To achieve the maximum performance and stability inherent in each EMT VCO, it is essential that a good RF ground be established and maintained between the VCO substrate and the mounting surface. Sufficient decoupling for the input supply voltage, a clean tuning supply voltage , and proper output isolation are also critical parameters needing to be addressed. The following are suggested design practices and mounting/removal instructions to help the engineer ensure optimal performance and mass production repeatability.

PCB Layout and Isolation Techniques:

Most VCO performance problems are related to insufficient RF grounding typically caused by incorrect mounting techniques or improper circuit topography. Problems such as low power output, frequency pulling, skipping or hopping, and degraded phase noise performance, can result from improper grounding.

The PCB layout should incorporate ground plane covering the full mounting surface of the device and feed-through holes or vias around the periphery. Solder mask should not be used as this may cause power output variations and increase noise susceptibility. The diameter of these holes should be carefully considered so as not to generate parasitic inductances. A simple rule of thumb equation is provided below to help determine the proper via diameter based on board thickness:

Minimum Via Diameter = Minimum Board Thickness

Noise on the dc input voltage supply can AM and FM modulate the output of the VCO. Therefore the supply should be adequately decoupled to reduce as much supply noise as possible. Capacitor values of 100 pF, 0.1 uF, and 10 uF should be placed as close as possible to the VCC input contact and along the supply line of the final assembly. Additional improvement can be gained by isolating the VCO from an unregulated primary power source by the use of voltage regulators and simple R-C filtering at the output of the supply.

Output isolation, like good RF grounding, is critical to achieve the published performance characteristics. Proper selection of components and PCB layout can minimize load pulling and noise susceptibility caused by external circuitry.

Matching the output of the VCO to external circuitry using a 10 dB pad located as close as possible to the output of the device will improve isolation and minimize an impedance mismatch. Lower attenuation levels may be used; however, this will result in decreased load isolation. Standard RF design practices should be followed regarding the use of a 50 ohm microstripline coupling the RF Out to the 10 dB pad.

Manual and Automated Soldering Techniques:

Although EMT VCOs are designed and manufactured using materials and processes that produce a mechanically stable device capable of withstanding automated assembly procedures or manual installation, some care should be taken to not reflow the components within the device. All VCOs are assembled using an SN96 solder alloy which should be taken into consideration when selecting soldering iron temperatures, IR, or convection reflow profiles.

The surface of the final assembly substrate should be a clean, smooth, flat area free from any deformities or residues. Solder masks should not be used, it may degrade the output performance of the device. To prevent mechanical stress over temperature extremes, the FR-4 VCO substrate should be soldered directly to a final assembly substrate with similar thermal expansion coefficients with both surfaces at the same temperature.

Manual Assembly Process:

When manually soldering, the assembler should select a soldering iron and tip of the appropriate size and shape to permit soldering with ease and control without damage to the VCO, adjacent areas, or connections. This process should be executed in a timely manner to prevent internal component damage or reflow. Therefore, due to the considerable difference in surface area to be heated between RF OUT, VCC, and VT compared to the ground area, it is recommended that two appropriate soldering irons be selected when manually soldering the VCO.

Surface mount devices should sit flat and be pressed firmly against the final assembly substrate prior to soldering to prevent mechanical stress during the process. All radial ground connections around the periphery of the VCO should be first soldered using a standard SN63 rosin core solder. Note: All ground connections should be used to ensure proper RF grounding. The other three contacts may now be made, ensuring that good solder connections are made without the use of excess solder.

Similarly, pin mount devices should sit flat and be pressed firmly against the final assembly substrate prior to soldering to prevent mechanical stress during the process. The contact pins of the device should be trimmed to their proper length prior to soldering. When available, all radial ground connections around the periphery of the VCO should first be soldered using a standard SN63 rosin core solder. The pins of the device may now be soldered with special care taken not to overheat the contact pins, as this may cause their upward movement into the device. All solder joints and positioning of the VCO should be inspected to ensure that the device is flat against the final assembly substrate for a good RF ground. Should additional grounding be necessary, copper tape or braid may be used to provide a ground from the metal cover to the surrounding ground area.

EMT strongly recommends that a solder fillet from the metal cover to ground NOT be attempted. The excessive heat may cause internal component damage or reflow, or it may cause the metal case to reflow and shift.

IR / Forced Convection Reflow Process:

Solder stencils may be used to dispense solder around the outer edge of the VCO and at the contact points for surface mount devices. Solder paste should not be dispensed in the center of the mounting area, as this may wick or spread out causing ground shorts between the inputs or output when compressed. This practice also creates extreme difficulty and possible damage should the need arise to remove the VCO from the final assembly substrate.

The installation profiles for either process should be based on the requirements of the final assembly substrate, taking into consideration the solder reflow temperature of the VCOs internal components. SN96 solder alloy is utilized for all components and contact pins in every VCO with a melting temperature of 225°C. The reflow profile should ensure proper wetting of the VCO substrate to the final assembly substrate. The reflow profile may be 215°C to 220°C for 20-30 seconds with the maximum device temperature exposure not to exceed 10 seconds at 230°C. This is only a suggested profile and should not be considered absolute.

Solvents and Cleaning:

The solvents or aqueous cleaners used should be selected to remove both ionic and nonionic contamination. Some components within the VCO are encased in plastic packages, which are susceptible to Trichlorethylene or similar solvents. Therefore, the use of these solvents should be restricted or carefully controlled. Immersion, vapor, or ultrasonic cleaning methods may all be employed without damage to the device.

Most surface and pin mount devices have a slotted cover design that allows solvents or cleaners to flow through easily, removing any fluxes or residue that may have entered during the assembly process. Residual cleaning solvents remaining on, or trapped in the VCO may be easily baked away from the assembly at +85°C. This step is recommended to prevent internal contamination or operation difficulties. VCO performance may be temporarily affected in the form of erratic behavior or low power output if any liquid cleaning solvents remain in or on the assembly.

Removal from your board:

NOTE: The technician should confirm the failure of the device prior to removal as the heat required to remove the device may cause damage to the internal circuitry.

Removal of leaded devices simply requires wicking the solder away from the leads and ensuring the leads are free from the holes prior to removing the VCO from the board. Care should be taken not to apply too much heat, as this may cause the leads to reflow internally or be pushed free of their internal connection.

Removal of surface mount devices are considerably more difficult and require experienced personnel. The removal method listed below is only a suggestion and should be used with caution to prevent damage to the board or device.

Heatgun approach: