Updated May 20,
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A superheterodyne receiver contains a combination of amplification
with frequency mixing, and is by far the most popular architecture
for a microwave receiver.
To heterodyne means to
mix two signals of different frequencies together, resulting in
a "beat" frequency. Actually, two signals are always created,
the sum frequency and the difference frequency. These are referred
to as the two sidebands. The sum frequency is the upper sideband,
and the difference frequency is the difference sideband. In most
microwave receivers, the upper sideband is ignored.
The word "superheterodyne"
is often hyphenated to "super-heterodyne". A common contraction
of the word is simply "superhet".
Here's a clickable index to our
super material on superhet!
of the superheterodyne receiver
components in a superhet receiver
topics on superhet receivers
of the superheterodyne receiver
The superheterodyne receiver
is still the most popular microwave receiver, and it was invented
during and directly after the Great war and patented in 1918. Edwin Armstrong
(see him in our Hall of Fame)
was truly one of the great minds of the twentieth century. Unfortunately,
he had a bad habit of inventing stuff independently of the corporate
anti-Christ of the time, which was RCA (Radio Corporation of America);
RCA and others pretty much ruined Armstrong's life. From this perspective,
nothing's changed, big companies ruin many engineers' lives today,
especially in arguments over inventions. As an engineer you'd best
remember your place at the bottom of the corporate food-chain or
you'll be living in a cardboard house behind the Circle K pretty
soon. Don't ever think your services are as valuable as the work
of more deserving corporate lawyers! But we digress....
The word "heterodyne" was coined by Reginald Aubrey Fessenden, also in our Hall of Fame. Heterodyning is the same as mixing, we offer a little history on the topic here. Although Fessenden was able to receive signals by mixing them, there is no evidence that he used a detector to sort out the baseband; therefore Fessenden only managed to put together just one of the five pieces of Armstrong's superhet receiver (RF amp, mixer, local oscillator, detector and audio amp). See "Man of Fidelity: Edwin Howard Armstrong" by Lawrence Lessing to corrobrate that fact.
Prior to the superhet receiver,
radio listeners had to constantly play with a set of knobs on a
radio to keep locked on to radio programs of the day. Prior attempts
at amplifying radio signals included the regenerative and super-regenerative
receivers (both invented by Armstrong, but stolen by greedy RCA),
which at least offered some gain to boost far-away signals, but
were prone to drift. The superhet solved this problem for all time
by using frequency conversion that was controlled by a local oscillator.
By tuning the LO frequency, the RF frequency that you received was
thereby tuned. It turns out it is much easier to control the LO
frequency and filter out unwanted channels at IF, rather than try
to filter out unwanted channels at the RF input. This characteristic
is known as receiver selectivity.
To learn more about the history
of radio, go to our book page and click
on the link to Empire of the Air.
components of a superheterodyne receiver
The block diagram below shows
typical components of a superhet microwave receiver. The pre-LNA
components all have a direct affect on noise figure, so low-loss
is a key characteristic of these parts, because loss adds directly
to noise figure. Often, waveguide components are used in front of
the LNA because it offers the lowest loss available. Stuff after
the LNA doesn't have such a drastic affect, but everything must
be taken into account in maximizing dynamic range.
This component restricts the frequency band that is permitted to
enter the receiver.
Limiter (receiver protector)
This component is what protects the LNA if a stray high-power signal
makes its way into the RF input. This function can be accomplished
by a passive or an active limiter. An active limiter, also known
as a blanking switch, must be commanded somehow to protect the LNA.
This is often the case of a co-located transmit/receive system such
as a radar. A passive limiter doesn't need to be commanded, it automatically
reduces the signal whenever it is hit with enough power.
The power that leaks out of the
receiver protection circuit must be specified and measured under
both pulsed conditions (spike leakage) and CW condition (flat leakage).
A pre-LNA switchable attenuator can be used to increase the dynamic
range. When signals are strong enough to saturate the LNA, the attenuator
can be switched in to reduce the signal strength to the LNA.
Low noise amplifier
Nothing is more critical to receiver performance than the LNA.
This is the active component that increases the power of the received
signal (it produces gain). The power handling of the LNA
means how much power can it be hit with before it is permanently
damaged. Typical MMIC LNAs can handle from 10 milliwatts to half
a watt. The damage to an LNA is not necessarily catastrophic, only
a small change in gain can result from overdriving it. The noise
figure usually does not degrade by much unless the gain of the device
is reduced by several dB. Designing an LNA for high-power use is
Image rejection filter
Used to reduce image noise foldover.
