Low-cost, programmable oscillator operates at 2 GHz

-May 11, 2000

All wireless communication systems need a local oscillator. Figure 1 shows a low-cost PLL circuit for the 2-GHz band for such applications as industrial, scientific, and medical (ISM) and satellite personal-communication systems (S-PCS). The central IC in the circuit is the LMX2325 (National Semiconductor, www.national.com) frequency synthesizer, which features RF operation to 2.5 GHz. This IC incorporates an N-programmable divider and a phase comparator. The JTOS-3000P (Mini-Circuits) VCO constrains the oscillation frequency to 2.3 to 2.5 GHz. The VCO's tuning-voltage range of 0 to 5V is compatible with the internal charge-pump D0 output of the LMX2325. The COP8SAA7 µC programs the synthesizer's internal registers. You can calculate the register values using the program LMX2325.EXE (National Semiconductor). The values depend on the desired oscillation frequency, the external crystal, and the reference-frequency values. R1, C1, and C2 implement a second-order loop filter, and you can determine the component values using the LOOPFILT.EXE (National Semiconductor) program. The values depend mainly on the oscillation frequency and the VCO tuning sensitivity.

R2, R3, R4, and L1 form a T-network. The design in Figure 1 assumes that a 50 W load connects to ac-coupling capacitor C3. The T-section matches the synthesizer FIN pin's input reactive impedance of 40-j the 50 W output impedance of the VCO's RF OUT pin and output load. If f is the oscillation frequency in hertz, X is approximately equal to
(–36×10–9)f+162. Therefore, the following inductor value for L1 cancels the reactance:

You must place the matching network components close to each other on the pc board. A power supply with good regulation characteristics is necessary, and, therefore, you should use a 7805 IC on the pc board.

A prototype of Figure 1's circuit operating at f=2.45 GHz has an output power of 7 dBm and a phase noise of –75 dBc/Hz at a 10-kHz offset. Click here to download the source program for the µC. You can then use the previously mentioned support software (www.national.com/cop8/) or use the .hex file. (DI #2526)

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