Playing with a signal demodulator

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Last week I got a Christmas present! Actually, it’s not a present, it’s a tool for work. But it is a brand new spectrum analyzer! Actually, it’s not brand new, it’s at least 15 years old. The basement guy found it in the company’s basement and brought it to my office to see if I could use it for something and stop distracting coworkers for a while.

The name is FSIQ 3, made by Rhode & Schwarz. People usually use it to analyze spectra of different signals but it also has a possibility to be used as a vector analyzer and as a demodulator. First one – yeah whatever, but the second one sounds intriguing: it demodulates a decent set of some standard modulation signals and shows off I and Q in the time domain. It also shows constellations and binary data! How cool is that?

This is something I haven’t seen before so I needed to check it out. I found some signal generator (R&S SMIQ 03B) and plugged in some coax cable. I’ve set some modulation parameters as:

• carrier frequency: 1 GHz
• simbol rate: 100 k/s
• squared cosine filter/0.35
• IF bandwidth: 10 MHz

and after playing a while with the buttons got some nice images:

PSK

BPSK, or the most simple digital phase modulation, is when the carrier turns upside down on every bit change. There is only and I component and it has two values, 1 and -1. Therefore, we see two clusters on I axis in a constellation diagram:

QPSK: brings in the Q signal with two discrete values (1 and -1) and the carrier will change the phase for 90° on every bit change. Advantage: you have four symbols now and each consists of 2 bits.

8-PSK Now, do double the data rate, divide phase in I-Q plane again…and you get the idea. Result will be the 8-PSK.

QAM

More interesting and vastly more used in communications is QAM, where alongside the phase you can modulate the amplitude too. In how many ways? Only the sky is the limit, but for this machine the limit is 16 symbols. Sorted in a nice looking grid.

FM Demodulation

This machine also supports the analog demodulation. And there are some speakers too. So, natural thing to see is if it can reproduce FM radio sound. I tucked a simple wire in an RF port (don’t tell it to any of my colleagues) and turned it on. The result pretty awesome.