# Implementing a bias tee

I'm pretty new to electronics and I'm having trouble understanding what's happening in this schematic:

I can see the inductor is going straight from the + to the SMA, however I'm not sure how to implement the capacitors, I can't see how he's routing them on the photo. How do I need to wire C15 and C16 if I want to supply the IC and oscillator with DC only?

• sorry, as interesting as this question is, that hand-soldered mess of a board is really impossible to analyze from your photo. There's a big (yellow) tantalum cap already in the photo, so I'd guess that is C15 already. But really, how should we make out what contacts what (and what not) from your photo? What has been done here clearly looks like something that should not have been done on perfboard, but on a quickly designed dedicatedly layouted PCB. Jul 29, 2017 at 16:29
• (seriously, does this look like the perfect photo for analyzing this circuit to you? Looks more like an angled, sharp shadows, out-of-focus thing to me.) Jul 29, 2017 at 16:31
• Right, unfortunately that's the only photo the author supplied of his upconverter, if there was a good one I'd likely be able to see how he routed it. Jul 29, 2017 at 16:38

According to the schematic, inductor L5 (56uH) has nothing to do with the upconverter integrated circuits...it supplies DC power to the signal source via the input SMA connector. If the input signal source doesn't need this "Bias-T", then L5 is not needed and should be removed.

DC input +5V should have a direct path to all these points:

• NE612 pin 8
• D3 anode
• C16
• C15(+)
• C1

There's another possibility: The signal source is also the source of +5V. In this case, the bias-T is required, and L5 feeds +5v to the nodes mentioned above.

• May I ask what the purpose of C15 and C16 is, they just go to ground? Does it matter in what order the positive rail will make contacts with the points you referenced? Jul 29, 2017 at 16:44
• C15 and C16 are bypass capacitors (yes...going to ground) to ensure that the input signal doesn't find its way to NE612 pin 8 (DC supply pin). C15 position doesn't matter so much. C1 should be close to NE612 pin 8. Use good RF grounding rules: all capacitors should have short, wide paths to ground. Jul 29, 2017 at 16:52
• Thanks, and just a little side note - it's the other way around - the upconverter feeds power to an active antenna. Jul 29, 2017 at 18:00

You cannot test a portion of the circuit for DC without affecting RF properties. DC is supplied on 2P header , and then C15,C16 reduce the Z(f) wrt. local ground then is distributed via ground plane as per schematic to reduce LO noise , provide low noise DC bias etc. Various parts provide impedance ratio filters for optimal DC and DC in RF noise blocking and signal filtering.

There is no good reason to do what you are asking for, that I can see.

The RF filters provide low (Z*f) for RF where DC is needed and high Z(f) where RF signal is used. Thus LPF for DC and BPF for signal are BOTH needed.