Tunable bandreject filters compared to tunable notch filters

In the early days of radio and microwave communications, tunable notch filters were invented to filter out narrow band interferers. For example, there could be an external signal or internally generated spurious mixing product that disturbed the performance.

The typical tunable notch filter transmission looks like this:

and we can clearly see that signals at or very close to 5900 MHz are rejected.

The notch filter can be either a mechanically or a digitally tunable filter, so that the notch can be put at any desired frequency within a given range.

The main drawback of the tunable notch filter is the narrow bandwidth and that the bandwidth will change with the frequency tuning. Some efforts have been made with cascaded YIG oscillators to increase and control the bandwidth up to 80 MHz., but the tuning range is limited and the filter slopes are not sharp enough. There is a limitation from the Q value of around 200.

Modern testing requires modern tunable bandreject filters

Modern communication equipment, such as a mobile phone or WiFi device, uses hundreds of channels and bandwidths up to 100 or 160 MHz. Requirements on near-channel spurious emissions are very hard. To measure the very weak emissions close to the channel is a challenge for the test systems and suppressing the strong signal within the channel with a tunable bandreject filter is necessary.

Another application of the tunable bandreject filter is for receiver blocking tests. A blocking is generated with a syntheziser in a band adjacent to the channel and the device should still be able to receive a weaker desired signal. A complication in the testing is that the syntheziser will have phase noise in the receiver channel. This unwanted phase noise can be suppressed by the tunable bandreject filter.

The Ranatec RI 260 series of tunable bandreject filters have very stringent filter characteristics. Measured filter performance curves are shown in the figure below.

The frequency response of a Ranatec tunable bandreject filter

Now we see that all signals within 5850-5950 MHz are attenuated and that the passband starts very close to the reject band, namely at 5840 and 5960 MHz, respectively.
This filter is exactly 100 MHz wide and with 10 MHz slopes, independently of which frequency it is tuned to. This is achieved by active frequency conversion of a state-of-the-art high performance metal cavity filter, which has a Q value of around 2000. The digitally tunable filter can be set with a resolution of 1 kHz.

Due to the historic development, tunable notch filters and tunable bandreject filters are not always properly distinguished, but either expression is often used for either instrument. In practise, however, it is definitely a digitally tunable bandreject filter that should be used in your test equipment for modern communication devices.

The latest Ranatec model, RI 268 Tunable bandreject filter, supports all bandwidths from 3 to 160 MHz for all frequency channels within 600 to 8000 MHz and it facilitates measurements of spurious emissions and receiver blocking from DC to 40 GHz.