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A noisy switching regulator on the DC rail of your Low Noise Amplifier will modulate the amplifier's operating point and push noise and spurious content into your signal path. USB power supplies are particularly bad offenders. If you are seeing a forest of spurs that do not correspond to any real signal, check your supply rail on an oscilloscope before you start suspecting your antenna or your filters. The problem is often on the bench, not in the air.     The fix is simple: replace the switching supply with a clean linear regulator that outputs on USB. A battery-backed...

If your AIS receiver seems to be working but you are consistently missing vessels that other stations are seeing, your antenna is probably not the problem. Neither is your SDR dongle or your coax run. The problem is almost certainly something you cannot see on your waterfall at all: FM broadcast interference compressing your LNA before a single AIS packet has a chance to arrive.     This post explains what gain compression is, why FM broadcast is particularly dangerous in coastal environments, and what the attenuation numbers from a real bandpass filter actually mean in practice. What Gain Compression...

  There is a persistent myth in GPS and GNSS system design: more gain ahead of the receiver is always better. Stack enough amplification in front and the problem goes away. It does not work that way. Why Gain Placement Matters GPS signals arrive at the antenna at around -130 dBm, well below the thermal noise floor. The receiver relies on spread-spectrum processing gain to pull the signal out of the noise, not raw signal strength. This is why the LNA's position matters as much as its gain value. Place the LNA close to the antenna and it sets the...

Your GPS receiver is not broken. The satellite signal is fine. The problem is sitting between your antenna and your chip, and it is almost certainly caused by a signal that has nothing to do with GPS. Out-of-band interference is the most common cause of unexplained GPS degradation, and it is widely misunderstood because the interfering signal is not on the GPS frequency. It does not need to be. The Problem: Your LNA Has a Breaking Point GPS L1 sits at 1575.42 MHz. LTE cellular downlink occupies bands from roughly 700 MHz to 2700 MHz. Wi-Fi operates at 2.4 GHz...

  There is a fundamental tension at the heart of every RF receiver front end. The trade-off between selectivity and sensitivity sounds abstract, but it shows up in entirely practical ways the moment you start specifying a bandpass filter. What the Terms Mean Selectivity is the receiver's ability to discriminate between a wanted signal and an unwanted one at a nearby frequency. A highly selective receiver — one with a very narrow bandpass filter ahead of its LNA — rejects out-of-band interference before it can cause problems downstream. Sensitivity is the receiver's ability to detect weak signals, limited primarily by...