Low Noise Amplifier: REV B
Specifications
| Absolute Maximum Ratings | MIN | MAX | UNITS |
|---|---|---|---|
| Supply Voltage | -60 | 24 | V |
| Input Voltage | -30 | +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 | 24 | V |
| x250 Output Termination | 1 | MΩ | ||
| Main Output Termination | 1 | MΩ |
| Electrical Characteristics | MIN | NOM | MAX | UNITS |
|---|---|---|---|---|
| Supply Current | 8 | mA | ||
| x250 Output Resistance | 4.99 | kΩ | ||
| Main Output Resistance | 2 | kΩ | ||
| Input Noise Density | 3.3 | nV/√Hz | ||
| Input Noise | 66 | nVpp | ||
| Input Noise | 9.7 | nVrms | ||
Rev A PCBA
Input Protection: Features and Limitations
The input protection circuit allows continuous input voltages of 0V to +30VDC and temporary inputs down to -30V to be applied. Within this input voltage range the LNA will draw up to approximately 2mA from the input source, slowly charging up the input capacitor. This makes this LNA safe to hot-plug into sources of 0 to +30V, so long as the source can safely supply at least 2mA of current.
Rev B Input Protection Circuit
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 turns off Q4 which limits the reverse current to ≤30µA (the current through R42). If the LNA is powered on, then a red warning LED will light up to indicate that a reversed input is present. The user must quickly disconnect the input to avoid degrading or damaging the input capacitor.
If the LNA is not powered on the LED will not light up! In that case the reverse current will be limited by a 1MΩ 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.
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.
Testing
Every LNA is tested with an automated procedure. This procedure checks battery current consumption, AC gain at 1Hz, frequency response, and noise floor.
