modify preamp for bias-t - Page 1
Forum section: electronics / rf-microwave

author: bob808
author_rank: Frequent Contributor
timestamp: March 06, 2017, 03:42:54 pm
content: Hello,I just got into SDR and bought a preamp from ebay, the cheap ones.As I see, it has a separate 5V supply. Since my sdr dongle has bias-t support, I'd like to send the power through the coax to power the preamp.After seeing some other devices online I figured I can just add a choke between the output to the sdr and the 5V input on the board.It uses the SPF5189Z amplifier.PDF here:http://www.rfmd.com/store/downloads/dl/file/id/28188/spf5189z_data_sheet.pdfAnd the application on my board seems to be identical to the one in the datasheet. There's three 0 ohm resistors added, but the rest seems to be the same.Now, I see two ways of powering the amp via the coax. One would be to remove the output capacitor and the inductor feeding the voltage to the chip, and bridge the capacitor pads. But that is not the recommended application.The second way would be to add an inductor between the output sma pin and the 5V pad. I can wind an aircore inductor at about 2uH or so. Would that be ok?

author: kg4arn
author_rank: Unknown
timestamp: March 07, 2017, 08:50:12 am
content: Quote from: bob808 on March 06, 2017, 03:42:54 pmWould you draw a schematics of what you are proposing?

author: babysitter
author_rank: Frequent Contributor
timestamp: March 07, 2017, 09:01:41 am
content: I recommend using a choke from output-coax center pin to a cap, feeding a low-noise 5V regulator from that voltage, and use the regulator output to feed the amplifier. Use a higher voltage on the feed line. This way you can supress some noise which gets introduced to your coax line, I guess one doesn't want the bias of a RF am to jump around.

author: bob808
author_rank: Frequent Contributor
timestamp: March 07, 2017, 11:56:50 am
content: Something like this

author: bob808
author_rank: Frequent Contributor
timestamp: March 07, 2017, 11:59:34 am
content: Quote from: babysitter on March 07, 2017, 09:01:41 amI see why this would be recommended. But I have 4.5-5V barely to work with. And the amp chip is 5V at full power. I may need to experiment a bit, as maybe full gain would not help me and I could use 3.3V or so. A 3.3V regulator should work very good.

author: babysitter
author_rank: Frequent Contributor
timestamp: March 08, 2017, 10:47:24 am
content: If you are really fighting for every hundred mV, this might be the way to go:http://www.wenzel.com/documents/finesse.html(No experience with this, just aware of its existence)

author: whiyne
author_rank: Newbie
timestamp: January 23, 2018, 09:15:53 am
content: Firstly, sorry for the thread revival - this was the only related topic I can find!OP, did you even manage to successfully complete this mod? I have the exact same LNA and am hoping to do the same.Or, if anyone else has advice on whether the mod shown above is a good idea, i'm all ears! Given the price of the LNA I'm leaning towards just having a go and seeing what happens.Cheers

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 09:03:19 pm
content: Hint: try Mini Circuits. They have Tees for all kinds o frequencies and even ones with SMA connectors.