to modify the LNB circuit itself to change the output, but I wouldn't know how.

Re: LNB Power insertion

Posted: Thu Jan 11, 2018 10:47 pm
Thank you for your response, I was thinking that since the v3 official rtl-sdr dongle has an integrated bias tee, wouldn't I be able to amplify that or at least feel fairly assured that it has some sort of protection circuit in it already? I read on the user guide it has a safety device in it. Also, I may have accidentally bought 2 of those thinking they were only LNAs but they have a dc input on them. ... +amplifier. I'm sure the v3 can run this off the internal bias tee. if it's only set up for 5v maybe I can mod the other one considering it has some smas already soldered on.

Re: LNB Power insertion

Posted: Fri Jan 12, 2018 11:37 pm
by rtlsdrblog
The RTL-SDR is powered by USB, so the maximum voltage out can only be 5v. There is an LDO on the bias tee for switching and filtering that takes it further down to 4.5v. You could try buck boosting it with an external circuit, but no idea if it would work well or if it would be super noisy.

The input of the RTL-SDR V3 can withstand voltages on the input, but the ESD diode will probably break down at 12v if voltage appears there. The whole point of a bias tee or power injector is to put voltage to the antenna side ONLY, and protect the receiver, so this shouldnt be an issue, assuming that you get the connections the right way around.

Those LNA's you've bought don't have any bias tee input, you need to power them externally. Of course you could build a bias tee for them and take the power out to them that way. I've done this before with those and it's as simple as adding a ~4.70uH inductor from the RFout center pin to the VCC connection of the LNA PCB.