Calibrating An Fm Modulation Monitor: A Step-By-Step Guide

Calibrating an FM modulation monitor is a complex process that requires specific equipment and technical knowledge. The procedure involves three main steps: calibrating a modulator, calibrating a demodulator, and measuring the unknown transmitter. The first step involves using a spectrum analyser to find the calibration point and adjust the modulation level accordingly. This is followed by calibrating a demodulator using the calibrated modulator, which involves setting up a receiver to demodulate the signal and measuring its output voltage. Finally, the unknown transmitter can be measured using the calibrated modulator and demodulator. While there are software solutions available, such as MpxTool, that can assist in monitoring FM modulation, a dedicated modulation monitor like the Inovonics Model 531 is essential for accurate metering of an FM carrier deviated by a complex program waveform.

Using a spectrum analyser to find the calibration point

To calibrate an FM modulation monitor, one of the steps involves calibrating a modulator. This is done by using the spectral properties of an FM signal.

Prepare the carrier source

Prepare to modulate the carrier source (the transmitter) with a 1kHz sine wave modulation source, adjust to zero modulation level and key the transmitter up.

Couple the carrier to an SSB receiver

Couple a small amount of the carrier to an SSB receiver and tune in the carrier to a beat note of about 800 Hz.

Increase modulation slowly

Slowly increase the modulation until the carrier beat disappears. Carefully find this null position of the carrier beat note. Note that you will also hear one or more sidebands when the modulation is applied, ignore these and just listen for the null of the carrier.

Find the modulation index

The modulation index is now 2.4, and therefore the deviation is 2.4kHz.

This technique is very sensitive and accurate, and any errors will mostly be due to the accuracy of the modulating frequency.

Calibrating a modulator

Calibration of a modulator is a critical process in the signal chain for digital transmitters. The following steps outline the procedure for calibrating an IQ modulator:

Set Control Values:

Set one or more control values of the IQ modulator corresponding to a desired constellation point of a constellation diagram.

Generate IQ Modulating Signal:

Mix the IQ modulating signal with a carrier signal to generate an IQ modulated transmit signal.

Transmit and Receive Signal:

• Transmit the IQ modulated transmit signal towards a predefined object at a fixed location.
• Receive the reflected signal from the object.

Down-Convert and Compare:

• Mix the received reflection with the carrier signal to generate a down-converted receive signal.
• Compare the amplitude and/or phase of the down-converted signal with the desired constellation point.

Adjust Control Values:

Adjust the control values of the IQ modulator until the deviation between the amplitude and/or phase of the down-converted signal and the desired constellation point falls below an acceptable threshold.

This process can be repeated iteratively for each constellation point to achieve a comprehensive calibration. Additionally, curve fitting techniques can be applied to generate an estimated curve based on the adjusted control values, reducing memory requirements.

The calibration of modulators is particularly important in applications such as radar systems, where the performance of the IQ modulator directly impacts the overall system performance. Calibration helps to compensate for non-ideal behaviour and improve the accuracy of the system.

Calibrating a demodulator

To calibrate a demodulator, you need to set up a receiver to demodulate the signal from the calibrated modulator and calibrate its output voltage against the known deviation of the source. This can be done by connecting an oscilloscope to the receiver output and adjusting the volume until the peak voltage is 2.4 divisions, corresponding to a peak deviation of 2.4kHz. The instrument used does not need to be a real scope; it could be a soundcard oscilloscope or sound recording software.

The process is very sensitive and accurate, with errors mostly attributed to the accuracy of the modulating frequency.

Measuring the unknown transmitter

Now that we have calibrated the receiver and display, we can move on to measuring the unknown transmitter(s). This process involves using the calibrated equipment to analyse the signal from the transmitter and determine its modulation level.

To begin, connect the calibrated receiver to the unknown transmitter. You will need to set up the receiver to detect the modulation on the carrier signal. This can be done by tuning the receiver to the carrier frequency and adjusting the bandwidth to capture the signal.

Next, observe the signal on an oscilloscope or other display. You are looking for the peaks and nulls in the carrier signal, which indicate the modulation level. The modulation index can be calculated by measuring the peak frequency deviation and dividing it by the modulation frequency. This will give you an idea of how strong the modulation is.

For example, if you measure a peak frequency deviation of ±2.88 kHz and have a modulation frequency of 1.2 kHz, the modulation index would be 2.4 (2.88/1.2). This corresponds to a certain amount of deviation in the carrier signal, which can be looked up or calculated using the Bessel function.

By comparing the modulation index and deviation to known standards, you can determine if the unknown transmitter is within the acceptable range. Adjustments can then be made to the transmitter's settings to bring it into compliance if needed.

It is important to note that this process may need to be adjusted depending on the specific equipment and signals being used. Additionally, the accuracy of the measurements relies on the calibration of the modulator and demodulator, so care should be taken to ensure accurate calibration.

Using a mod-monitor to identify issues in the RF chain

A mod-monitor is a crucial tool for broadcasters to ensure compliance with broadcasting regulations and maintain positive relationships with other broadcasters in the market. The Inovonics Model 531 is a highly regarded mod-monitor that can help identify and remedy various issues in the RF chain.

Firstly, the mod-monitor must be a high-quality off-air FM receiver to fully recover the 100 kHz FM baseband. This means its RF bandwidth must be greater than a typical consumer radio, which can compromise its performance as a standard radio receiver. The FM demodulation function must also be highly linear, ensuring the recovered FM baseband is an exact representation of carrier deviation. The Inovonics 531 excels in this regard with its dual conversion receiver and linear-phase filtering.

Secondly, the mod-monitor must provide precise stereo decoding to measure stereo separation and crosstalk across the audible range. The Inovonics 531's stereo decoder is a phase-locked-loop (PLL) design, surpassing the capabilities of a consumer-radio chip. It displays left and right audio channels on separate quasi-peak-responding meters, adhering to either the 75 or 50-microsecond de-emphasis characteristic.

Thirdly, a top-tier mod-monitor will display the injection levels of popular FM subcarrier services, such as RDS and audio SCA. This feature is vital for setting these subcarriers to the required levels for reliable operation. The Inovonics 531 provides excellent functionality in this area, displaying peak injection levels for various subcarriers.

Lastly, a valuable mod-monitor feature is the display of incidental, synchronous amplitude locations, alerting personnel to potential faults. The Inovonics 531 has proven its worth in this area, with a track record as a reliable "workhorse" in the industry.

In conclusion, a mod-monitor is an indispensable tool for broadcasters, not only for compliance with regulations but also for optimising their RF chain. The Inovonics 531 is an excellent example of a mod-monitor that meets and exceeds the desired features, making it a popular choice for broadcast groups and regulatory authorities.

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