A Primer on RF Filters for Software-defined Radio

An RF filter is a hardware device that is used to limit signals in certain frequency bands while allowing signals of interest at other frequencies to pass through. Filters are used in both SDR transmitters and receivers for different reasons.

In a receiver, a filter is used to limit signals that would otherwise degrade the performance of the receiver in terms of its ability to process the signal. So for instance a band pass filter can be used to limit the effect of a strong out-of-band interfering signal that might otherwise saturate or damage the receiver.

Band pass filter used in SDR Receiver

When a transmitter generates a signal to be sent out over the antenna, it can also generate several spurious (spurs) signals in the process. Transmitting these spurs can be damaging to other radio systems. A transmit band pass filter is used to limit such spurs.

Band pass filter used in SDR transmitter

There are mainly four different types of filters as shown in the picture below.

Four different types of filters

Each of the filters has a pass band and stop band which are the frequency ranges of signals that are allowed through or attenuated by the filter, respectively.

Filter Specifications

There are three key attributes of a filter. The table below lists these specifications for Band pass, Low pass and High pass filters.

Filter SpecificationWhat is this a measure of?
Center Frequency and BandwidthRange of frequencies the filter will pass through. The signal of interest is within this range
Insertion LossHow well the filter passes signals of interest and rejects everything else
Max RF PowerHow much input power the filter can withstand

In the case of a notch filter, the center frequency and bandwidth refers to the range of frequencies that the filter will reject.

The Insertion Loss and Rejection are specified in decibels (dB). In general, the pass band insertion loss should be as small as possible and in real-world filters typically this number varies between 1 dB and 5 dB. The out-of-band rejection should be as high as possible. Theoretically, the further away in frequency a signal is from the pass band, the higher the rejection. However this is not generally true in practice. Below is a plot of a Mini-Circuits filter’s rejection.

Insertion loss of a Mini-Circuits band pass filter

There are two things that really stand out here:

  • The filter is not symmetric in its ability to reject signals, i.e. a signal that is 2500 MHz to the left of the center will be rejected more than a signal that is 2500 MHz away to the right.
  • The rejection of the filter increases with an increase in frequency but only up to around 5300 MHz. Beyond that, the rejection actually decreases. Note that the rejection is only 9 dB at 12500 MHz.

Pre-select Filters

Some SDRs have built-in filters called pre-selectors or pre-select filters. These filters are used to eliminate signals that are outside the range of frequencies at which the SDR is being operated. An SDR that operates over a wide frequency range typically has a bank of pre-select filters that it switches across depending on the frequency of operation. Let’s say for instance an SDR operates from 25 MHz to 1700 MHz. In this case a pre-select filter bank could potentially be comprised of three filters as shown in the picture below. The first filter has a pass band between 25-500 MHz, the second between 500-1200 MHz and the third between 1200-1700 MHz.

Pre-select Filter Bank

Now when the SDR is tuned to operate anywhere between 25-500 MHz, the first filter in the bank of three is selected. The second or third filter in the bank is selected depending on whether the SDR is operating in the range of 500-1200 MHz or 1200-1700 MHz, respectively. The filters are specified and designed to both minimize interference to the receiver and the transmission of spurious signals.

Most SDRs on the market at the sub-$500 price point don’t have built-in pre-select filters as can be seen in the table below.

Software-defined RadioPre-select filter bank?
RTL-SDRNo
LimeSDRNo
HackRFYes. With limited rejection capability
BladeRFNo
ADALM PlutoNo
SDRPlayYes. With very good rejection

Most if not all of the above lower-cost SDRs have limited receiver dynamic range and in the absence of filters they are easily saturated by strong signals. In another post we wrote in more detail about the importance of FM notch filters to prevent saturation. On the transmit side there are almost always spurious signals generated as a result of clocks or other signal processing in the SDR. It becomes very important to filter these spurs and reduce their levels prior to transmission as they can interfere with other wireless systems.

Although ideal, it is very difficult to add a pre-select filter bank to an SDR that doesn’t already have one built-in. On the other hand, when an SDR is used in a specific band and for a particular application it is relatively easy and always advisable to use an external RF filter for best performance. The type of filter used depends on the application.

For instance if an SDR is being used to read water meters that communicate at 869 MHz, then it’s a good idea to use an 869 MHz band pass filter to limit out-of-band signals. If an SDR is being used for many different applications such as ADS-B, AIS, etc. and is being operated near a strong FM transmitter, an FM notch filter would be very beneficial.

Summary

In this post we discussed the different types of filters and why they are required for SDRs. If an SDR doesn’t have a built-in preselect filter bank or pre-selector it is recommended that an appropriate external filter be connected and used with the SDR.

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