Mixers

Updated August 18, 2006

Hey MITEQ or WJ, how about sponsoring this page and a few more? While we are building up our discussion of mixers, we recommend that you check out WJCommunications.com, they have a great selection of application notes on mixers, but don't forget to come back to us in the future! And go to our book recommendation page and order Stephen Maas' masterpiece on mixers!

Here's a clickable index to our growing material on mixers:

Mixer mess! (new for April 2006!)

Mixer waveforms

Single-balanced mixer

Double balanced mixer

Sub-harmonic mixers

Spur search calculator instruction

Baluns

Image reject mixers

I/Q mixer

What's a mixer? It's a device that performs the task of frequency conversion, by multiplying two signals (why do you think that the schematic symbol for a mixer is an "X"?) Mixers are needed in most microwave systems because the RF signal is way too high to process its information (for example, looking for a Doppler shift in an X-band radar application, you won't find many A/D converters than can handle 10 GHz!)

Schematic symbol for a mixer

A mixer can be as simple as one that uses a single diode, or it can get far more complicated for improved performance. Two broad categories of mixers commonly used in microwave applications are switching mixers and nonlinear mixers. Switching mixers include single-balanced and double-balanced mixers are the most prevalent and have the most predictable performance, but nonlinear mixers allow you to go to much higher frequencies (well into the millimeterwave spectrum). Even in switching mixers you still need a nonlinear device. The nonlinear device within a mixer is most often a Schottky diode, but can also be a FET or other transistor. PIN diodes are never used for mixers, they switch too slowly. (Thanks, Miles!)

Mixer ports

There are three ports on a mixer,the radio frequency (RF) port, the local oscillator port (LO), and the intermediate frequency port (IF).

The RF port is where the high frequency signal is applied that you want to downconvert it, or where the high-frequency signal is output in an upconverter.

The local oscillator (LO) port is where the "power" for the mixer is injected. In this case, the power that is applied is RF, not DC like it would be in an amplifier. The LO signal is the strongest signal, and is used to turn the diodes on and off in a switching mixer (which is nine out of ten mixers). The switching action effectively reverses the path of the RF to the IF.

The IF port is where the RF signal that was modified by the LO signal is passed, and its waveform is filtered to become the IF signal.

Active versus passive mixers

Even though a fair amount of LO power is used to operate a mixer, a mixer is considered a passive device unless it contains a means of amplifying the converted signal using DC power. One type of active mixer that you might have heard of is a Gilbert cell mixer, which we promise to describe another time!

Downconverters versus upconverters

You can use a mixer to convert a signal down in frequency (as in a receiver) or up in frequency (as in an transmitter or exciter) because it is a reciprocal device.

Sidebands

When two sine waves are beat against each other, you get both sum and difference frequencies. In its simplest form, the math behind a mixer is shown in the following trigonometry identity that you should have learned in college:

sin(₁t) x sin(₂t)=1/2{(cos[(₁-₂)t] - cos[(₁+₂)t]}

Thus a mixer multiplies two signals, which results in sum and difference frequencies. If your RF signal is 10 GHz and your LO is 9 GHz, you will get two output signals, one at 1 GHz and one at 19 GHz. In a receiver design you would discard the 19 GHz signal by means of a simple low-pass structure.

RF frequency versus image frequency

By similar analysis, there are two RF signals that will downconvert to the same IF frequency, which are known as the RF and the image frequencies. The image can cause you all kinds of havoc in a receiver system, for two reasons. First, it is possible that an interfering signal at the image frequency will be mistaken for the "true" signal, and may even saturate your receiver if it is strong enough. And second, noise at the image frequency will add directly to your signal-to-noise ratio, messing up your noise figure, even if you put a low-noise amplifier in front of the mixer. Two ways to get rid of the image and to remove it with a filter in front of the mixer, or to use an image rejection mixer.

Double sideband mixer versus image rejection mixers

Ordinarily a mixer will process both sidebands, and its up to you to eliminate the undesired sideband, using a preselector filter. A mixer that processes both sidebands is called a double-sideband mixer, an image rejection mixer is called a single-sideband mixer.

High side mixer versus low-side mixer (HSM/LSM)

In a single-sideband receiver you can process either one of the sidebands. A high-side mixer is one in which the LO is higher in frequency than the RF, a low-side mixer is one in which the LO is lower in frequency.

Conversion loss/gain

This is the difference in amplitude of the available RF signal to the IF signal output (downconverter), or from the IF signal to the RF signal (upconverter). For passive mixers, the conversion gain magnitude is always less than unity, perhaps -5 to -10 dB.

Speaking of conversion loss data in dB, we know that many engineers get all twisted when someone refers to something with loss as having "minus something dB loss", when of course it must have positive loss if it is passive, because the minus sign is implicit in the word loss. People who get hung up on this have underwear that is way too tight. In our opinion, loss can be expressed with our without the minus sign, unless you are submitting an IEEE paper, and then you might want to get it right if you want it accepted.

Mixer isolation measurements

In a perfect mixer, the RF and LO signals would not be present at the IF port, and the LO would not be present at the RF port. There are three important isolations to consider:

RF to IF

LO to IF

LO to RF

The LO in particular presents a problem because it is usually a much stronger signal that the other two. The problem with LO (or RF) at the IF terminal is that these signals may cause other spurious products later in the chain, and perhaps saturate the IF amp if they are strong enough. The problem with LO at the RF port is that it can cause your receiver to radiate RF energy at the antenna port and get you in trouble with the FCC, or worse, get your fighter aircraft spotted by a surface-to-air missile.

Spurs

The word "spur" is an abbreviation for "spurious signal". These are unwanted products that can creep into your IF bandwidth unless you know what you are doing when you come up with your system's frequency plan, and know the limitations of the type of mixer that you are planning on using. Any nonlinear device, when presented with two or more input frequencies, will output not only the input frequencies but harmonics and intermodulation products of the input frequencies as well. The expression normally used for the mixing products is:

F_IF = M * F_RF + N * F_LO

Basically, there are in infinite number of possibilities that are due to multiples RF and LO frequencies. We have a separate page that covers this topic.

The "pump"

The LO is sometimes called the "pump" signal. Subharmonic mixers are said to be subharmonically pumped. This probably is somehow related to the English referring to tubes as "valves", they seem to be fixated on plumbing references...

Some mixer rules of thumb:

The one dB compression point of a mixer is typically 6 dB less than the specified LO power.

Mixer noise figure is roughly equal to the magnitude of its conversion loss, or just a little bit less. For example, a mixer with -6 dB conversion gain might have 5.5 dB noise figure.

You should measure the return loss of a mixer's three ports at the specified LO drive level, or you will see bad results because the diodes won't be turned on. This means that you need to measure the LO return loss at the proper power, which is not usually possible using a network analyzer.