A Wideband, Varactor-tuned Microstrip VCO

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Home A Wideband, Varactor-tuned Microstrip VCO

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A Wideband, Varactor-tuned Microstrip VCO

The design of a varactor-tuned VCO covering an octave frequency range up to 4 GHz and more

By Matjaz Vidmar

June 01, 1999

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ICUWB 2012 (September 17, 2012 - September 20, 2012)

A Wideband, Varactor-tuned Microstrip VCO

Matjaz Vidmar

University of Ljubljana

Slovenia

When electrical tuning of an oscillator is required over a wide microwavefrequency range, yttrium iron garnet (YIG)-tuned oscillators usually provide the

widest frequency coverage as well as relatively low phase noise. While YIG-

tuned oscillators generally provide a tuning range of more than one octave, the

tuning range of varactor-tuned VCOs is much more restricted. Furthermore, all

available varactors are rather lossy capacitors with low Q values at frequencies

above 1 GHz, degrading the phase noise of the oscillator.

Wideband, varactor-tuned microwave VCOs are usually built as hybridmicrocircuits to reduce device parasitics. Expensive GaAs varactor chips are

used to simultaneously increase the frequency coverage and reduce the phase

noise due to the better Q of GaAs varactors when compared to their silicon

counterparts. Although varactor-tuned VCOs cannot compete with YIG VCOs

directly, significant improvements can be made with a better varactor-tuned VCO

circuit design.


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This article presents the design of a varactor-tuned VCO covering an octave

frequency range up to 4 GHz and more. The VCO design uses standard surface-

mount device (SMD) components, including inexpensive silicon varactors.

Furthermore, the VCO is built using conventional microstrip technology on

inexpensive FR-4 circuit-board laminate. Finally, the phase noise of the described

VCO is reasonably low (approximately 20 dB worse than a free-running YIG

oscillator).

Wideband VCO Design

Wideband VCO design with negative-resistance active devices

is difficult due to circuit parasitics. Two-port amplifier devices

offer the designer more freedom. A basic microstrip oscillator

design with a two-port amplifying device is shown in Figure 1.

Any oscillator design requires an amplifier and a frequency-selective feedback

network. Besides the desired frequency response, the feedback network should

also provide the correct phase shift so that the total phase shift in the feedback

loop equals an integer multiple of 360. Of course, this total phase shift also

includes the phase shift inside the active device, which is usually much larger

than 180 due to device and package parasitics at microwave frequencies.

In the case of a microstrip oscillator, the feedback network may include aninterdigital bandpass filter to determine the oscillator frequency. Additional

phasing microstrip lines are required to bring the total phase shift to an integer

multiple of 360 at the desired operating frequency. In addition, the feedback

network should be designed so that oscillation is only possible at the fundamental

mode of the interdigital bandpass filter, while oscillations at higher order

resonance are suppressed.

A fixed-frequency oscillator can be modified into a VCO by inserting one ormore varactors into the circuit. If a single varactor is inserted in series with the

central finger of the interdigital bandpass, frequency coverage of approximately

10 percent around a central frequency of 2 GHz can be obtained. Although a

single varactor may tune the interdigital bandpass over a wider frequency band,

the desired phase shift cannot be maintained over the wider range to keep the

oscillator running.


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A wideband microstrip VCO can be built only if the phase velocities of all

microstrip lines are controlled. The phase velocity of a microstrip line can be

controlled using several series or parallel variable reactances distributed along

the microstrip line. A series connection is usually used at microwave frequencies

due to the relatively high capacitance of available varactors.

A tunable microstrip oscillator design is shown in Figure 2.The phase velocity of the microstrip lines is controlled by

several variable capacitors connected in series in the

transmission lines. The capacitors adjust both the center

frequency of the microstrip bandpass filter and the overall

phase shift of the feedback network. If the spacing between

adjacent variable reactances is kept sufficiently small (less than one-quarter

wavelength at the maximum operating frequency), very wide tuning ranges are

possible.

A Practical S-band VCO

The circuit diagram of a practical wideband, varactor-tuned microstrip VCO

operating in the S-band (2 to 4 GHz) is shown in Figure 3. The

feedback network includes an interdigital bandpass filter tuned

with six type BB833 silicon, hyperabrupt varactors (0.75 pF

minimum capacitance, 1.8 W series resistance, SOD-323 SMDpackage). Several 22 kW resistors are used as resonance-free

RF chokes to apply the same tuning voltage to all six varactors.

