Question

1. Please explain the operation principle of YIG resonator and MSW thin film resonator.

2 .Please explain the operation of the dielectric resonator based oscillator.

3. Please answer the following questions for the image rejection mixer.

a. Why the image signal is important in the heterodyne receiver or transmitter not in direct conversion receiver or transmitter?

b. What is benefit to use image rejection mixer instead of image rejection filter?

c. What is limitation in using image rejection mixer?

4. Please explane about three different types of FOMs of VCO based on your knowleage and explain how each FOM can be used for VCO performance.

Answer 1

Answer 1)The operation principle of YIG resonator is the field from an electromagnet changes the resonance frequency it will allow to pass.

A simple resonant cavity for sustaining magnetostatic waves (MSW) in a resonator device is achieved by depositing a ferrimagnetic film on a substrate and cutting two substantially parallel edges in the film. The substantially parallel edges serve as simple reflectors to replace the complex reflector arrays that are etched or otherwise formed in the prior art thin film resonators.

Answer 2)An oscillator that uses a Dielectric Resonator as the frequency determining element, to produce signals with excellent signal stability, high Q and very low micro-phonics is called as Dielectric Resonator Oscillator (DRO)

Answer 3)

(a)image signal is important because it needs the signal to be transmitted and it mixes the two signals to get a new one.

(b)the benefit of using image rejection mixer is image spectrum is suppressed before the mixer shifts it to the intermediate frequency. This is an effective solution.

(c)there are two limitations of image rejection mixer.they are:

(1)The Trade-Off
The image-reject filter won’t be very useful if it attenuates the desired spectrum and the image spectrum. Thus, the filter’s response must transition from low attenuation at the desired band to high attenuation at the image band. As with any filter, rapid transitions from passband to stopband are challenging, and thus it will be easier to design an image-reject filter if there is a large frequency separation between the desired band and the image band.

However, the separation between the desired band and the image band is proportional to the intermediate frequency (more specifically, it is twice the intermediate frequency). This means that more separation corresponds to a higher IF. This is not catastrophic, but we have to remember that we want an intermediate frequency to be significantly more convenient, from a signal-processing perspective, than the high frequency used for RF transmission. If we increase the intermediate frequency too much, the difficulties created by the higher IF may outweigh the benefits of improved image rejection. Thus, image-rejection filtering entails a fundamental trade-off between image suppression and the desire to maintain a lower intermediate frequency.

(2)Integration, or Lack Thereof
Image rejection is typically accomplished by means of a filter that is not incorporated into an integrated circuit. In other words, image-reject filters consume PCB area and design time, and in the context of modern electronics, both of these resources are valuable and in short supply.

Answer (4)

The VCO types include LC oscillator, RC oscillator, crystal oscillator, multivibrator, relaxation oscillator and ring oscillator.The difference between these VCO types are also covered.

About VCO: The VCO is the short form of Voltage Controlled Oscillator. It is an electrical circuit which generates oscillatory output voltage. VCO is oscillator whose output frequency changes in direct proportion to the applied input voltage. The VCO devices can be constructed which can oscillate from few Hz to hundreds of GHz. VCO are of many types based on its construction as explained later.

The VCO has a characteristic gain, which can be expressed as ratio of the VCO output frequency to the VCO input voltage. VCOs utilize variable control voltage input in order to give frequency output. The control voltage input can be tuned such that desired operational frequency can be produced by the VCO. There are different tuning methods for the VCO as explained below.

VCO in PLL
The figure-1 depicts one of the application of VCO in PLL circuit. As shown PLL (Phase Locked Loop) consists of VCO, PD (phase detector), loop filter, digital frequency divider by N and RF mixer. In steady state output frequency of VCO is expressed as follows.
Fo = Fm + N*Fr
Where, Fo = Output Frequency of PLL
Fm is the frequency input to the RF mixer used in order to downconvert Fo to some lower frequency input for the divider. Frequency Divider block further applies down conversion by factor of N. The output of the divider is fed to the PD input.
Fr is the reference frequency for the PLL.
From the expression it is imperative that VCO output frequency depends on N, Fr, and Fm. Usually frequency of the VCO can be controlled by various methods. The popular among them is by varying Fm & N and keeping Fr constant.