You Need a Tunable Bandreject Filter with Sharp Slopes for Spurious Emission Measurements
Today’s society is overwhelmed by communication devices. To coexist, these devices must take turns in time, so-called time division duplex (TDD), or use different frequency bands, frequency division duplex (FDD). Both are used for mobile communication devices. Therfore it is very important that a device has sufficient suppression of spurious emissions, i.e. transmissions outside of the operating frequency band.
The requirement for spurious emissions according to the certification standard is that they should be lower than 50 dBm in any 1 MHz band everywhere except in the operating band and its nearest neighbouring bands of the same width. So for a 100 MHz wide operating channel, the emissions need to be suppressed except within 100 MHz of the operating channel band edges and for a 10 MHz channel within 10 MHz of the edges. The most severe case is at maximum transmit power, which is +23 dBm.
To verify that emissions are lower than 50 dBm a spectrum analyzer or eqivalent instrument is used which can be tuned to proper frequencies and bandwidths. Unfortunately, the measurement instrument will have non-linearities of its own and the strong operating channel signal will generate spurious harmonics that cannot be distinguished from emissions from the device-under-test (DUT).
You Need a Bandreject Filter with Sharp Slopes
To eliminate the harmonics of the test instrument, the operating band signal should be suppressed with a bandreject filter. Then the spurious emissions from the DUT can be tested properly. A block diagram of the setup is shown below.
It is imperative that the filter has wide enough bandwidth to suppress the complete operating channel of the DUT. At the same time it must have sharp slopes to allow nearby frequency bands through, where the measurements are to be performed. A bandreject filter is sometimes called a notch filter, but it is important to distinguish from the simpler, narrow band filters that are no longer sufficient for modern communication testing.
Modern communication devices have adaptable channel bandwidths for adjusting to whatever data transmission rate that is needed for the moment. This means that the bandreject filter of the test equipment must support all these bandwidths.
The Ranatec RI 260 series of digitally tunable bandreject filters combine wide suppression bandwidth with very sharp slopes. This facilitates measurements for several channel bandwidths with the same filter. For example, the RI 268 tunable bandreject filter has a 160 MHz wide suppression bandwidth with 10 MHz slopes. It can be used for test of spurious emission tests of all channel bandwidths from 10 MHz up to 160 MHz, including all sub 8 GHz WiFi and 5G frequency bands.
You Need a Digitally Tunable Bandreject Filter
The combination of the many operating bands and operating channel bandwidths lead to the availability of many hundreds of communication channels for mobile communication devices. All of them need to be certification tested for compliance to the standards. This calls for many hundreds of bandreject filters, which is not realistic in practise.
Instead, the filters should be digitally tunable. The Ranatec RI 260 series of digitally tunable bandreject filters can be tuned over the full range of all the DUT operating bands. The bandreject performance is unchanged over the full range. Measured filter performance curves are shown in the figure below.
The frequency response of a Ranatec tunable bandreject filter
We note 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.
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.
Visit us at ranatec.com and follow our latest news on LinkedIn and Twitter.
If you would like more information about this topic, please call or e-mail Charlotte Ornstein at +46 317061660 charlotte.ornstein@ranatec.com.
