UHF-Satcom.com - GPS 1.5GHz distribution amplifier (17/02/2012 22:31:20 -0000)
Having 10MHz GPS locked oscillators in the shack is both useful and problematic – once it is locked up, how do you know the quality of the 10MHz output? The simple answer is that you build two or three GPS locked oscillators, and compare the outputs, but this means you need 1.5GHz RF feeds to each receiver from a common GPS antenna in order to avoid time of arrival based errors. Andy G4JNT came up with a nice design using three Wilkinson dividers which worked well, but it was thought that a preamp should be added to overcome losses in long antenna cable runs and of course losses in the dividers.
The above image shows the copper side PCB layout. It’s made on standard 1.6mm thick FR4 and the resulting image should end up 79mm by 59mm. The original artwork is on-line here - use ‘Irfanview’ for Windows to print this off at the correct size. The layout shown above is marked with letters indicating what components need to be placed where. The capacitors are mostly 100pF in the RF line, and whatever suitable DC decoupling capacitors you have laying around for the others.
The MMIC goes directly in the RF path - A MAR-6 was used here, along with the appropriate bias resistor. The MMIC has its DC generated by a 7805 regulator, again use appropriate de-coupling capacitors to make sure it doesn’t self oscillate. On the PCB layout, where there is an ‘earth’ symbol, a hole should be drilled through the PCB and a Vero pin inserted and soldered both sides. The whole PCB can be soldered into a tin plate or PCB enclosure. The two inductors were salvaged from an Ionica board, but could be made by winding two or three turns over a 2mm former. The board runs from +13v, and feeds DC up the coax to an old marine active GPS antenna. There is a resistor in the DC feed to the antenna, which should be a low value, 10 ohms or so. This is only so if there is a short in the cable the resistor will take the hit and not the tracks on the PCB. Facilities exist on the board for +5V to be fed to the antenna.
DC feeds from the GPS receivers could also be used to power the MMIC and the antenna by some simple re-wiring. . As the circuit is so simple, and can be built with junk box components, a schematic diagram was not necessary – looking at the above picture should give enough clues as to where the components are placed. Most of the parts can be salvaged off of old Marconi LNB boards. The output connectors can be whatever is convenient. SMA females are ideal, but BNC’s and TNC’s will be just as good.
(above) The completed distribution amplifier.