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19-1248; Rev 1; 5/98
MAX2620 Evaluation Kit
________________General Description
The MAX2620 evaluation kit (EV kit) simplifies evaluation of the MAX2620 integrated oscillator with buffered outputs. It includes a varactor-based tank circuit that allows the VCO to tune across an approximately 30MHz band in the 900MHz frequency range. Outputs utilize 50 SMA connectors. The EV kit has a test port that facilitates complete characterization of the MAX2620 tank port, enabling resonators to be designed for frequency ranges other than that supplied with the EV kit.
____________________________Features
o Complete, Tunable VCO Test Board with Tank Circuit o Tuning in 900MHz Frequency Range o Low Phase Noise (-110dBc/Hz typical at 25kHz offset from carrier) o Operates from Single +2.7V to +5.25V Supply o Two Output Buffers with 50 SMA Connectors o Low-Power Shutdown Mode o Test Port for Oscillator Tank Port Characterization o Fully Assembled and Tested
Evaluates: MAX2620
____________________Component List
DESIGNATION QTY C1, C7-C10, C12 C2, C11, C14 C3 C4, C6 C5, C13, C17 C15 D1 JU1, VCC, GND L1 L4 L3 OUT, OUT, TEST PORT R1, R3 R2 R4 R5 SHDN U1 VCONT None None None 6 0 1 2 3 1 1 3 1 0 1 3 2 1 0 1 1 1 1 1 1 1 DESCRIPTION 1000pF, 10% ceramic capacitors Not installed 2.7pF, 10% ceramic capacitor 1pF, 10% ceramic capacitors 1.5pF, 10% ceramic capacitors 10F, 10%, 25V tantalum capacitor Sprague 293D106X9025D2 Varactor diode Alpha Industries SMV1204-34 2-pin headers Ceramic coaxial resonator Trans-Tech SR8800LPQ1357BY Not installed 10nH inductor Coilcraft 0603HS-10NTJBC SMA connectors (edge mount) 10, 5% resistors 1k, 5% resistor Not installed 51, 5% resistor 3-pin header MAX2620EUA SMA connector (PC mount) Shunt MAX2620 circuit board MAX2620 data sheet
______________Ordering Information
PART MAX2620EVKIT TEMP. RANGE -40C to +85C BOARD TYPE Surface Mount
______________Component Suppliers
SUPPLIER Alpha Industries Coilcraft Sprague Trans-Tech PHONE (617) 935-5150 (847) 639-6400 (603) 224-1961 (301) 695-9400 FAX (617) 824-4579 (847) 639-1469 (603) 224-1430 (301) 695-7065
_________________________Quick Start
The MAX2620 EV kit is fully assembled and factory tested. Follow the instructions in the Connections and Setup section.
__________Test Equipment Required
*
NOTE: All capacitors and resistors are size 0805 unless otherwise noted.
Power supplies. Low-noise power supplies are recommended for oscillator-noise measurements. This is especially important for the tuning voltage supplied to the varactor (VCONT). Noise or ripple on the tuning voltage frequency-modulates the oscillator and causes spectral spreading. Batteries can be used in place of power supplies, if necessary. --DC supply capable of supplying +2.7V to +5.25V at 20mA. Alternatively, use two or three 1.5V AA batteries. --DC supply capable of supplying 0V to +3V, continuously variable, for VCONT. Alternatively, use two or three 1.5V batteries with a resistive voltage divider or potentiometer.
________________________________________________________________ Maxim Integrated Products
1
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MAX2620 Evaluation Kit Evaluates: MAX2620
* HP8561E spectrum analyzer, or equivalent highsensitivity spectrum analyzer with approximately 3GHz frequency range. Contact the instrument manufacturer for information regarding phase-noise measurement capabilities. Digital multimeter (DMM) to monitor DC supply and VCONT, if desired Male SMA 50 terminator Network analyzer such as HP8753D (required only if additional device characterization for oscillator tank design at other frequencies is desired) 6) Set another marker to measure the difference between this peak and the signal level at 25kHz offset from the peak. (Phase noise can be observed at frequencies other than 25kHz offset.) 7) Under the Marker function, select marker noise and turn it on. This automatically scales the spectrum analyzer's output to take into account the resolution BW filter's non-ideal characteristics. If your spectrum analyzer does not offer this feature, contact the manufacturer for proper scaling for noise measurements. 8) Verify that the resolution bandwidth is 1kHz. 9) Verify that the video bandwidth is 1kHz. 10) Read the measurement directly from the screen. Phase noise will be about -110dBc/Hz. In some environments that have ambient pulse noise, this measurement may be difficult to achieve without additional shielding or the use of a shielded enclosure.
* * *
____________Connections and Setup
1) Verify that the shunt on jumper SHDN is installed between pins 1 and 2 (SHDN = V CC ). Placing the shunt between pins 2 and 3 (SHDN = GND) puts the MAX2620 into low-current shutdown mode. 2) Connect the spectrum analyzer to either OUT or OUT. Connect a 50 terminator to the output (OUT or OUT) not connected to the spectrum analyzer. 3) Connect a +2.7V to +5.25V supply across VCC to GND. VCC should be the most positive terminal. 4) Connect the tuning voltage supply to either VCONT or JU1. This supply should be positive when referenced to ground.
_____________________________Outputs
The MAX2620 EV kit is assembled with OUT matched to 50 (at approximately 900MHz) using L3 and C13. OUT is resistively pulled up to the supply with a 51 resistor, R5. R5 provides a simple broadband 50 output match but offers less output power than OUT. The EV kit provides additional component pads at R4, C14, L4, and C11 to accommodate any output match configuration for OUT and OUT. Refer to the Output Matching Configuration section in the MAX2620 data sheet for more information.
