Unveiling the secrets of the FM radio using PROMAX analyzers

September 24, 2021.

Digital radio is available nowadays in many countries, however the analog FM radio is still among us... and for years to come. So, how does FM stereo work? What can we measure on those signals? Let's have a better understanding of all those topics using the advanced FM analyzer from PROMAX TV spectrum analyzers.

What's inside an FM channel?

An FM channel is, in fact, a multiplex (MPX) containing these subcarriers:

Subcarriers of an FM channel

  • L+R subcarrier: Sum of the left and right channels. This is what we listen in FM mono receivers.
  • Stereo pilot: Its presence denotes a stereo channel and also allows to regenerate the carriers of the two following subcarriers.
  • L-R subcarrier: Subtraction of the left minus the right channels. By combining this information with the L+R subcarrier, the stereo FM receivers can restore both audio channels.
  • RDS subcarrier: Digital information on the channel for compatible receivers.
  • SCA services: Reserved to encode information in many ways for professional or paid applications.

How to display the FM subcarriers?

The FM multiplex (MPX) has a 100 kHz bandwidth. We can display the location of the carriers using the MPX Spectrum function from PROMAX field strength meters:

Subcarriers of an FM channel

  1. L+R subcarrier.
  2. Stereo pilot.
  3. L-R subcarrier.
  4. RDS subcarrier.
  5. SCA services.


RANGER Neo analyzers

In this article we are using the advanced FM analysis option of the RANGER Neo analyzers. PROMAX analyzers, with model-dependant scalable features, are a equipment range with models both for installers and for RF spectrum authorities.


Advanced measurements on stereo FM radio signals

The following screen shows at a glance the left and right channel levels and the level of each of the subcarriers mentioned (L+R and L-R):

Measurements of the FM radio subcarriers deviation
Measurements of the FM radio subcarriers deviation

The lower part of the screen displays the basic measurement of the signal/carrier level and the frequency deviations of all the pilots that constitute the FM multiplex (MPX).

MPX pilots frequency deviations

What is the frequency deviation and why do we need to measure it?

FM stands for Frequency Modulation. This means the information is encoded altering the signal frequency: Increasing and decreasing the carrier frequency within certain boundaries. This alteration is the frequency deviation.

An excess of deviation (this is to say, a carrier whose frequency is too much altered) may produce interference in two adjacent channels. We can avoid this problem by requiring by law certain limits of deviation the broadcasters must respect. Therefore, the RF authorities need to measure the excess of deviation and having an analyzer capable of measuring the FM deviation for all carriers is a must.

Distribution of the MPX deviation along the entire FM frequency
The analyzer can display histograms according to the TU-R SM.1268-2 and the TU-R SM.1268-4 norms.

In the previous screen we can see a histogram as an example of the ITU-R SM.1268-2 norm: The yellow color depicts how many samples of the total have reached a certain frequency deviation. In green (cumulative), how many have exceeded a given deviation.

Using the analyzer as a monitoring station of the FM deviation

The webControl feature of PROMAX analyzers allows not only to remotely control the devices but also to monitor the signals and to send e-mail alarms based in user-defined quality thresholds.

webControl is a free function included as standard in all RANGER Neo+ and above analyzers and boosts its capabilities: it allows to use them not only as field instruments but also as lab equipment with full remote control capabilities and as measuring stations to be located in fixed or mobile locations.

Analyzing the deviations of each MPX carrier using the remote control of PROMAX TV analyzers
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PROMAX is a leading manufacturer of test and measurement systems, broadcast and TV signal distribution equipment. Our product lines include measuring instruments for cable TV, satellite TV, broadcast, wireless and fibre optics networks, FTTH and GPON analyzers. DVB-T modulators, IP streamers or IP converters (ASI, DVB-T) are among the company’s latest developments.