G0MJW Bias-T and reference-injection boards

For those among us who would like to feed external frequency reference to their LNBs, Mike G0MJW has designed two boards which combine the usual Bias-T with the ability to send the reference signal on the same feeder. This of course means having to adapt the LNB accordingly - but given you will likely have to modify it to accept external frequency reference in the first place, it shouldn't be a problem.

Both boards feature pads for Pi attenuators on both IF and reference lines, with resistor values as shown in the schematics providing 10dB attenuation on the former and 3dB attenuation on the latter. These can be omitted if not necessary, modified should different attenuation be desired, or possibly replaced with Pi filters.

Note on the LNB connectors

The boards have been designed so that one can easily substitute SMA or even F-type for Mike's preferred BNC. As a result the LNB connector must have the "ground pins in a square plus signal pin in the centre" geometry which while typical of SMA panel connectors is somewhat unusual (and more pricey) for BNC ones.

Furthermore, keep in mind that the maximum ground-pin spacing which can be accommodated by the boards is about 9mm outer edge to outer edge. This is not a problem for most SMA and F-type connectors, however many BNC connectors of this sort feature ground-pin spacing of over 10mm and will therefore not fit. Bear that in mind should you aim to replace the BNC connectors listed in the BoMs with something cheaper.

External reference

Described in CQ-TV 263. Useful if you have a stable 25MHz reference source such as a GPSDO.

Bill of materials: File:G0MJW BiasT ext ref-BoM.ods

A blank PCB for this project is available from the BATC Shop.

Inductor pads can accommodate a wide range of different packages, both SMD and through-hole ones.

To omit attenuator on IF out: replace R3 with a 0 Ohm jumper or a solder bridge, omit R1+R5.

To omit attenuator on Ref in: replace R4 with a 0 Ohm jumper or a solder bridge, omit R2+R6.

Onboard reference

Described in CQ-TV 266.

Bill of materials: File:G0MJW BiasT TCXO ref-BoM.ods

A blank PCB for this project is available from the BATC Shop.

The TCXO has all the pads underneath and is therefore suggested to be fitted first, tinning the pads and with hot air or a hotplate.

C3 can be fitted as needed to tune the crystal filter. For the crystals specified in the BoM it shouldn't be necessary.

To omit attenuator on IF out: replace R13 with a 0 Ohm jumper or a solder bridge, omit R11+R15.

To omit attenuator on TCXO reference: replace R14 with a 0 Ohm jumper or a solder bridge, omit R12+R16.