Due to the low Q of the varactors used, the insertion loss of the feedback network

is rather high. Therefore, a type BFP420 high ft silicon bipolar transistor (ft = 25

GHz, SOT-343 SMD package) is required as the active device. The DC bias is

provided by a simple resistor network. In order to isolate the oscillator, only a

very small fraction of the RF signal is taken through a resistive divider from thecollector to the output.


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Some components are required only to simplify the printed-

circuit-board layout (for example, the series connection of two

4.7 kW resistors for the base bias). Other components are

required to suppress unwanted resonances, such as the two 100

W resistors in the varactor bias circuit. Finally, the VCO is

followed by two buffer stages to further isolate the output, as

shown in Figure 4.

The microstrip circuit was etched on 0.8-mm-thick, double-sided FR-4 glass

fiber-epoxy laminate. Two different layouts were tested, both

with dimensions of 20 mm x 80 mm, as shown in Figure 5. The

narrowband version uses higher impedance microstrip lines in

the feedback network, resulting in stronger coupling and lower

insertion loss. The wideband version uses wider microstrip

lines, resulting in weaker coupling and higher insertion loss, butwith wider frequency coverage using the same varactors. All passive SMD

components (resistors and capacitors) are standard size 0805 parts.

If the described VCO is to be redesigned for different frequencies and/or different

varactors, the circuit size first should be scaled. Second, the center finger of the

microstrip bandpass should be tuned to obtain the correct phase shift in the

feedback loop. If tuning of the center finger shifts the operating frequency range

too much, the entire circuit must be scaled again. Next, tracking of the amplitude

and phase response should be checked while adjusting the varactor tuning

voltage. Finally, the amplitude response should be checked both at the desired

frequency and at the undesired higher order resonances of the microstrip feedback

network. This design procedure applies equally to computer simulation and

practical circuit tests.

The VCO's Measured Performance

Three versions of the described VCO were built. The first version was built on

the narrowband PCB 1 with BB833 varactors. The second version was built on

the wideband PCB 2 (also with BB833 varactors). The third version was built on

PCB 1 with better BB857 silicon hyperabrupt varactors (very small SCD-80

SMD package, 0.55 pF minimum capacitance and 1.5 W series resistance).

Several samples of all three versions were built and tested.


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The typical tuning curves of all three VCO versions are plotted

in Figure 6. The first version covers 2 to 3.85 GHz, depending

on the tolerances of the varactors used. The tuning range of the

second version is approximately 150 MHz wider, covering 2.05

to 4.05 GHz with the same varactors. The tuning range of the

third version extends from 2.4 to 4.6 GHz thanks to the

improved BB857 varactors.

The tuning range of all three versions can be extended by approximately 50 MHz

on the lower end by allowing the tuning voltage to turn negative to -0.7 V. The

tuning curves are quite nonlinear. The tuning slope exceeds 100 MHz/V at tuning

voltages of approximately 7 V and falls below 10 MHz/V at tuning voltages of

approximately 30 V. Since the VCOs were designed for the widest frequency

coverage, no attempt was made to linearize the frequency/voltage response in the

RF circuit.

Since specialized phase noise test equipment was not available, the phase noise

of the available model HP8593EM spectrum analyzer was first roughly estimated

The phase noise of the analyzer was determined to be sufficiently low to

accurately measure the phase noise of the described VCOs. Two samples of the

first and second versions (with BB833 varactors) were packaged in shielded

cases and connected to well-filtered supply and tuning voltage sources.

The single-sideband (SSB) phase plots, shown in Figure 7, demonstrate that the

described VCOs are roughly 20 dB worse than the YIG

oscillator inside the spectrum analyzer. Interestingly, the phase

noise is approximately 5 dB stronger at the band center than at

the band edges, suggesting that at least part of the phase noise is

caused by thermal noise voltage generated in the 22 kW

resistors that is modulating the VCO frequency at the point

where the tuning slope is the steepest.

Possible VCO Improvements

Although the frequency coverage of the described VCO exceeds the advertised

performance of commercially available hybrid VCOs, many improvements to the

described circuit are still possible. In particular, the phase noise performance

probably could be improved. Last, but not least, the described VCO design can


10/14



be readily adapted to other two-port active devices, such as GaAs FETs, high

electron mobility transistors, heterojunction bipolar transistors or MMIC

amplifiers.

