PEMP VSD538
Session 5
Mixer Circuits
Session Speaker: D. Varun
© M. S. Ramaiah School of Advanced Studies, Bangalore
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PEMP VSD538
Session Objectives • To expound on MW and RF Mixer architectures • To design and develop a Single Balanced / Double Balanced Diode Mixer & Gilbert Cell Mixer • To illustrate various mixer performance parameters for the developed design • To expound on various Oscillator architectures for MW and RF frequencies • To design and develop Hartley, Colpitts and Differential Oscillators • To illustrate Oscillator phase noise parameters and Leesons Equation 2
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Session Topics • • • • •
Introduction to RF/MW mixers Types of Mixers Unbalanced Mixer Balanced Mixer Image Reject Architectures
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PEMP VSD538
RF Mixers Primer • All RF transceiver systems require a fixed range of electromagnetic frequencies at any given time • Baseband frequency at a transmitter or receiver is encoding of data to be transmitted/received into a signal at a frequency close to zero • The process wherein signals are multiplied together and new frequencies (IF) are generated is called RF or radio frequency mixing • Up-conversion is the process of translating lower frequency (IF) to a higher frequency (RF) and downconversion is the process of translating RF signal to a lower frequency (IF) thus enabling data extraction 4
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Introduction to Mixer • Mixers perform frequency translation by multiplying two signals • Mixers are non-linear devices used in systems to translate (multiply) one frequency to another. • All mixer types work on the principle that a large Local Oscillator (LO) RF drive will cause switching/modulating the incoming Radio Frequency (RF) to the Intermediate Frequency (IF). • The multiplication process begins by inputting two signals:
• The resulting multiplied signal will be:
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PEMP VSD538
Introduction to Mixer • This can be multiplied out thus:
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PEMP VSD538
Mixer Definitions • Conversion Gain: This is the ratio (in dB) between the IF signal (usually the difference frequency between the RF and LO signals) and the RF signal • Noise Figure: Noise figure is defined as the ratio of SNR at the IF port to the SNR of the RF port. • Single sideband (SSB): This assumes the only noise from the signal ω1 and not the image frequency ω11, this would be the case if a band-pass filter was added in front of the mixer eg. – RF = 1694 MHz, LO = 1557MHz to give an IF of 137MHz. 7
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Introduction to RF Mixers
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PEMP VSD538
Mixer Output Example
• When two or more signals of differing frequencies are input to a nonlinear system, the basic nature of nonlinear system produces a multiplicity of output frequencies that appear at frequencies nominally equal to the sum and difference in frequencies of the original signals 9
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PEMP VSD538
Types of Mixers • Mixers can be classified into two broad categories depending on the type of nonlinear component used viz., diode, MOS,HEMT etc.. • Active and Passive mixers are the classification based on nonlinear components • Mixer classification of mixers can also be based on type of input applied • Unbalanced and Balanced are the classes based on input methods
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PEMP VSD538
Passive & Active Mixers • ON/OFF switch is the most common form of a RF mixer • When ON, the RF mixer allows the signal to pass through, and when OFF it does not • Such a mixer can be realized using any passive device like diode provided the local oscillator input is high enough to switch the device ON • In a passive mixer no signal propagates through when the device is in OFF state • In a passive mixer the conversion loss will be 3 db • In reality conversion losses of 6 dB to 9 dB are commonly encountered • Active mixers have a significant advantage over passive mixers 11
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Passive & Active Mixers • Passive resistive diode mixers operate over a wide band and offer conversion loss to the system • Varactors operate over a narrow band and offers low conversion gain • These factors make the passive mixers unreliable for operation under certain conditions • Active multiplier mixers are used to abate the losses of passive mixers • An active mixer operates over broad band widths and provides conversion gain • The other major advantages of high-frequency FET or BJT mixers are: multipliers devour little dc power; output is more devoid of spurs and dissipates little heat 12
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Unbalanced Mixer • A mixer functioning in any frequency requires isolation between the ports to avoid any intermodulation distortion • The mixer becomes unbalanced when designed without any port isolation • Unbalanced mixer can be designed using a single diode • The local oscillator and RF signal input appear along with the intermodulation products at the mixer output
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PEMP VSD538
Unbalanced Architecture
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PEMP VSD538
