RF Mixer

by Sanjay Mishra

The need for RF Mixers

All RF transmitters and receivers work on a narrow range of electromagnetic frequencies at any given time. At a transmitter the data to be transmitted is encoded into a signal at a frequency close to zero, also known as baseband frequency. This signal is then shifted up to the frequency where the transmitter is allowed to transmit. This process is known as upconversion and is accomplished using RF Mixers. Similarly at the RF Receiver the received RF signal has to be converted to a signal at a lower frequency so that the transmitted data can be extracted. This process is known as downconversion. This down-conversion is also accomplished using mixers.

This process of frequency conversion is also called heterodyning.

Modern RF receivers may contain two (superheterodyne), one (Near Zero Intermediate Frequency) or zero physical mixers. In case of direct sampling of RF Signal the mixing process is performed in software. A secondary mixing process may also be performed in software in case of a Near Zero Intermediate Frequency architecture applied in RF receiver.

Theory of RF Mixers

Consider the following result from trignometry -

A cos(xt) * B cos(yt) = (A+B)/2 cos( (x+y) t) + (A+B)/2 cos( (x-y) t)

if a sinusoid waveform with a frequency of x, is multiplied with another sinusoid waveform of frequency y, a resulting waveform is produced that is a sum of two sinusoidal waveforms one with a frequency of (x+y) and the other sinusoid with a frequency of (x-y) .

If instead of a sine wave we use a square wave of frequency Flo and a sinusoid input of frequency Fin we will still get the (Flo + Fin) and (Flo - Fin) together with a bunch of other frequencies. This is because a square wave can be thought of as the sum of a sine wave at its primary frequency and a increasing smaller sinusoidal waves of decreasing frequency known as harmonics. Each of these harmonics will combine with the input sinusoid to generate sum and difference sinusoid waveforms.

In a mixer we choose only one of the resulting sinusoid waveforms by filtering the output with a low pass or bandpass filter. Usually either (Flo + Fin) frequency or (Flo - Fin) frequency is chosen. If (Flo+Fin) is the chosen frequency, then the (Flo - Fin) frequency is the known as the image frequency and must be suppressed. Conversely if (Flo+Fin) is the chosen frequency, then the (Flo - Fin) frequency is the image frequency and must be suppressed

Design of RF Mixers

A mixer with no active gain elements is known as a passive mixer . Using such a mixer results in a conversion loss - some of the input power is lost. This conversion loss is the ratio of output power at converted frequency to input power at original frequency.

An active mixer provides gain and using in provides a conversion gain.

• Both active and passive mixers can be further classified into
  
• Unbalanced Mixers
  
• Single Balanced Mixers
• Double Balanced Mixers
• Triple balanced Mixer also known as double double balanced mixers
• Image Rejection Mixers

Passive RF Mixers

The simplest RF mixer is an ON/OFF switch. During the on state the RF mixer allows the signal to go through and during the off state it does not. Such a mixer can be implemented using any device (Diode, Field Effect Transistor, Bipolar Junction Transistor) as long as the local oscillator input is high enough to turn the device ON.

In this simple mixer, no signal goes through when the device is off. Its conversion loss therefore must be 3 db. In reality conversion losses from 6 db to 9 db are common.

In practice Schottky diodes are used so that the time spent in switching the diode from OFF state to ON state is minimal. The strong local oscillator signal (LO) is used to drive the diodes. In one arrangement when the LO signal is positive the diode is driven to saturation and the input signal passes through. When the LO signal is negative the diode is off and no input signal passes through. This effectively modulates the incomig signal with the LO signal.

It is cheaper and easier to purchase Double Balanced Mixers from manufacturers who are better able to match the diodes.

In an Image Reject Mixer, the sinusoid wave at the image frequency is removed by phase canceling it. Image Reject Mixers make possible Near Zero Intermediate Frequency (NZIF) receiver architectures which are increasingly becoming common. Image suppression is not usually better than 35 dB, hower, so any strong signlas at image frequency will casue in-channel interference.

Active RF Mixers

Active mixers supply a conversion gain instead of a conversion loss. In addition they provide better isolation between the 3 ports, require less LO power and the output is more spur free. Gilbert cell is a common active mixer architecture.

However active mixers have a high noise figure and not very good linearity over different input signal strengths. These drawbacks usually limit the active mixer's role.

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Custom Design

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