Narrow vs. Wide IF Bandwidth - How it Impacts Your VNA Measurements + Wave Winners

Spoiler: IF Bandwidth affects measurement accuracy and speed
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Whenever you read about improving measurement accuracy and dynamic range, you’ll see a lot about IF bandwidth. But what exactly is IF bandwidth, and how does it affect your measurements?
The textbook definition of IF bandwidth is the span of the center frequency of the intermediate frequency filter.
Okay.
So what does that actually mean? First, we have to talk about mixers.
When you put two signals at different frequencies, f1 and f2, into a mixer, the output will be two more signals: one at a frequency of f1+f2 and one at a frequency of f1-f2.
Here’s where it gets interesting. You can use a local oscillator as one of your mixer inputs to tune input frequencies to your desired output frequency. This output frequency is called the intermediate frequency, or IF.
Those of you with a radio background are probably saying “this is just your standard superheterodyne receiver”. And you’re right. Network analyzer receivers operate in a very similar way to radio receivers. Modern network analyzers have very wide frequency ranges, like 100 kHz to 53 GHz on our new midrange network analyzers. It’s impossible for the instrument to actually analyze all of those frequencies, so the receivers convert the input, piece by piece, into the instrument’s intermediate frequency using the mixer.
So on the block diagram we have a bandpass filter at the output of the mixer. This is our intermediate frequency, or IF, filter. When you change the IF bandwidth of your instrument, you’re changing the bandwidth of this filter. A wider IF bandwidth means bigger portions of your measurement sweep can be converted to the intermediate frequency, meaning your measurement is faster, but there is a tradeoff in accuracy. A narrow IF bandwidth means only small parts of your measurement are converted to the intermediate frequency. Analyzing smaller portions of the signal reduces noise and improves your dynamic range, at the cost of measurement speed.
Let’s look at a visual example. When it comes to image processing, IF bandwidth is analogous to pixel size. If the text is a signal, you can think of each pixel as a piece of the signal that has been converted to the intermediate frequency and displayed on the page. In this one, the pixels are very large. This is like having a wide IF bandwidth. This image is quickly processed on a computer, but may lack some of the detail we need for deeper analysis.
This looks like our classic “hello world” message, but let’s decrease the pixel size and take a closer look.
At a higher resolution, we can see we actually have a typo in this message that we couldn’t see with the large pixels. When you decrease the IF bandwidth, you increase the resolution of your measurement. Your application will determine what degree of accuracy you need.
In summary, IF bandwidth determines the resolution level of your measurement. A wide IF bandwidth gives you faster, low resolution measurements. A narrow IF bandwidth gives you slower, high resolution measurements.

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