Thursday, December 10, 2015

DIY Bias-T

I am receiving more and more requests for the separate Bias-T unit that can be accompanied with the LNA4ALL. The LNA4ALL has the Bias-T powering option but if you do not have such a possibility on your receiver side you need to buy one or to build one. Hack-RF, FCD, AirSpy support the DC over the coaxial cable but the cheap DVB-T dongles are without such an option.

Bias-T is nothing more than a simple diplexer but sometimes simple things are not simple to design if you want to have a proper performance. The bias-T that we need should be quite broadband and this is not a trivial task. To make things as simple as possible the compromise is required. This project should be good enough for our needs. All the parts can be retrieved from an old hard disk drive, but if you are ordering a new one the shipping will be much more then the parts itself.  I use the "round" values that are most common.

Recently we introduce the ADS-B filter where on the same PCB the bias-T is integrated. If you want to use only the bias-T option without filter you can still use the PCB designed for the filter on the picture above. The parts placement and the SMD standard size components are clearly visible. Series capacitor 1206 size is created by parallel 100pf and 1nF capacitors.  Bias-T maximum frequency declared is 2 GHz. The Insertion Loss on the higher frequencies is not a problem and found to be low, but above 2 GHz frequencies the Return Loss start to fall bellow 10dB value and this may create some problems with the unit sensible to good matching and impedance. This is why we declare the operating range from 1-2000 MHz only. Find the Insertion losses related to the frequency in the following table.

The filter PCB only can be ordered following the link : ADS-B filter
If you prefer the DIY bias-T older design here is the part placement for the same unit but assembled on the LNA4HF PCB that can be ordered following this link: LNA4HF

I assemble mine on the LNA4HF PCB as this was handy and offer a lot of possibility and extra option, as installing the voltage regulator or extra filter, but this will not be explained now. All the parts are SMD 0805 size except the middle parallel 1nF+100pf capacitors. They are 1206 size just because the gap for the MMIC which should be in place is to big to accommodate the 0805 size. This two capacitors are mounted one top the other. The green parts are simple 0ohm bridges, also 0805 size or just a piece of wire. You can design your own PCB where this green bridges will be replaced with the PCB trace, so even simpler. The PCB can be cut with the X-acto knife like the one on the following picture.

This one is made on the Teflon substrate and with the 0603 size components to reach the 6GHz. You can do the same with the FR-4 laminate but the maximum frequency will be 2GHz. The Laminate should be double sided and the main track should be calculated for the 50ohm impedance. You can use the 1.8uH, 2.2uH, or 3.3uH whatever you find on your cannibalised hard disk drive PCB.

When connecting the Bias-T pay attention on the right orientation of the ports. The DC port is simple, just apply the DC voltage. The RF ports can not be swapped, means that the RF port goes to the DVB-T dongle or receiver where only RF will pass and the DC will be insulated by means of the two parallel capacitors. The RF+DC port should be connected to the coaxial cable going to your remote device (LNA). Of course, this device should have Bias-T option enabled (another Bias-T).

Limitations and performance
This Bias-T is designed to supply the LNAs or other low current consumption devices. The main current limitation is the Bias-T inductor used, so consult the datasheet for the coil you intend to use and note the maximum current.
What about the RF power? You should not have the problems with the LNA or low power up to 500mW. For higher RF power levels the quality ATC capacitors should be used instead.
The losses are well bellow the 1dB over the range from 1MHz to 2GHz as per my quick measurements on the network analyzer. The Bias-T can be used even higher if you need that.


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