Notes on Skew Control for LNB  (for two Pansat FTA Receivers)
11/9/14 - Trimble


A big dish (LNB) skew control was created to use a 3.3vdc voltage available on the Pansat 9200HD, Linkbox 8000Hd (and others) (0vdc for horizontal polarity and 3.5vdc for vertical polarity).  For the set top boxes this voltage normally controls the voltage to an LNBF (to yield 13 or 18v).  This polarity control design assumes that all satellites require the same polarity setting - except for AMC 1 Ku band which is about 19 degrees off of the others.  An independent set of controls is included for AMC-1, manually switched when needed. 

Figure 1 and figure 2 shows the component side and bottom side of the milled circuit board used.

    Figure 1 - Skew control component side      Figure 2 - Skew control milled circuit side

There are 2 variable resistors, one for horizontal and one for vertical, to allow tweaking for the best signal angles, and once set should not have to be adjusted again.  An independent pair of variable resistors are used for AMC-1 Ku band.  The power supply should have a voltage of 5vdc and sufficient amperage capability to eliminate any controller chatter due to slight variation of pulse length.

 For this design the vertical adjustment is independent of the horizontal and the final settings need not yield exactly 90 degrees between their directions.  The output pulse occurs at about 30ms periods.

To obtain the 3.3vdc signal from the Pansat 9200HD see Figure 3 and for the LInkbox 8000HD Local see Figure 4.  On the Linkbox Local a small pc board has to be lifted from its socker to access the portion of the board shown.  The wire connecting the 3.3vdc point on the Pansat board should be isolated from the outside with a 100K resistor as shown in Figure 5 so it can not be exposed to any significant load.

        Figure 3 - 9200HD 3.3vdc source               Figure 4 - Linkbox Local 3.3vdc source                   Figure 5 - Isolation board

The lifting of the transistor noted in Figure 3 for the 9200HD may not be necessary for many LNBs as they may operate on the 13vdc as well as the 19vdc which is the case for the LInkbox 14vdc/18vdc output.


Parts list:

Resistors – (see photo of top)
Variable resistors (4) all 1k ohm

Timer chip 555
Comparator chip LM311
Transistors 2N4401 and 2N3904E
Electrolytic 1000uf - 10v capacitor
Capacitor - Tantulum 1uf (>5v)
Reed Relay - single pole, double throw ECG RLY F71C05C (5-6v, 0.25 amp coil)
Power supply - 5vdc 2500MA or better (most for DLINK Routers work - ACY096 JTA0302B Power Supply on Ebay)