Examcrazy Logo
MBA in India CAT How to Prepare for Exams Technical Freshers Jobs
  Follow us|  twitter  Orkut  facebook
Electronics II Tutorials
   Overview Analog Wireless Communcation
   Resistors, Capacitors, RC Networks
   Diodes, Amplitute Modulation, Diode Detection
   RL Circuits, Inductive Kicks, Diode Snubbers
   RC filters. Series resonance and quality factor, Matching, Soldering
   Ladder filters Butterworth and Chebyshev filters Filter tables ADS
   Bandpass ladder filters Quartz crystals
   Impedance inverter
   Ideal Transformers
   Transformer shunt inductance
   BJT-Large signal models
   Transistor switches. Voltage regulators
   Transistor switches. Voltage regulators
   Common emitter amplifier. Max. efficiency of class A amps. Transformer coupled loads
   Available power. Distortion. Emitter degeneration. Miller effect
   Emitter follower and differential amplifiers
   JFET Source follower amplifier
   Oscillators. Clapp oscillator. VFO startup
   Variable frequency oscillator. Gain limiting
   Receiver incremental tuning. Crystal oscillators
   Mixers. Gilbert cell
   Superheterodyne receivers. Spurious responses of mixers
   Decreasing channel bandwidth by using CW
   Audio amplifiers
   JFETs as variable resistors
   Automatic gain control
   Noise, SNR, MDS, noise power density and NEP
   Nyquist noise formula. Cascading noisy components. Noise figure
   Receiver intermodulation and dynamic range
Other Electronics 1 Tutorials
   Diode Tutorials
   BJT Tutorials
   MOSFET Tutorials
   Applied Electromagnetics Tutorials
Free Electronics Tutorials
   Diode Tutorials
   BJT Tutorials
   MOSFET Tutorials
   Electronics II Tutorials
   Applied Electromagnetics Tutorials
   Microwave Tutorials
GATE preparation tips
   GATE Books & How to prepare
   Objective Solving Tricks
   Other GATE links
   IES exam preparation
   All about DRDO-SET
More Engineering Links
   Directory of coaching Institutes
   Govt engg college rankings
   Private engg college rankings
   Admission notifications for Mtech/PhD
   All Engineering Colleges in India
Mixers Gilbert Cell

Mixers Gilbert Cell
Mixers shift the frequency spectrum of an input signal. This is an essential component in electrical communications (wireless or otherwise) if we wish to use RF signals to convey audio or data signals over long distances, for example. The circuit symbol for a mixer has three ports

Notice that all three ports have signals at different frequencies! “Mixing” has a couple of connotations. One is that of combining (by summing) signals from different channels (or sources), as in the recording industry. from the ARRL Handbook on the next page illustrates this principle. Clearly, this is not the type of “mixing” that’s needed in communications. We need to shift the frequency. This type of mixing is the result of multiplying signals in the time domain, as shown of the ARRL Handbook.

There are two general types of mixing circuits, those involving:
1. Nonlinear components, such as diodes, etc.
2. Linear, but time-varying circuits. These circuits can shift
the frequency spectrum of a signal, in contrast to linear and time invariant circuits (which cannot).
Gilbert Cell
All mixers in the NorCal 40A are based on the Gilbert cell. The Gilbert cell uses a linear, time-varying circuit to achieve timedomain multiplication, and hence, frequency shifting. A Gilbert cell is shown The RF signal is input to a long-tailed differential amplifier, which we studied in Sec. 9.8. The collectors of Q1 and Q2 have a cross connected set of four transistors, which are driven by a local oscillator (LO). To see how the Gilbert cell operates, first consider what happens when the voltage VLO1 is large enough so that Q3 and Q5 turn on and VLO2 is small enough, that Q4 and Q6 turn off:

Here we see that Q3 and Q5 act as closed switches so that:
• Q1 is connected to R1, and
• Q2 is connected to R2
as in a “typical” differential amplifier configuration with the output taken at, what we will call here, the “IF” terminals. Second, consider what happens when the opposite input occurs. Specifically, suppose VLO2 is large enough so that Q4 and Q6 are on while VLO1 is small enough that Q3 and Q5 are off:

