UHF-Satcom.com - Surplus X band low noise amplifier information and modifications (17/02/2012 22:25:55 -0000)
|Surplus X-Band low noise amplifiers have appeared on the
market in the UK, having been removed from Racal VSC501 type down
converters. The converters have a waveguide input, a tuneable oscillator
inside, and an IF output, all running from 24V DC. The amplifiers in an
unmodified state cover 7250MHz to 7750MHz. Modified, they have a useful gain
from 5.8GHz to 9GHz, and so may be ideal for DSN operations at 8.4GHz.
The X-Band LNA's are made by Ferranti, part number 5820-99-744-0560. The amplifiers are solidly built and housed in a machined aluminium casing which is of sturdy construction. The amplifier needs +15V and -15V DC supply, but with a simple modification, it can be made to run of +12V (see mod info below).
The X-Band RF output is an SMA socket adjacent to the waveguide input, and the output is driven with a pair of NEC9001 MESFET's. Power and control is via a 7 pin 'mil' connector.
In the above picture, a DC-DC converter has been added, along with a 7805 regulator. Measuring the FET supply rails, it was found that +12V provided sufficient voltage to correctly bias the positive rails, and -9V from the DC-DC was sufficient for the negative voltage rails. The picture below shows the modified LNA. The 1st band pass filter has been removed and a SMA socket installed, so as to take advantage of the broad band front end. Since we are not interested in using these LNA's in conjunction with a transmitter, the out of band filtering is less important. The other modification performed was to remove the FET drain power supply lines for those device in the output chain, this dropped the LNA's power consumption by 100mA. Note that the 'plugs' in the metal casing need to be removed to allow access to the hex socket cap screws that hold the band pass filter in place - this is done by chipping away the paint, then digging out the filler from the plugs and unscrewing them.
A short test was conducted to check the useful gain over the 6GHz to 9GHz band, with the following results;
It should be noted that the above gain figures are for the modified LNA with the RF output taken after the initial 3 gain stages, as per the above picture.
The band pass filter that was removed from the LNA has excellent pass band characteristics, allowing 7250MHz to 7750MHz to pass. These filters are therefore perfect for integration into your X-Band down converter unit in order to cover the milsatcom downlink band. These satellites which are mostly geostationary, make perfect dish alignment / calibration points. The following picture shows the BPF during the modification process;
The holes previously used to mate the filter unit to the LNA are ideally sized to be tapped with an M2.5 tap. This is what has been done to the unit in the above picture, in order to screw the SMA socket securely in place. As can be seen, the other SMA socket has yet to be attached.
A simple mod can be performed to make the LNA run off a single +12V supply, this involves fitting a DC-DC converter into the PSU side of the LNA; -12V is fed to the end of C5 closest to the PCB edge, whilst +12V is present at C1, on the pin also closest to the edge of the PCB.
The modified LNA was tested with 'off air' signals from the Stereo-A spacecraft, which at the time was around 130 million miles away from Earth. Reasonable quality signals were received, meaning that this LNA is an ideal first step for those wanting to start experimenting with 8.4GHz reception. The LNA was modified as described above. The next picture below shows the LNA mounted on the dish. A simple transition was made to convert the waveguide LNA input to a circular 28mm waveguide (copper pipe) as used for the X-Band receive system.
FFT Screenshot showing Stereo-A carrier as received (Note: Frequency step is a retune, not spacecraft)
If you are interested in obtaining one of these LNA's, please contact UHF-Satcom.com who will gladly put you in touch with the seller. (The LNA's do not come from uhf-satcom.com - we are simply a very happy customer!)