Low Noise Amplifier: REV A
Contents
Rev X1 enclosed LNA, Custom Gain and Frequency Response
Specifications
| Absolute Maximum Ratings | MIN | MAX | UNITS |
|---|---|---|---|
| Supply Voltage | -60 | 12 | V |
| Input Voltage | 0 | 30 | V |
| x250 Output Current | ±10 | mA | |
| x250 Output Short Circuit Duration | Continuous | ||
| Main Output Voltage | ±30 | V |
| Recommended Operating Conditions | MIN | NOM | MAX | UNITS |
|---|---|---|---|---|
| Supply Voltage | 6 | 9 | 12 | V |
| x250 Output Termination | 1 | MΩ | ||
| Main Output Termination | 1 | MΩ |
| Electrical Characteristics | MIN | NOM | MAX | UNITS |
|---|---|---|---|---|
| Supply Current | 7 | mA | ||
| x250 Output Resistance | 4.99 | kΩ | ||
| Main Output Resistance | 2 | kΩ | ||
Rev A PCBA
Input Protection: Features and Limitations
The input protection circuit allows inputs of 0V to +30VDC to be safely applied. Within this input voltage range the LNA will draw approximately 2mA from the input source, slowly charging up the input capacitor. (Note: this current limit scales with LNA supply/battery voltage.) This makes this LNA safe to hot-plug into sources of 0 to +30V, so long as the source can safely supply (and sink) 2mA of current. However, caution is required whenever the LNA input is connected, especially when dealing with voltages higher than +15V.
Rev A Input Protection Circuit
After a test, the input capacitor will store the test voltage for a long time. The time constant of the self-discharge rate is approximately 40 minutes. This means that the LNA input may actually output current when its input is switched from a higher to lower voltage. The input protection circuity also limits the output current to approximately 2mA, but care must be taken to ensure that does not cause damage to any devices being tested.
Negative Input Voltages - Don't Do it!
Negative voltages should never be applied to the LNA input, but since this is a common setup error, the input protection circuit offers some damage mitigation features. If a negative voltage as low as -30V is applied, the input protection circuit limits the reverse current to 2mA if the LNA is powered on. The reverse current will begin to break down the oxides in the electrolytic input capacitor, but the process is slowed due to the limited current, and the risk of irreversible damage or explosion is reduced. If the LNA is powered on, a red warning LED will light up.
If the LNA is not powered on the LED will not light up! In that case the reverse current will be limited by a 100kΩ resistor (R38) instead of a fixed current limit. This will further reduce the rate of damage, but it will not prevent damage.
IF THE RED LED IS LIT, UNPLUG THE INPUT!
The red LED is just a warning light, the negative input voltage must be removed manually to avoid damage to the LNA input capacitor. The warning light also does not work if the LNA is off, so turn on the LNA before connecting it’s input, and pay attention to the red LED. If you soldered the LED’s on your LNA yourself, it’s worth checking if they were installed with the correct polarity. This can be done by briefly and carefully applying a very small -20mV to -50mV voltage to the input, checking that the red LED illuminates, and immediately removing the input voltage.
Working With +15V and Higher
After a test, the input capacitor stores the full test voltage for a long time. If a new test is started at the same voltage, but the input is accidentally reversed, a negative voltage of double the test voltage is generated inside the LNA at the negative terminal of the input capacitor. If the test voltage is +15V or lower, than the most negative voltage that can be caused is -30V, which will be current limited to 2mA and is likely safe for a short duration. However, if the test voltage is above +15V, there is risk that a reversed input could cause the TVS diode, D2, to clamp. This can draw very large currents, damage D2, the input capacitor, and/or the input source.
To mitigate this risk, between any tests of +15V or higher, short the input of the LNA and leave it powered on for 60 seconds. This will discharge the input capacitor, greatly reducing risk of damage when connecting the LNA for the next test.
Building an Enclosure
If you have just an LNA board, you may want to make an enclosure for it. Here is an example of the LNA in a Hammond 1590Y diecast aluminum box. The simplest mounting method is to just drill the three holes for the BNC connectors and use their jam nuts to hold the entire board in place. Then the LED holes can be drilled, and the LED’s can be inserted and soldered into place. Download links to drill guides are provided at the top of this page. When printed at 100% scale, they can be cut out and used to mark or center-punch the hole locations on your choice of chassis. In addition to mounting the board a power switch will need to be mounted and soldered inline with the battery/power cable.
Rev A Enclosed LNA – Hammond 1590Y Aluminum Box
Alternatively, the board has six mounting holes that can be used. Aligning both the board mounting holes and the BNC mounting holes on a chassis may be difficult, or impossible if the chassis walls are not all perfectly perpendicular, like the die-cast enclosure pictured above. The enclosure does not need to be metal, a plastic enclosure can still provide valuable physical protection and keep the air around the LNA still, which appears to be a larger factor than EMI.