Now we see that Q4 and Q6 act as closed switches so that
• Q1 is connected to R2, and
• Q2 is connected to R1.
This also is a differential amplifier configuration, but with the outputs interchanged wrt to the previous case. In other words, the output (the IF) is almost the same as before. It has just been multiplied by the factor –1. The overall function of the Gilbert cell is to multiply in the time domain the input RF signal (at the RF frequency) by a square wave with value +1 or –1 at the LO frequency! This is mixing. The Gilbert cell is also an active mixer in that the IF output signal is amplified because of the differential amplifier gain. (Active mixers are very nice in the sense that they accomplish two jobs at once: they mix and they amplify.) A Gilbert cell is the active mixer inside the SA602AN IC used in the NorCal 40A (see the datasheet on p. 415). As we’ve already seen, this IC also has part of an oscillator circuit built inside. What a versatile IC! From Fig. 4 of the SA602AN datasheet we can see that certain subsystems are internally biased. Consequently, we don’t need to construct an external bias circuit. However, we must capacitively couple to the SA602AN so we don’t disturb this biasing (examples of this are C4, C5, C13, C15, C31 and C33).
Mixer Mathematics
It is helpful to study the mathematic basis behind the Gilbert cell. This will help us understand this circuit better, as well as develop an appreciation of the mixing process in general. Let’s define the RF input voltage as
Vrf(t) =Vrfcos( ?rft)
and define the mixing signal (which is not the LO signal, as is stated in the text) by the square wave:

Since this mixing signal is a periodic waveform, we can expand it in this Fourier series

as derived in Sec. 2 of Appendix B.The Gilbert cell effectively multiplies both of these signals (12.2) and (12.3) in the time domain as
V(t) =Vrf(t) ?Vmax(t)

?+= ? 10?rf ? - |? 10?rf|

Note in (12.5) that we have the sum and difference signals present in the output (IF) voltage signal:

as well as the third-harmonic terms:

and the fifth-harmonic terms:


and all higher-ordered odd harmonics. Observe that the amplitudes of these harmonics are decreasing with increasing harmonic number. Also note that the RF signal, the LO signal and the even mixer harmonics are not present in the output. Nice! This occurs because the Gilbert cell is a balanced mixer. However, in reality some (or all) of these signal components will be present in the output since we won’t have a perfectly balanced mixer.
NorCal 40A Mixers
There are three mixers in the NorCal 40A. You’ll install the:
1. RF Mixer in Prob. 28,
2. Product Detector in Prob. 29, and
3. Transmit Mixer in Prob. 30.
In addition, using the measured spectrum from the output of the Transmit Mixer shown in you will identify the various harmonics using
fmn = mfvfo ± nfto
There is a misprint of this equation in the text.
onversion Gain
Gain (or loss) of a mixer is characterized with a power gain expression similar to any amplifier

where G is called the conversion gain, P is the output IF power and P+ is the available power from the RF source. Here, however, the input and output frequencies of the two signals are different.

Discuss about MOSFET here
Discussion Board for MOSFET
You can discuss all your issues on MOSFET here
Thread / Thread Starter Last Post Replies Views
fourier transform
plz send me the notes for fourier transforms its very urgent.

Posted By :-
Aug 31, 12:14:03 PM 0 69204
Fourier Transform
Sir I want tutorial on Fourier Transform.........

Posted By :-
Jul 15, 3:24:49 PM 0 72731
fourier transform
sir i want tutorial on fourier transform

Posted By :-
Jul 11, 10:08:19 AM 0 68478
match filter
heloo sir ,i want a tutorial for match filter.plz send it as soon as possible it is very urgent.

Posted By :-
Jun 4, 2:25:18 AM 0 90144
Electic circuits
I want lecture notes for single phase ac & 3phase ac circuits

Posted By :-
May 21, 11:32:46 AM 1 116656
electro statics
what is the work done to move a charge? derive an expression for assembling a configuration of point charges

Posted By :-
May 12, 8:28:18 AM 0 68882
electromagnetic waves
i want lcr circiut teorems derivations

Posted By :-
May 12, 8:23:58 AM 0 67399
i want oscillator frequency derivations for all. plz let me know from where i can get that

Posted By :-
May 4, 5:21:01 PM 0 68920
Equivalent circuit Models
I've got a question in one of my revision papers,

Explain the advantages of representing a transistor by means of an equivalent circuit circuit model

The only thing i can remember is that you can take complex circuits and break them down into simpler circuits which are easier to understand,

Is this the only advantage or do you have any more?

Please help!

Posted By :-
Apr 12, 6:33:57 PM 2 149164
temperature Vs reverse satuation current
I want to know the variation of reverse saturation current with the increase in temperature for both germanium and silicon diodes

Posted By :-
Jan 28, 7:17:23 PM 0 74550
communication system
analog and digital communication system, fiber optic communication, telecommunication technique and application, mobile communication

Posted By :-
Dec 17, 10:38:27 AM 0 80862

Posted By :-
Dec 14, 3:29:22 PM 0 81289
coaching in ies in indore
what about coaching

Posted By :-
Dec 7, 5:27:52 PM 0 85514

To start your new thread you must login here.
New user signup at ExamCrazy.com Exam Crazy
To reply/post a comment you need to login, Use your user name and password to login if you are already registered else register here

(Members Login)

  About us | Privacy Policy | Terms and Conditions | Contact us | Email: support@Examcrazy.com  
Copyright © 2014 Extreme Testing House, India. All rights reserved.