Software-defined radio
Present software-defined radios (SDR) employ front end circuits that contain multiple receivers and transmitters for each band of interest, which is inflexible, expensive and power inefficient. A programmable front end circuit is implemented on a CMOS device and is configurable to transmit and receive signals in a wide band of frequencies, thereby providing an adaptable transmitter and receiver operable with current and future wireless networking technologies.
1. A circuit comprising: a frequency synthesizer configured to provide a clock signal having a variable frequency; a transmit path including: a first anti-aliasing filter for receiving an analog signal; an upconverter configured to upconvert an output of the anti-aliasing filter according to the clock signal; and a first programmable bandpass filter configured to filter an output of the upconverter; and a receive path comprising: a second programmable bandpass filter configured to filter a received signal; a downconverter configured to downconvert an output of the anti-aliasing filter according to the clock signal; and a second programmable anti-aliasing filter configured to filter an output of the downconverter; and
one or more of the first and second anti-aliasing filters, first and second bandpass filters and the transversal filter include at least one programmable biquad circuit;
the biquad circuit includes at least one attenuator, at least one integrator and at least one summer; and
at least one attenuator includes: a plurality of M attenuator blocks, each block comprising: a first switch connected between a signal rail and an output node; a second switch connected between an offset rail and the output node; and a resistive element connected in series between the output node and the first and second switches; the attenuator providing N-bit accuracy, wherein N is less than M.
2. The circuit of claim 1 , further comprising a programmable transversal filter configured to receive an output of the first programmable bandpass filter and a signal corresponding to the received signal, the programmable transversal filter providing echo cancellation of the transmit signal from the received signal.
3. The circuit of claim 1 , further comprising a combiner circuit in the receive path, the combiner circuit configured to combine the received signal with an output of the transversal filter.
4. The circuit of claim 1 , further comprising a balanced hybrid circuit at both the transmit path and receive path, the balanced hybrid circuit configured to provide electrical symmetry between the transmit path and the receive path.
5. The circuit of claim 1 , wherein the balanced hybrid circuit is further configured to limit transfer of energy from the transmit path to the receive path.
6. The circuit of claim 1 , further comprising a roofing filter in the receive path, the roofing filter configured to limit a frequency of the received signal to a selected bandwidth.
7. The circuit of claim 1 , wherein the at least one integrator includes: first and second p-channel transistors including respective sources coupled in parallel to a first voltage supply terminal and respective drains configured to provide complementary output signals; first and second variable resistors including respective first terminals coupled to the drains of the first and second p-channel transistors, respectively, and respective second terminals coupled to gates of the second and first p-channel transistors, respectively; and first and second n-channel transistors including respective drains coupled to the second terminals of the first and second variable resistors, respectively, respective gates configured to receive complementary input signals, and respective sources in electrical communication with a second voltage supply terminal.
8. The circuit of claim 1 , wherein the at least one summer includes: a plurality of N switches connected in parallel between an output node and a ground rail; and a resistive element connected in series between a source rail and the output node; each of the switches being controlled by a respective voltage input.