This Low Noise Amplifier Helps Spectrum Analyzers Measure Signals Better Closer to DC
Amplify from DC to 600 kHz in a stylish custom-milled case.
Spectrum analyzers are exceptional tools for making low noise measurements across a wide range of frequencies. However, they tend to handle high frequency better than low frequency. For example, their inputs cannot tolerate a DC bias voltage. This limitation makes it difficult to measure something natively, like a resistor's thermal noise. So, electronics geek Mathieu Stephan (Limpkin) designed, built, tested, and thoroughly explained a low-frequency low noise amplifier design that works down to DC!
"My arbitrary goal for this project was to clearly measure the thermal noise of a 50 ohms resistor."
Stephan's low noise amplifier (LNA) design amplifies frequency content from DC to 600 kHz with a gain of 34 dB, or a 50.125 times (50X) increase! Since this design is DC-coupled, a clipper circuit protects the spectrum analyzer. It does not entirely eliminate DC components, but it does reduce them to below 200 millivolts.
The LNA consists of an LT102B precision operational amplifier, OPA1622 output buffer, and discrete clipper circuit on a 60 by 20 millimeter PCB. A custom-milled aluminum enclosure houses the board and two 9 volt batteries. These batteries provide the positive and negative rails for the amplifier. Based on the nominal current draw, Lithium-based batteries should last up to 47 hours.
Stephan used a Signal Hound SA44B spectrum analyzer throughout the LNA's extensive testing. While the datasheet says the input should have a 0 volt DC component, Signal Hound confirmed it could safely tolerate up to 200 millivolts. Fortunately, testing of the LNA showed that it effectively clipped its output signals to respect that requirement.
While the LNA can amplify down to DC (0 Hz), most spectrum analyzers cannot measure that low. So, this amplifier helps by reducing the need for a DC-blocking cap between the amplifier and the spectrum analyzer. Plus, many spectrum analyzers' displayed averaged noise floor (DANL) is higher at the low end. So, this amplifier helps move the low-frequency content above the analyzer's noise floor. You could also use the amplifier with a dc-coupled instrument like an oscilloscope, which typically has much higher noise floors than a spectrum analyzer.
You can use the KiCad files on this GitHub repo to build the PCB, or you can visit Stephan's Tindie Store to purchase a fully assembled low-frequency LNA. Prices start at $249 before shipping.
