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:
FIF = M * FRF
+ N * FLO
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...
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.
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