Part of the phase noise is caused by the 22 kW resistors used as resonance-free

chokes to bring the tuning voltage to the varactors. Lower resistor values cannot

be used since the RF circuit losses increase. True RF chokes (inductors) shouldbe selected carefully to avoid parasitic resonances in the frequency range of

interest.

The phase noise performance could also be improved by replacing the simple

resistor bias network of the BFP420 transistor. Since the S parameters of bipolar

transistors depend mainly on the DC bias currents through the transistor, the

transistor's operating point should be accurately stabilized to further improve the

phase noise performance.

Both the phase noise and long-term (thermal) stability of the VCO could be

improved by using a better microstrip material than the suggested FR-4 glass

fiber-epoxy laminate. This material has a high temperature coefficient, shifting the

VCO frequency downwards by a few megahertz for each degree of temperature

increase. Besides lower temperature coefficients, suitable microwave substrates

should also provide lower losses and higher Qs for the microstrip resonators.

The nonlinear frequency/voltage response of the described

VCO design may require a linearizer. A simple tuning slope

linearizer circuit is shown in Figure 8. The gain of the two

operational amplifiers is set to the lowest value around the

reference voltage of +7 V DC (where the VCO tuning slope is

the steepest). At lower and higher tuning voltages, some

positive feedback is switched in so that the overall gain increases to compensate

for the decay that occurs in the tuning sensitivity.

The requirements for the linearizer response could also be estimated form the

varactor diode capacitance (CT = f(VR)) curves shown in Figure 9. Curves for

the BB833 and BB857 varactors clearly show that the maximum relative

capacitance change occurs between 5 and 10 V. Above 15 V, the capacitance

curves become flat, resulting in a decrease in the VCO's tuning slope.


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The presettable potentiometers P1 to P8 permit the switching points to be set.

Since the upper part of the tuning curve is much more nonlinear

than the lower part, six switching points are used in the upper

part (resistors R+) and only two switching points are used in the

lower part (resistors R-). Besides using low noise operational

amplifiers, the resistor values should be selected carefully to

avoid excessive noise generation in the linearizer circuit (resistor values too high)or excessive power dissipation (resistor values too low).

Conclusion

A varactor-tuned VCO has been designed that uses standard SMD components

and inexpensive silicon varactors. The VCO's frequency tuning range is over one

octave and extends above 4 GHz. The VCO is built using low cost FR-4 PCB

material and achieves good SSB phase noise performance.

Acknowledgment

The author wishes to thank Knut Brenndoerfer of Siemens Semiconductors,

Munich, Germany, for supplying the many different varactors and other SMD

semiconductor samples without which the described VCO could not have been

developed.

Matjaz Vidmar received his BSEE and MSEE from the University of Ljubljana,

Slovenia in 1980 and 1983, respectively. He received his PhD in 1992, also

from the University of Ljubljana, for developing a single-frequency GPS

ionospheric correction receiver. Vidmar is currently teaching undergraduate

and postgraduate courses in electrical engineering at the University of

Ljubljana. His current research includes high speed electronics for optical fiber

communications. Vidmar is also taking part in amateur satellite projects. He

developed very high efficiency VHF and UHF transmitters that weresuccessfully flown in space on the Microsat mission in 1990.

Recent Articles by Matjaz Vidmar

Microstrip Resonant Phase Shifters

A Microwave Analog Frequency Divider


12/14



Company Profile

Contact Information

Contact:

Phone: (508) 665-4400

Fax: (508) 665-4401

Email: [email protected]

Web Site: http://www.cst.com

Location

CST of America Inc.

492 Old Connecticut Path Ste #505Framingham MA 01701

US

Company Profile

Contact Information

Contact:

Phone: (800) 829-4444

Fax: (800) 829-4433


Web Site: http://www.agilent.com

Location

Agilent Technologies

5301 Stevens Creek Blvd.

Santa Clara CA 95051

US

Company Profile


13/14



Contact Information

Contact:

Phone: (781) 376-3000

Fax: (781) 376-3100


Web Site: http://www.skyworksinc.com

Location

Skyworks Solutions Inc.

20 Sylvan Rd.

Woburn MA 01801

US

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A Wideband, Varactor-tuned Microstrip VCO