Balanced Mixer • Grossly divided into two classes: Singly-Balanced Mixers (SBM) Doubly-Balanced Mixers (DBM) • Singly-Balanced mixers use two devices, and are usually realized as two single device mixers connected via a 180-degree or 90-degree hybrid • Double balanced mixers usually consist of four untuned devices interconnected by multiple hybrids, transformers or baluns
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PEMP VSD538
Merits and Demerits • The advantages of balanced mixers over single-device mixers are: Rejection of spurious responses and intermodulation products Better LO-to-RF, RF-to-IF and LO-to-IF isolation Rejection of AM noise in the LO • The disadvantage of balanced mixers is their greater LO power requirements • Balanced mixers often used to separate the RF and LO ports when their frequency overlaps and filtering is impossible. In practice a perfect doubly balanced mixer give 10- 30dB isolation without any filtering 16
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Mixer Definitions • Double sideband (DSB): In DSB both sidebands are available thus it has twice as much power available at the IF port compared to the SSB signal. As a result, it’s conversion loss is 3dB less than that of an SSB signal, as shown:
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PEMP VSD538
Mixer Definitions • Isolation: These parameters define how much signal leakage will occur between pairs of ports. – ie RF to LO, LO to IF and RF to IF. So if for example RF to IF isolation was specified at 35dB this means that the RF at the IF port will be 35dB lower than the RF applied to RF port.
• Linearity – 1dB Compression point : Like other non-resistive networks, a mixer is amplitude-nonlinear above a certain input level resulting in a gain compression 18
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PEMP VSD538
Gain Compression Characteristic
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1dB Compression point • Above this point the If fails to track the RF input power level normally a 1dB rise in RF power will result in a 1dB rise in the IF power level. The 1dB compression point is measured by plotting incident RF power against IF power • 1dB compression point the input signal level at which the output of the mixer has fallen 1dB below the expected output level. • For typical double balanced mixers this figure is ~ 6dB below the LO power level. 20
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PEMP VSD538
Inter modulation (IM3) • Inter modulation: When two signals with different frequencies are applied to a nonlinear system, the output in general exhibits some components that are not harmonics of the input frequencies, called inter modulation (IM) • It is measured by applying two closely spaced input tones at frequencies F1 and F2. • Third order products from the mixing of these tones with the LO (at frequency FLO) occur at frequencies given by: – (2F1 ± F2) ± FLO and (2F2 ± F1) ± FLO
• In the case of the mixer, the third order products of most interest are (2F1-F2)- FLO and (2F2-F1)-FLO as they fall in, or close to the IF band. 21
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PEMP VSD538
IM3 intercept point • As a rule of thumb the IM3 intercept point is approximately 10dB above the 1dB compression point.
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PEMP VSD538
IM3 intercept point • For mixers the measurement is referred to the input (IP3,in) and is given by
• Where IMR = Inter modulation ratio (The difference in dB between the desired output and spurious signal) and n = the IM order • Typically, for double balanced mixers IM3,in is ~ 14dB greater than the single tone 1dB compression point and ~ 8dB greater than the LO power. 23
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PEMP VSD538
Balanced Architecture
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PEMP VSD538
Image Rejection
Image Frequency: Frequencies with high PSD and close to RF such that the IF due to RF generated is overshadowed by the IF due to image or frequencies that are translated into the same IF band 25
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PEMP VSD538
Image Reject Architectures • A single stage mixer is susceptible to image distortions corrupting the IF signal and making the detection of signal difficult • The concept of image rejection was realized out of dire necessity • Traditional mixers used a SAW bandpass filter for image frequency rejection • Image Rejection Mixers are useful particularly when the desired and image are very close (low IF frequencies) and a narrow-band channel pre-selector (SAW) renders impracticable 26
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Image Reject Mixer
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PEMP VSD538
Up / Down Conversion
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PEMP VSD538
Mixer Characteristics • Conversion Gain or Loss of the RF Mixer is dependent by the type of the mixer (active or passive), but is also dependent by the load of the input RF circuit as well the output impedance at the RF port • The typical conversion gain of an active Mixer is approximately +10dB when the conversion loss of a typical diode mixer is approximately -6dB • The Conversion Gain or Loss of the RF Mixer measured in dB is given by: Conversion[dB] = Output IF power delivered to the load[dBm] – Available RF input signal power[dBm] 29
