I would like to document my single cable GPSDO feeding modification made to my VENTON EXL-S LNB.
While modifying this LNB for external GSPDO feeding is extremely simple (just remove original XTAL and feed GPSDO signal externally via a short SMA pigtail into the original pins where the XTAL was previously located from the bottom side of the PCB) and was working here well for a few weeks, I did not want to have a separate cable, so I was looking for a cleaner solution.
The VENTON EXL-S is using a 25 MHz PLL and due to it’s long circular waveguide I find it extremely well suited for use with the famous dual band POTY feeds. Also due to PCB and case layout it has quite some convenient space inside for adding modifications. This LNB type is also nice since it comes with a very efficient dielectric lens that can be removed and used for the POTY dualband feed also.
For feeding the GPSDO signal to the LNB via the main coax cable I am using the triplexer circuit described by OH2AUE here in the forum and on his website:
Here are roughly the steps that I followed:
1. Open LNB case and remove all screws and open the lid
2. Unsolder center pin of F-Connector (removing the solder works well using solder wick) until the centre pin is noticeable loose inside the PCB hole
3. Cut carefully with a very sharp thin cutting tool several times around the PCB edge into the silicon that is used to glue the PCB into the LNB case.
4. Use a very tiny and thin watchmaker screwdriver to VERY GENTLY lever the PCB out of the case. Do not apply to much force as the PCB is very thin and may break.
5. Once PCB is removed turn it upside down and remove the 25MHz XTAL. This works best with a hot air station (cheap one for 35 EUR works well for this purpose) or use two soldering irons applied left and right of the Xtal. It will be very difficult to remove with just one soldering iron.
6. Now turn PCB to upper side and cut the trace leading to the meander line (see photo)
7. Also scratch away some of the solder mask and tin it
8. Now place a 11 ohms resistor in series from the F connector pin to the meander line (see photo)
This approach I just copied from OH2AUE modifications documented here in the forum.
9. Now add a 120pF capacitor (0603) in on top of the line leading from the F connector pad towards the PLL chip. Following this trace if useful because there is a convenient space available inside the lid.
10. Add a 330nH (0603) inductor in series to complete the series LC circuit.
11. Now just use a short piece of thin enamelled wire to connect the just created LC circuit with one of the two inputs of the PLL chip. I simply connected it to one of the conveniently located load capacitors from the original XTAL inputs (see photo)
12. Put the PCB back into the case and solder the F connector centre pin
13. Check for any short circuits on the F connector J
14. Close the lid – LNB mod complete
With this mod and the OH2AUE triplexer I can feed my Leo Bodnar GPSDO directly into the triplexer (approx. 6-7dBm into the LNB) and the LNB PLL locks without any problems.
However, I noticed that with approx. 0dBm at the LNB it does not lock anymore….so this mod needs quite some drive. But since the output of the Leo Bodnar GPSDO was adequate I did not bother at first to improve this any further.
I will try to improve this later by adding a parallel LC circuit in series of the meander line and document here is successful.
Also going to try a TCXO mod for this LNB type to be documented here hopefully soon.
73 de Oscar DJ0MY
This post has been edited to correct the wrongly posted 12pF into corrected 120pF values (I will also try to update the photos)