Salil Tembe Walks Through the Design and Testing of a VHF-Band Low-Noise Amplifier
From picking the right part to a fully-tested PCB, Tembe takes you on a journey of LNA design for SDR use.
Radio amateur Salil Tembe has designed a low-noise amplifier (LNA) for use with software-defined radios (SDRs) — and offers a walkthrough of the process, from part selection and schematics to the PCB and testing.
"There are plenty of amplifier designs available online. You can simply purchase one and be done with whatever you plan on doing," Tembe writes. "However, designing your own low-noise amplifier can be fun. For someone into ham radio, you might have come across weather satellite LRPT [Low-Rate Picture Transmission] reception using an RTL-SDR. Receiving images from a satellite may require a good low-noise amplifier. On the other hand, someone into radio astronomy definitely requires a very low-noise amplifier to receive the hydrogen line frequency emanating from the Milky Way."
Rather than picking something up off the shelf, Tembe opted to design his own — aiming for a noise figure below 1dB. The Mini-Circuits PGA-103+ monolithic amplifier, rated at a typical 0.6dB at 1GHz rising to 0.9dB at 2GHz, was chosen as the part around which the LNA would be built, along with a bandpass filter targeting the frequencies required for satellite imagery reception and the 2m amateur radio band.
Tembe's build log walks through the design of a circuit for the PGA-103+, a bias tee, and a bandpass filter, before the schematic is translated into a PCB design and sent off for manufacturing and assembly. The next step was, of course, testing — which Tembe carried out using hobbyist-friendly tools including the NanoVNA vector network analyzer and TinySA Ultra spectrum analyzer, plus a real-world test connected to an RTL-SDR software-defined radio.
"The low-noise amplifier isn't a complicated design," Tembe admits of his creation, "but it takes quite some effort to design, assemble, define tests, and actually conduct tests to prove it's working according to the design."
The full build log is available on Tembe's website Nuclear Rambo, while the maker is selling the LNA itself on Tindie for $20.