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Mixer Characteristics • Input Intercept Point (IIP3) is the RF input power at which the output power levels of the unwanted intermodulation products and the desired IF output would be equal • From an RF System point of view, a Mixer linearity is more critical than Noise Figure • The Third-Order intercept point (IP3) in a Mixer is defined by the extrapolated intersection of the primary IF response with the two-tone third-order intermodulation IF product that results when two RF signals are applied to the RF port of the Mixer 30
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
IP3 (Intercept Point 3)
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PEMP VSD538
Mixer Characteristics • Isolation is the amount of local oscillator power that leaks into either the IF or the RF ports • There are multiple types of isolation: LO-to-RF, LOto-IF and RF-to-IF isolation • Noise Figure is a measure of the noise added by the Mixer itself, noise as it gets converted to the IF output • In a mixer noise is replicated and translated by each harmonic of the LO that is referred to as Noise Folding 32
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Diode Mixer
Single-device Mixer using one diode is primarily a process of matching the pumped diode to the RF input and IF output, terminating the diode properly at LO harmonics and unwanted mixing frequencies (other than the RF and IF), and isolating the RF, LO, and IF ports Isolation, and in some cases the termination, can be provided by using filters, a balanced structure, or both The choice depends on the frequency range and the intended application. 33
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PEMP VSD538
Single Balanced Active Mixer
• This configuration provides gain • More noisy at higher frequencies • No isolation • Simple and used low end application
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PEMP VSD538
Diode Double Balanced Mixer • The local oscillator, LO, signal turns on first one arm (D3, D4), and then the other (D1, D2) within the diode ring • As the points where the LO signal enters the diode ring at the junction of D1 and D4 appear as a virtual earth to the RF signal, this means that the points where the RF signal enters are alternatively connected to ground as the diodes turn on and off 35
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PEMP VSD538
Balanced Mixer Gilbert Cell
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PEMP VSD538
Gilbert Cell • This Bipolar Double Balanced Gilbert Cell mixer is sometimes referred two as a four-quadrant multiplier because when signals are "multiplied" their frequencies are mixed • This mixer is realized using two differential transistor pairs that share a current source controlled by one of the input signals • Implementation involves the differential current sources are driven by the RF signal using a transformer as a balun • The LO drive is also applied using a transformer and the IF output is taken with a transformer 37
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Gilbert Cell • The impedance level of all ports is generally higher than 50 ohms, particularly at lower frequency • The baluns may also serve as impedance transformers • The term Zo is the port impedance and the terms Zif, Zrf and Zlo in the equations are factors that affect the port impedance up to the internal impedances of the mixer • This mixer exhibits conversion gain at frequencies where the device gain overcomes the loss associated with the mixing process • The differential pair transistors provide gain so the required LO drive level is as low as -20 dBm at low frequencies 38
© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Image Reject Mixer Topologies
Hartley Architecture
Weaver Architecture 39
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PEMP VSD538
Balun • For the purposes of the simulation we need to convert the differential inputs and outputs of the mixer to single ended source and load impedances. The device that achieves this balanced to un-balanced transformation is known as a ‘Balun’.
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PEMP VSD538
Test setup
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PEMP VSD538
Inputs to mixer • As the inputs and outputs are differential balun trans-formers have been added to convert to single ended inputs and outputs. • The 500-ohm load Term3 correctly terminates the mixer 500-ohm output impedance. • The RF frequency is set to 2500MHz (RF_freq), Local oscillator frequency to 2250MHz (LO_freq) , resulting in an IF frequency of 250MHz (IF_Freq).. • For correct switching of the LO transistors the variable vg needs to be set to 1V – running the simulation this gives Vgs across the switching 42 transistor of ~ 1V. © M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Simulation Results
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PEMP VSD538
Simulation Results
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Simulation Results
PEMP VSD538
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© M. S. Ramaiah School of Advanced Studies, Bangalore
PEMP VSD538
Simulation Results
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PEMP VSD538
Summary • RF/MW mixers and Oscillators are the important building blocks in any RF system • Mixers are classified in to Unbalanced Mixer & Balanced Mixer depending on the input phase • Image Reject Architectures has been described as class of mixers which can combat image frequencies
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PEMP VSD538
THANK YOU
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