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Microwave receivers
Updated July 8,
2011
Click
here to go to our page on receiver blocking (new for July
2011!)
Click
here to go to our page on distortion measurements
Click
here to go to our page on noise figure
Click
here to go to our page on low noise amplifiers (LNAs)
Click
here to go to our page on limiters
Click
here to go to our page on mixers
Click
here to go to our page on cascade analysis
What's a microwave
receiver? It is used to amplify the weak signal that is gathered
by the antenna in a radar, radio or other communications system
or sensor. The most common receiver architecture by far is the superheterodyne.
But before you design a receiver, learn to spell the word. So many
microwave engineers spell it "reciever" it ain't funny!
Here's a clickable
index to our growing content on microwave receivers:
Receiver
terminology
Selected
receiver topics
Homodyne
receivers (separate page)
Superheterodyne
receivers (separate page)
Radiometric
receivers (separate page)
Dual-channel
receivers (separate page, new for July 2008!)
Dynamic
range (new for September 2010!)
Receiver
terminology
Baseband frequency
the baseband is the frequency at which the information you want
to process is. This is almost never a true microwave frequency,
it could be an audio frequency, as in a radio. Being a microwave
engineer, we almost never have to get our hands dirty at the baseband
frequency, that is some other Dude's problem!
RF, IF and LO frequencies
When a receiver uses a mixer (as in a superheterodyne
receiver) , we refer to the input frequency as the RF frequency.
The system must provide a signal to mix down the RF, this is called
the local oscillator (because it is local to the system). The resulting
lower frequency is called the intermediate frequency IF), because
it is somewhere between the RF frequency and the baseband frequency.
Preselector filter
A preselector filter is used to keep stray radiation from saturating
a receiver. For example, you don't want your cell phone to pick
up air-traffic control radar. Quite often, the antenna element can
act as the preselector filter, because it might have a narrow-band
response.
Receiver channels
The term "channel" has dual meanings. Most often a receiver
channel refers to a particular frequency band. However, there are
applications where two receiver chains are required, and they are
also known as receiver channels. For example, two receivers are
needed to receive two antenna polarizations simultaneously. Two,
three or even four separate receivers could be used in a monopulse
radar system.
Amplitude and phase matching
versus tracking
In a multi-channel receiver (more than one receiver), it is quite
often important for the channels to match and track each other over
frequency. We'll add a separate page on this topic some day, and
a spreadsheet. For now we'll just define the two terms:
Amplitude and phase matching
means that the relative magnitude and phase of signals that pass
through the two paths must be within a certain amplitude and phase
of each other. Any little difference between the two receivers
will cause a matching error, for example, if the physical lengths
of some 50-ohm transmission lines or cables within a receiver
are different, the matching will suffer.
Amplitude and phase tracking
means that you are allowed to add or subtract a fixed electrical
delay from one receiver chain before you compare their amplitude
and phases. The trick is to calculate the electrical delay that
minimizes the tracking error. Once again, this is a good
job for Excel!
Matching is usually harder to
meet than tracking.
Tunable bandwidth versus instantaneous
bandwidth
Instantaneous bandwidth is what you get with a receiver when
you keep the LO at a fixed frequency, and sweep the input frequency
to measure the response. The resulting bandwidth is a function of
the frequency responses of everything in the chain: the preselector
filter, low noise amp, the mixer, the IF amp and any filters that
you have. Their instantaneous bandwidth has a direct effect on the
minimum detectable signal.
Tunable bandwidth implies
that you are allowed to change the frequency of the LO to track
the RF frequency. The bandwidth in this case is only a function
of the preselector filter, the LNA and the mixer. Tunable bandwidth
is often many times greater than instantaneous bandwidth.
Gain
The gain of the receive is the ratio of input signal power to output
signal power. Almost always, the gain is expresses in decibels.
Gain magnitude and phase of a
device that does not contain frequency conversion is straightforward,
and can be done on a network analyzer. Once you add frequency conversion
the fun really starts! More on this later....
Noise figure
Noise figure of a receiver is a measure
of how much the receiver degrades the ratio of signal to noise of
the incoming signal. It is related to the minimum detectable signal.
One dB compression point
This is the power level where the gain of the receiver is reduced
by one dB due to compression. This is regarded as the upper limit
on receiver dynamic range. Calculating the one dB compression point
of a chain can be a complicated assignment; it can be approximated
using our cascade analysis tool.
Linearity
A receiver is said to be operating linearly if a one dB increase
in input signal power results in a one dB increase in IF output
signal strength. See our page on dynamic
range.
Minimum
detectable signal
The MDS is calculated, not measured. It is a function of receiver
noise figure and instantaneous bandwidth. See our page on dynamic
range.
Intermodulation distortion
Another topic in itself! Click here
to learn more about distortion measurements!
Receiver protection
Take a look at our page on limiters for
an introduction to this topic.
Dynamic
range
See our new page on dynamic
range.
Selected
receiver topics
Input VSWR versus noise figure
and sensitivity
The VSWR of a receiver does not affect its
noise figure directly. However, the mismatch loss caused by a poorly-matched
receiver will affect the minimum detectable signal, so the input VSWR
of a receiver is an important characteristic if you are designing
a sensitive system.
Another property of good receiver
VSWR is that it may help in designing a "stealthy" system,
by creating a good termination to the antenna, it is less likely
to re-radiate impinging radar signals.
How does a noisy LO affect
a receiver?
From a true subject matter expert: "The receiver is a circuit
with input and output, so it has a noise figure. The LO generates
a signal and a signal does not have a noise figure (by definition).
If the LO signal has a high AM and/or FM noise, it could degrade
the receiver noise figure because, far from the carrier, the AM
and FM noise originate from thermal noise. Remember that the effect
of LO AM noise is reduced by the balance of the balanced mixer.
The FM noise shows up in all of its glory."
Gain compression due to nearby
strong signals
When an interfering signal makes its way into a receiver, and is
able to saturate the chain to a high power level, the gain of the
receiver folds up like a cheap umbrella. This is often the answer
to the question "where is the missing 20 dB gain?"
Temperature effects
Coming soon!
EMI effects
Coming soon!
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