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Microwave Receivers

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!

 

Author : Unknown Editor

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