Phase 3D X-band receiver project

X-band receiver for the Phase 3 D Satellite

Brief description of system

The X-band receiver is based on a Ku-band satellite TVRO low noise converter of commercial design. Much of it has changed though. Firstly the DRO is heavily modified into a buffer amplifier. This was a major modification ! The input to the buffer stage is via a male SMA connector. the input is DC blocked with a 0.7 pF high Q chip capacitor to the gate of the GaAs-FET. The gate is at 0 DC volts and the source is grounded via a source resistor. The drain voltage is 5 V. I chose the IF to be 432 MHz instead of 144 MHz ( too low ) and 1296 MHz ( too complex as I would then need a second converter ). The mixer and the image rejection filters are brought down in frequency to cover the 10368 and 10451 MHz bands by simply loading them with pieces of PTFE ( teflon ). The preamplifier stages are manually tweaked with thin flakes of silver plated metal foil. The noise figure of the converter is around 1.5 dB. I am planning on replacing the first stage GaAs-FET with a P-HEMT when time permits. The image rejection of the overall converter is over 35 dB which is ample. Modification of the IF amplifier was also necessary as the original IF strip had a very sharp cutoff below around 600 MHz. The IF strip has four broadband transistor stages and the frequency response was achieved originally with the DC bias circuitry. By increasing the L I could decrease the lower frequency cutoff. There also was a series resonant circuit between stages 1 and 2. I simply bypassed this with a 27 pF chip capacitor. After modification the overall gain of the converter was in the order of 45 dB's at 432 MHz and peaking at over 50 dB's at 1296 MHz. The IF amplifier operates reliably and is stable over a large variation of temperature. The converter can be used for 10450 MHz satellite operation but also for 10368 MHz and others if a scanner is used at the IF. The original DC feed via coax was modified so that the IF connector ( an N connector ) is DC free and the 12 V supply is fed via a feedthrough capacitor of 1 nF. The local oscillator is a modified 6 GHz phase locked multiplier ( PLM ) from Microwave Associates. I changed the crystal to 104.3541667 MHz. This is a 60 degree C oven crystal and I am heating it with a PTC resistor that stabilizes at this temperature with the -20 V supplied. The VCO is running at 1669.6667 MHz as is locked at the 16th harmonic of the local oscillator. The VCO frequency is multiplied by six with a step recovery diode multiplier followed by a digital filter. I shortened the filter resonators in a milling machine in order to get the filter to tune to 10018 MHz. Tuning is simple and multiplication is reliable with no parametric oscillation observable. The SRD is loaded via a small attenuator so it sees a fairly resistive load during alignment. The multiplier output level is +13 dBm and is fed to the converter buffer via a small attenuator. The whole system is built into an Eddystone diecast box with a waveguide input on the end wall. The waveguide input to the converter is depolarized from circular to liner by a TVRO depolarizer ( this is also modified from LHCP to RHCP by rotating the Rexolite delay line in the circular waveguide by 90 degrees ). The diecast box is attached directly to the feedhorn of a 90 cm prime focus dish and is light enough not to require any other support than the original tripod mount of aluminium spokes. Haven't measured the sky/ground noise ratio yet, because I still need to build a +12 V to -20 V inverter into the box to enable relaxed portable operation with 12 V only... (well, since writing this I have certainly built the converter and used the receiver in a few experiments. The stability is quite good even without a crystal oven and the sky/ground noise level is very easily heard with my FRG9600 receiver and using the loudspeaker. Sun noise is several dB with a 90 cm dish - easily copiable with the ear - 14.02.2000/MF) Updated 13.04.1998