____________________________Analysis
1) Using the spectrum analyzer, observe the voltagecontrolled oscillator's output. With 1.5V applied to VCONT, the fundamental output frequency will be near 900MHz. The output power level will be approximately -2dBm at OUT, or -12.5dBm at OUT. Varying the voltage applied to VCONT between 0V and VCC changes the fundamental oscillation frequency. (Increasing the voltage applied to VCONT increases the frequency, and vice versa.) The typical tuning range is a 30MHz band centered near 900MHz with VCONT between 0.5V and 3V. To avoid damaging the varactor, do not apply voltages greater than 15V to VCONT. (The varactor on the EV kit board has a 15V breakdown specification.) 2) Allow the oscillator to operate for about 5 minutes to thermally stabilize the frequency. Since the frequency is not phase-locked to a reference, this minimizes frequency drift and measurement error. 3) Center the fundamental on the spectrum analyzer and set the frequency span to 100kHz. 4) Set the spectrum analyzer for single sweep. This minimizes errors due to oscillator frequency drift. 5) Set the marker on the waveform's peak.
2
___________Resonator and Varactor
The resonator tank circuit is critical in determining VCO performance. It typically contains a varactor (voltagevariable capacitance) for voltage-tuning the center frequency. For best performance, use high-Q components and choose values carefully. The external resonant circuit on the MAX2620 EV kit has been designed to operate near 900MHz. To synthesize the component values for other frequency ranges, use the following procedure. On the EV kit, C3 and C4 are feedback capacitors that set the oscillator's negative resistance and impedance. Their values have been chosen to provide adequate performance over a 650MHz to 1050MHz frequency range. To optimize the values of these components for a specific application, refer to the Feedback Capacitors section in the MAX2620 data sheet. Measure the MAX2620 TANK pin's input impedance with feedback capacitors C3 and C4 but without the resonant circuit. This measurement takes into account parasitic circuit elements that are specific to board lay-
_______________________________________________________________________________________
MAX2620 Evaluation Kit
out. Use the test port provided on the MAX2620 EV kit to facilitate measurement by installing a 1000pF capacitor at C2 and removing C5. (Remove C2 and install C5 to use the MAX2620 as an oscillator.) When using the test port, subtract an approximately 586ps electrical delay from the S11 measurement (this delay can be compensated for on most modern vector network analyzers) to account for the delay of the transmission line from the test port to the MAX2620 TANK pin. The test port should provide a negative input resistance and thus return gain when S11 is measured on a vector network analyzer. This return gain provides measurement data that is outside the unit circle of the Smith chart. A useful technique is to configure the vector network analyzer to display 1/S11 for this measurement. The vector network analyzer displays the information inside the unit circle of the Smith chart. Most modern vector network analyzers perform this conversion. Input-impedance data presented in this format (1/S11) is the complement of the input impedance, which is the impedance desired to provide the MAX2620 with feedback to oscillate at a particular frequency. The Typical Operating Characteristics section of the MAX2620 data sheet contains a plot of 1/S11 for specific values of C3 and C4 provided in the MAX2620 EV kit. Also refer to the Tank Circuit Design section in the MAX2620 data sheet. The MAX2620 EV kit uses a low-voltage varactor. With the coupling capacitor C17 kept small, the oscillator circuit is less affected by losses in the varactor. However, keeping C17 small also reduces overall tuning range. L1 on the MAX2620 is a ceramic coaxial resonator, which provides the best phase-noise performance. For cost-sensitive applications, the layout for L1 on the MAX2620 EV kit is a dual pad that accepts either a spring coil or a ceramic coaxial resonator. When properly specified, coaxial resonators provide tight tolerance inductance at very high Q for best circuit performance. Spring coils, such as Coilcraft mini-spring coils, provide a good cost/performance compromise for costsensitive applications.
Evaluates: MAX2620
_____________Layout Considerations
The MAX2620 EV kit can serve as a guide for your board layout. To minimize the effects of parasitic elements, which may alter circuit performance, remove the ground plane around and under the components that make up the resonant circuit (C3-C6, C17, D1, and L1). Keep PC board trace lengths as short as possible to minimize parasitic inductance. Also keep decoupling capacitors C1, C7, and C9 as close to the MAX2620 as possible, with direct connection to the ground plane.
VCC R1 10 C1 1000pF 1 J1 TEST SMA PORT C2 50 OPEN C5 1.5pF C3 2.7pF C4 1pF VCC1
VCC C8 1000pF L3 10nH R4 OPEN C13 1.5pF
50 SMA OUT
U1 MAX2620
OUT
8
C14 OPEN
2 3
TANK FDBK
7 VCC2 GND OUT 6 C9 1000pF L4 OPEN R5 51 C12 1000pF 50
VCC
SMA VCONT JU1 R2 1k D1 C17 1.5pF C6 1pF L1
4
SHDN
5 C10 1000pF
SMA C11 OPEN OUT
R3 10 VCC C15 10F 25V VCC VCC SHDN 1 2 3
C7 1000pF
GND
Figure 1. MAX2620 EV Kit Schematic
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MAX2620 Evaluation Kit Evaluates: MAX2620
1.0"
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Figure 2. MAX2620 EV Kit Component Placement Guide-- Top Silk Screen
Figure 3. MAX2620 EV Kit PC Board Layout--Component Side
Figure 4. MAX2620 EV Kit PC Board Layout--Solder Side
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Figure 5. MAX2620 EV Kit PC Board Layout--Ground Plane
Figure 6. MAX2620 EV Kit PC Board Layout--Power Plane
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
4 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1998 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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