Monday, December 7, 2015

DIY DC block

I am receiving a lot of questions regarding the LNA4ALL usage. More options - more questions. More than year back we introduce the LNA4ALL with Bias-T 5V modification included. This way we can use the RTL.SDR dongle, Hack-RF, Airspy DC power to power remotely LNA4ALL through the coaxial line. Any other receiver that has 3.3-5V DC present on the antenna input connector can power the LNA4ALL using the same principle. Nice option saving some external power supply and extra wiring. And now we come to "but I want to..." section

1. I want to use the antenna that has shorted (DC short) 50 ohms connection !
If you have the AirSpy or HackRF or any other receiver that can switch off Bias-T through the software than this is not a real problem. Even then, if you forget to uncheck the Bias-T option the receiver can survive if any kind of hardware protection is engaged, like poly-fuse.

If you have the RTL.SDR dongle you have to disconnect the Bias-T supply by de-soldering the bridge on the hardware.

2. I want to use the external power supply for the LNA4ALL !
If you have the AirSpy or HackRF or any other receiver that can switch off Bias-T through the software than this is not a real problem.
If you have the RTL.SDR dongle you have to disconnect the Bias-T supply by de-soldering the bridge on the hardware.
The story does not end here. If you side power the LNA4ALL with Bias-T modification the DC will reach the receiver through the coaxial cable, same as RF signal. You have to undo the Bias-T modification on the LNA4ALL and this is not easy for all users. later if you need that option again, you will need to restore all SMD parts back.

Resolving the problems

The problem can be solved by introducing the RF DC coaxial block like the one from the picture bellow. The Minicircuits DC-BLOCK is rated for the frequencies from DC to 18GHz. Of course, quality comes with a price.

As named, DC block is blocking the DC component and letting through the RF component of the signal. This way the radio signal will pass through, to the receiver while the DC voltage from the LNA4ALL will not reach the receiver. This DC voltage may damage the receiver if there is no protection involved.

So what is the magic inside the DC-block? To block a DC we need a capacitor in series with the center conductor. So simple. The only problem is that the capacitor should not cause any losses in the declared range of operation. Basically, the SRF should be high enough, higher than the maximum frequency used. We can achieve that by using a small (size and value) capacitor. Such a capacitor may still introduce significant losses on lower frequencies. If you need good performance, then you will buy rather then build it.

The cheap DVB-T dongles aka SDR receivers we are using are covering the range from 25-1700 MHz. This should not be a big problem for a simple SMD capacitor used as DC BLOCK so we can build one really cheap. The one I build is can be used from 50-3000 MHz without problems.

This is simple DIY project where you will need two SMA (male/female) connectors, a piece of PCB laminate and a capacitor.

SMA connectors
You can use the type of connectors you have or handy for connection to your present equipment but SMA became a kind of standard for the mainly used cheap SDR equipment. Male or female type? Up to you but having one male and another female will cut the need for an extra jumper cable.

PCB laminate
As we are using the frequencies from 50-3000 MHz the losses introduced by the short length of 50 ohm track will not dominate so FR-4/G10 or Teflon type can be used. I re used some surplus RF PCB and cut out a peace with a 50 ohm track. Important! Double side laminate with ground bottom layer must be used.
Just cut the notch on the 50 ohm track where the series capacitor will be soldered.

To make a long story short, just use the SMD 100pF 0603 size C0G capacitor.
If you have a better quality capacitor then use it.
100pF capacitor will give us working range from 50 MHz up to 3000 Mhz for sure. Note that 100pF capacitor will not work on the HF. For that you need to use at least 1000pF capacitor value.
If you have a network analyzer, you can find out the SRF of the capacitor and measure the S1,2 and S1,1 to get the working frequency range. The DC voltage you can block will be limited with the maximum DC rated capacitor value. Standard 50V SMD capacitors are more than good for 5V or 12V DC that are commonly used.

Assembling the DC BLOCK
Cut the PCB to suit your RF connectors. 
Notch the 50 ohm line and solder the 100pF SMD capacitor.
Solder the SMA connectors center pins to the 50 ohm track.
Solder the SMA connectors ground to the bottom PCB ground and top if any.

Using the mentioned 100pF 0603 SMD capacitor one can get the following frequency response. S1,2 or insertion losses. As mentioned, the losses are high bellow the 50MHz but low up to 3GHz. Observe the markers indicating the losses on 50, 1000, 2000 and 3000MHz. response is pretty flat.

Beside the insertion losses the return loss is also very important. Note the S1,1 through the same range and markers on the same frequencies. Quite satisfying for the amateur use.15dB or better RL through the most of the frequencies will not cause any problems on reception or transmitting through the DC BLOCK.

Yes, you can transmit also through the DC block. The power that is common in the most popular sdr devices is not higher then 10dBm. 50V 0603 size SMD capacitor can handle that without any problems.
Do not stay without your DC block, it may save your receiver, spectrum analyzer or even a signal generator.

1 comment:

  1. I am really impressed with this blog! Very clear explanation of issues is given and it is open to everyone. Thanks for sharing this post.
    Cheap essay writing service usa