Most software-defined radios on the market today are designed for communication applications. When receiving signals, they are used primarily for signal processing including demodulation and decoding. SDRs are not designed to make accurate signal level measurements like a spectrum analyzer or a power meter.
In many situations it’s useful to know what the RF signal level is prior to connecting your antenna to an SDR or connecting the SDR to a signal source. As discussed in this post, the signals from a two-way radio or a source might be strong enough to saturate or even damage your SDR. An SDR that is saturated will not function properly. This is one of the main reasons to make a signal level measurement.
In such situations, it’s always a good idea to connect a spectrum analyzer to the signal source prior to plugging in your SDR. This will allow you to measure the signal in advance. A spectrum analyzer typically has a higher RF input damage level. For instance this spectrum analyzer has a damage level of +33 dBm or 2 Watt. Always make sure you check this specification. If you suspect the source level is very high, then it’s a good idea to also plug in some RF attenuators at the input to the spectrum analyzer prior to connecting the source.
While a spectrum analyzer is a very useful tool for this application, most people don’t have access to one and most analyzers are expensive. This handheld spectrum analyzer is a cost-effective alternative to measure RF signals. Note however that its damage level is only +10 dBm or 0.01 Watt. This is not very high so it’s also recommended to buy some RF attenuators to go along with it.
Once you have made your signal measurements, you can make the appropriate adjustments to your SDR. For instance, if you find the signal levels are too high, you can connect an attenuator to the front-end of your SDR. Alternatively, if the signal levels are too low you can use a low noise amplifier to improve the sensitivity of the SDR. Additionally, the analyzer measurements will help you determine how much gain you require. In some situations you might only want 20 dB for instance and adding a 40 dB LNA will be too much and saturate your SDR.
A spectrum analyzer is a useful measurement tool to complement your SDR kit. It allows for easy signal strength measurements and allows you to make adjustments to your signal levels with the use of either an LNA or attenuator blocks.