Simple X-Band Receive Pre-Amplifier |
Overview
It's possible, without any complicated tuning, to turn an old HughesNet/DirecPC satellite low-noise block downconverter into a very simple 10 GHz receive pre-amplifier with approximately 25 dB of gain and a 0.8 to 1.2 dB noise figure.
The block downconverter model to look out for is from Japan Radio and is labeled "NJR2117FK," with slight differences in the model number based on their noise figure. Model NJR2117FJ has a noise figure of 0.9 dB and model NJR2117FK has a noise figure of 0.8 dB. These downconverters were originally used to convert the 11.7 to 12.2 GHz satellite band down to a 950 - 1450 MHz Intermediate Frequency (IF) range using a 10.75 GHz Local Oscillator (LO) signal and a simple diode mixer.
The stock downconverter's RF input is via a small WR-75 horn antenna and the entire low-noise block downconverter assembly was mounted at the focal point of the satellite dish. The local oscillator and mixer on this downconverter unit are both integrated into a single little 4-pin "can" which will need to be removed for this project. A SMA jack will then be added in its place and will provide the RF output from the downconverter's pre-amplifier stages.
The stock downconverter does have a 12 GHz stripline bandpass filter on the output of the last low-noise amplifier stage. This filter will need to be removed and "jumpered over" using a DC blocking capacitor. Remove the filter using an X-acto knife to cut and lift the traces, then solder in a high-quality, microwave-rated 1.2 pF American Technical Ceramics (ATC) capacitor. You may have to solder in little "flakes" to extend the stripline traces if you can't find the proper size capacitor to fill the gap.
The stock downconverter has an onboard 7808 voltage regulator and was originally powered by a +15 to +24 VDC bias signal coupled into the IF output connector. For convenience, we'll disable this method of powering and use a simple feed-through capacitor to run the voltage in.
The original work on converting these units to X-band pre-amplifiers was done by Chuck Houghton, WB6IGP and Ellwyn Angle, N6CA.
Pictures & Construction Notes

Stock NJR2117FK low-noise block downconverter on the right.
Internal overview on the left.

Closeup view of a stock NJR2117FK low-noise block downconverter.
The silver 4-pin "can" contains the local oscillator, which uses a Dielectric Resonator Oscillator (DRO) at around 10.75 GHz, and the diode mixer.
This unit, and the little bracket, will need to be unsoldered and removed. Keep the screws though, as they will hold the SMA connector which will be added for the pre-amplifier's final RF output.

Overview with the local oscillator/mixer can removed.
The RF input, from the WR-75 waveguide, is on the left.
Note the little lip next to the screw hole on the right. This will need to be machined or filed down so the SMA connector sits flat to the pre-amplifier's PC board.

Bottom view of the NJR2117FK low-noise block downconverter showing the IF output connector, voltage input bias tee, surge protection, and 7808 voltage regulator.

Top view showing the removal of the pre-amplifier PC board.
You'll need to unsolder it at two points, on the RF input antenna probe and the red +8 VDC bias line.

Bottom view showing the removal of the voltage regulator board.
This is probably optional, but you don't want to get any metal shavings on it.

Good overview showing the lip which will need to be machined or filed down so it is parallel with that center section.

After the lip is gone, secure a 2-pole SMA jack using a screw which originally held the bracket for the local oscillator/mixer can.
Apply a bit of solder to the SMA connector on the PC board side to further help secure it.
Note the little stripline bandpass filter.

Removing the stripline bandpass filter.
Use a sharp X-acto knife to carefully cut and lift the filter's traces.
A high-quality 1.2 pF ATC porcelain capacitor will then be added to bridge this gap.

Overview showing the addition of the 1.2 pF ATC capacitor and the RF output SMA jack.

The new +12 VDC input will be via a 1000 pF feed-through capacitor mounted just above the IF output F connector.
The center conductor of the F connector was clipped, but the F connector was left intact for use as a handy mounting point.
You'll probably want to stick a ferrite bead on that incoming DC power line.

Overview of the completed low-noise pre-amplifier.
A commercial SMA-to-WR-75 transition is used to feed the RF input into the pre-amplifier. A WR-90 transition will provide slighty better input return loss, but the mounting holes will need to be redrilled.
You'll also need to file down the pre-amplifier's cover around the SMA jack so it sits flush with the PC board and doesn't short the RF output.