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Selecting the Right VCO for Your Design

Thu, 09/27/2007 - 5:58am
Understanding the key parameters is a "must" before selecting a VCO for your design.

By Sanaz Sarfaraz, Z-Communications, Inc.
Senior Applications Engineer

Voltage-Controlled Oscillators (VCOs) are the heart of every signal source. They are used in fixed wireless designs, digital radios, cellular base stations, test and
measurement equipment, WiMAX and several other applications. Selecting the proper VCO for your design requires a good understanding of its most important parameters. The following list outlines the key parameters you need to consider when selecting a VCO.

Key Parameters for VCO Selection

1. Frequency Range (MHz): The range of a VCO's output signal is specified by a minimum and a maximum tunable frequency. It is not necessary to add buffers to the frequency range. VCO manufacturers usually calculate a margin on both lower and higher ends because of drift over temperature, supply voltage variations and production variation due to part tolerances. VCOs can generate fundamental up to a certain frequency, for frequencies higher than that a doubler can be used. Generally, a single VCO cannot cover much more than an octave (f1 to 2xf1). Wider tuning VCOs may also need a tracking filter to reduce second harmonics. They will usually have more variation in the RF output power as well.

2. Tuning Voltage (V): The minimum and maximum available tuning voltages specify the voltage that is required to tune the VCO over its specified frequency range. More tuning voltage for the same frequency bandwidth generally results in lower sensitivity with better phase noise and the opposite applies to less tuning voltage. VCOs are designed to oscillate at zero volts.

3. Output Power (dBm): The output power of the VCO refers to the RF output power of the VCO (in dBm) delivered to 50 ohm load. Customers should add at least 6 dB to the power specifications as eventually a minimum of 6 dB pad is required at RF output of VCO to isolate the VCO and reduce any pulling effect. Output levels may also have variation over the tuning range by a few dB for wide-band VCOs.

4. Tuning Sensitivity (MHz/V): Tuning Sensitivity (KVCO) is a measurement of how much the VCO output frequency will change with respect to a variation in its tuning voltage. The tuning sensitivity will be slightly larger than simply dividing the frequency bandwidth by tuning voltage, due to calculated frequency margins. Customers ordering customized VCOs do not necessarily have to specify the tuning sensitivity in the initial stages of the ordering process. This is one of specifications that can be determined by VCO manufacturer in a more precise way after the VCO is designed.

5. 2nd Harmonic (dBc): Naturally, all VCOs generate harmonic frequencies of the fundamental. Usually the 2nd harmonic is specified on a VCO's datasheet, as it has the highest level of all harmonics. 2nd harmonics are higher in octave tuning VCOs. Tracking filters can be implemented to reduce the level of 2nd harmonics in that case.

6. Pulling (MHz): Pulling is a measurement of the output frequency drift due to load variation. Using a minimum of 6 dB pads at the RF output port of VCO is recommended to decrease the pulling effect.

7. Pushing (MHz/V): Pushing is a measurement of output frequency drift due to supply voltage variations.

8. Input Capacitance (pF): Input capacitance is the value seen looking into the VCO's tuning port. For most VCOs the maximum value of input capacitance is specified on the datasheet. This value is also related to tuning or modulation bandwidth. Lower frequency ranges result in larger input capacitance values and vice versa.

9. Phase Noise @10 kHz Offset (dBc/Hz): A VCO's phase noise value is the measurement of how clean the output signal is. Here at Z-Communications we use our test fixtures or evaluation boards with Agilent's E5052A to measure the phase noise of our VCOs. Phase noise is also measured at other offsets such as 1 kHz and 100 kHz and these values can be presented to customers upon request. The phase noise measurement for offsets larger than 1 MHz will be limited by the phase noise floor of the test equipment rather than VCO's phase noise. For more details on phase noise measurements you can refer to our applications notes at www.zcomm.com

10. Supply Voltage (V): A clean voltage source with low AC ripple is recommended.

11. Supply Current (mA): The supply current defines the power consumption of the VCO. The DC current drawn by some VCOs (usually smaller packages) with lower DC supply voltage may be reduced upon request.

12. Operating Temperature (°C): The operating temperature refers to the temperature range over which the VCO specifications are guaranteed. Most of Z-Communications VCOs have an operating temperature range of -40 to 85°C.

13. Package Size: Depending on the PCB space limitation, a number of different package styles are available. The height of the shield can also be higher or lower depending upon the design and customers' specifications.

Common Questions

RF Design Engineers can find many answers to commonly asked VCO questions on our web site. For example, many customers ask about our evaluation boards and test fixtures and that information can be found under the products section. Customers asking for direction on the proper mounting of VCOs and reflow profiles can find those answers under the support section in our application notes. Information on pin configuration and RoHS compliance can be found on our datasheets which are available on our web site.

Z-Communications also has a team of application engineers ready to provide technical support. Customers looking for information on the feasibility of a customized solution, the maximum tuning voltage a VCO can tolerate, storage temperature, microphonic effects and cleaning methods should contact our technical support staff at applications@zcomm.com. For additional information contact Z-Communications at 858-621-2700 or email at sales@zcomm.com.

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