The device that converts the incoming RF frequency to intermediate
frequency is called a mixer.
The cleanup IF filter's purpose is to remove unwanted high frequency
signals such as the RF and LO, which could cause additional distortion
products when they pass through the IF amplifier. It is usually
a low-pass filter (LPF), and could use low-cost lumped components
(inductors and capacitors).
You should always design a receiver with extra gain, then you can
used attenuators here and there to
"pad" out poor VSWRs. Places that
are notorious for VSWR problems is the mixer/filter interfaces.
This is where gain can be added to the receiver without high cost.
At one GHz, gain can be purchased for less than 10 cents per decibel!
The local oscillator signal must be stable (time-invariant) and
maintained at a fixed or minimum power for the mixer to work properly.
Often an amplifier stage is put in the LO path, and kept in compression
to reduce variation in LO power coming from the exciter.
Advantages of superheterodyne
The advantages of superheterodyne
receiver are many. An obvious advantage is that by reducing to lower
frequency, lower frequency components can be used, and in general,
cost is proportional to frequency. RF gain at 40 GHz is expensive,
IF gain at 1 GHz is cheap as dirt.
The second advantage is in the
superior sensitivity that we almost take for granted. Filtering
out unwanted signals at IF is a much easier job than filtering them
out at RF, because the desired bandwidth is much higher after the
signal is mixed down.
Further advantage in that many
components can be designed for a fixed frequency (and even shared
between different receiver designs), which is easier and cheaper
than designing wideband components.
topics on superheterdyne receivers
Single down-conversion versus
A receiver that has only one frequency conversion device (mixer)
is said to be of single-downconversion variety. Quite often a high
performance receiver has two downconversion steps. In this case
we would label the frequencies RF, LO1, LO2, IF1, and IF2. There
are advantages to the second downconversion approach. Having a higher
IF1 frequency splits the RF and image frequencies far apart, which
makes an easier task of preventing image noise foldover. For example,
if your RF frequency was 10 GHz, and your IF was 3 MHz, in order
to filter out the image, you'd need a filter that passes 10 GHz
yet rejects 10.003 GHz. This is for all intents impossible. But
if your IF frequency was 3 GHz, it is quite easy to filter out incoming
noise at 13 GHz.
There is a solution to image
noise no matter how close the image is to the RF, that is an image
A mixer is used in a receiver to create the IF frequency. Because
a mixer creates both sum and difference frequencies,
The sidebands of a mixed signal occur at the sum and difference
of the RF and LO signals. Thus a 12 GHz RF signal mixed with a 10
GHz LO will produce a lower sideband of 2 GHz and an upper sideband
of 22 GHz.
Mixer spurs (MxN products)
This topic is coming soon!
A mixer is used in a superhet receiver to create the IF frequency.
Because a mixer creates both sum and difference frequencies, by
corollary, there are two RF frequencies that will produce the exact
same IF frequency. The unwanted signal frequency is called the image
frequency. An example:
RF is 12 GHz
LO is 10 GHz
Upper beat frequency is 22
Lower beat frequency is 2 GHz
Image frequency is 8 GHz
The RF passes through the mixer,
and two possible outputs are (sum and difference) are 22 GHz and
2 GHz. We don't have to tell you that the IF is 2 GHz, do we? The
22 GHz is the upper sideband, and is discarded by filtering the
IF (usually a low pass filter is all that is required). The image
frequency that would produce a 2 GHz IF is 8 GHz. What's up with
this? This is because an 8 GHz RF signal will mix to 18 GHz, and
minus 2 GHz. Guess what? the minus sign is unimportant, there is
no such thing as a negative frequency!
The two sidebands of a receiver can contribute to its signal-to
noise ratio. It's time for a Microwaves101 rule of thumb!
Twenty dB of image rejection is about all you need before you can
neglect image noise. Worst case, image noise foldover can degrade
receiver noise figure by 3 dB.
Image noise foldover can be prevented
in two ways: use an image filter, or use an image-rejection mixer.
The image rejection filter is located between the LNA and the mixer.
If you place it in front of the LNA, it doesn't do the job.