We had several discussions about this LNB. It's a TWIN and you can use it for receiving NB and WB transponder at the same time. Great for DATV with talk back then. My modification uses a triplexer indoor, feeding in DC (12V for NB transponder), 27 MHz LO (I use a GPSDO) and get an NB transponder IF out that is connected to a RTL dongle for reception.
- It uses two RDA3565ES IC sharing one 27 MHz SMD XTAL.
- This LNB is heavily wobbling - unmodified.
- Depending on temperature it is sometimes ok, sometimes bad.
- There is a mutual interaction of those two outputs, if there is 0V, 13V or 18V applied.
- These interaction results in LO drift or offset due to temperature change of power additional dissipation.
- And not only this...
- After cutting the horn the round waveguide can be mounted at a 22mm copper tube (POTY).
- It is relatively easy to modify to an external LO (GPSDO or TCXO).
- LO: 27MHz, ca. +-60mVpk at XTAL (in) measured. This is the input of an external LO.
L1=L2=470nH wire wound (mine has < 0.8 Ohm DC)
C1=C2=C3=100pF NP0 0805
R1=50Ohm (or 68Ohm ...)
Top: HD-BP2 with modification
Note: I did a simulation and used L, C etc with parasitics. Therefore I use 100pf.
I you calculate f_res it's different.
I put a foil (tesa of capton) into the cap to avoid short circuits because of the additional parts.
I've measured at the XTAL (in) +-60mVpk. (There is a connection to PIN 20 of the RDA via a capacitor)
So you have to put in about the same level by your LO or GPSDO.
In my triplexer I use a poti. The level was set on best MER of the DATV beacon A71A.
If you can't do this, try to measure the level XTAL (in) before and set the correct level.
Or do minimum a level calculation with cable loss.
MER is almost the same before modification.
But frequency is now stable and exact.