![]() In the examples above, we see that the filtered BPSK signal has a BT product of 0.8, i.e., ISI is beginning to become noticeable. This isn't bad in and of itself, providing the received signal level is sufficiently above the noise floor to demodulate properly, i.e., convert symbols back into bits. An oscilloscope snapshot of the output of the filter would show symbols beginning to blur together. When a BT product is less than unity ISI occurs. When a BT product is greater than unity a filtered signal retains crisp leading and trailing edges of its symbols. Generally, properly designed filters have BT products of between 0.8 and 1.1. Filter bandwidths should be designed as narrowly as possible without causing inter-symbol interference, or ISI. In a radio system design, normally systems engineers specify the radio signalling speeds first (in bits/sec) before specifying filter bandwidths. The BT product is used by radio systems engineers to specify filter bandwidths. Since a QPSK signal transmits 2 bits per symbol, the symbol period is twice as long as for BPSK, so for QPSK, T = 400 microsec. Another example is a 5 kb/sec quaternary phase shift keyed (QPSK) signal. ![]() For example, a 5 kilobit per second binary phase shift keyed (BPSK) signal's symbol period is 200 microseconds, so that T = 200 microsec. T refers to the symbol period of the signal passing through the filter. For example, a 4 kHz wide filter has a group delay of roughly 250 microseconds. (This is a good rule of thumb, but as with all rules of thumb, there are invariably some nuances that I'm glossing over.) So a qualitative understanding of the group delay of a filter is the amount of time it takes for a signal to pass through the filter. The group delay of a filter turns out to be roughly equal to the inverse of the delay time between an impulse at the filter input and the peak of the filter's impulse response at the output. A 4 kiloHertz wide bandpass filter has B = 4 x 10^3 Hz. Bandwidth-delay product is also called bandwidth-time product, or BT, by analog filter designers.Ī bandpass filter's bandwidth, B, is measured in Hertz, which are units of inverse seconds.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |