All radios used to be superheterodyne radios with downconversion to an intermediate frequency (IF) for filtering before demodulation. While the superhet hasn’t disappeared completely, the direct-conversion zero-IF receiver has mostly replaced it. In fact, most receiver ICs today are direct-conversion ICs.
For example, the versatile CML Microcircuits CMX994 direct-conversion receiver IC (DCRx) fits an incredible array of applications, including the next generation of multi-mode software defined radios (SDRs) for wireless data and two-way radio applications (see the figure). Its design and small size allow a small RF receiver to be realized with a minimum of external components in both zero-IF and low-IF systems.
A DCRx mixes the wanted RF signal down to 0 Hz in a single quadrature mixing process using a local oscillator (LO) tuned to the desired RF operating frequency. Selectivity filtering and gain takes place at baseband with practical, low-power analogue and digital circuits. A DCRx also eliminates the need for an image-reject filter.
The CMX994 offers an on-chip phase-locked loop (PLL) with a voltage-controlled oscillator (VCO) for VHF applications, an operating range of 100 MHz to 600 MHz, precision baseband filtering with selectable bandwidths, and a very small printed-circuit board (PCB) footprint. Also, its single-ended RF antenna connections mean no baluns are required before the low-noise amplifier (LNA). The receiver outputs are the I and Q signals needed by the baseband circuits for demodulation.
Direct conversion’s small size and flexibility make it a key element for the next generation of multi-mode SDRs for wireless data and digital two-way radio applications. Specific applications include multi-mode radios, data telemetry modems, and satellite and land mobile radio systems with 25-, 12.5-, and 6.25-kHz bandwidths.
Available now, the CMX994 operates from 3.0 to 3.6 V. It comes in a Q4 40-pin very thin quad flat no-lead (VQFN) package.