How to modify Bulls Eye LNB to inject 25MHz signal.

I am not going to explain basics – if you are here you probably know why I do it.

So, first of all, Bulls Eye is relatively stable anyway, however the frequency drift is about few kHz (in my case) and also the TXCO in my LNB is away by 35kHz so despite the fact I keep my TX of the Adalm Pluto stabilised with GPS DO (see article HERE) I still have to use a middle beacon stabilisation feature in SDR Console (see another article HERE).

Anyway, as GPS DO I use a double output Leo Bodnar GPS DO – brilliant device!!!

The configuration of the Leo Bodnar is shown below:

After pressing update, close the software, and reopen it to see the changes. Now, the next step is to disassemble the LNB – it is not very easy but it is not disaster either 🙂

As you can see I broke a clip:

Keep removing the silicone:

Use T9 size allen key

Pain in the bottom…

Now, there are two possibilities – the first one – I have NOT tried – you need to remove the PCB. It is hold only (apparently) by the F connectors. The problem is that if during the soldering back you do not push the PCB hard enough, then the paths will get broken when you push back the cover. Anyway, if you decide to go that way, what you have to do after flipping over the PCB – you need to remove the SMD resistor from the other side of the PCB. The storage place is where you can keep the removed resistor if you want to reuse it in the future. So possibility of getting the paths broken is disadvantage, however, there are some advantages of this solution – first one, some people complain, that removing TCXO (which is another way of modifying the LNB) is extremely difficult as when the PCB is not removed, it is difficult to de-solder the TCXO, as the aluminium chassis takes away the heat from the ironing gun. Also if you use the hot air soldering station, some people complain that the all near by element get de-soldered too 🙁 The other advantage of this solution is that the change can be reversed by moving back the resistor.

Anyway, the modification I decided to do, is…

removing the TCXO

The advantage of this mod is that you do not have to remove the PCB from the chassis. But, as mentioned before, the aluminium chassis takes away a lot of heat, so you have to use a hot plate or you need to do the trick which I did in the video below.

Once the TCXO has been removed, you can put back the cover, the best idea to make it perfectly sealed again is to use silicone Sikaflex 11 or any similar polyurethane based sealant.

Now your LNB is ready to “accept” 25MHz signal 🙂

However, there is a small problem. The signal from GPS DO Leo Bodnar is is square.

The LNB will NOT lock into this signal. It needs to be perfectly sinusoidal. So what to do? You need to spend another money and get a filter like this:

which is a bandpass filter 24-31MHz from Mini-Circuits.

Now just simply connect the cable from the Leo Bodnar output 2 and the filter into the red F connector in your Bulls Eye LNB.

The other much cheaper option is make the filter like that yourself, but it can be just a lowpass filter (first picture) or…

…or even the bandpass DIY version

DIY lowpass filter:

I design a nicer filter:

And real assembly 🙂 :

I did another small test – I used 20 meters long (or maybe even more?) 75 ohms satellite coax between GPS DO with filter and the LNB on the other end. With this length of the coax the LNB was stable and interference free. Conclusion – there is no need to keep GPS DO and the filter close to the dish.

There is one disadvantage I found so far – when my HF radio transmits on 24MHz band (which is very close to 25MHz) the LNB gets crazy – see picture below. All other bands are fine. And it happens to the both, DIY and Mini-Circuits filters.

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