MOD Tab

The MOD tab provides access to the settings of the amplitude and frequency modulation units. This tab is only available when the MF-MOD AM/FM Modulation option is installed on the Instrument (see Information section in the Device tab).

The MF-MOD AM/FM Modulation option requires the MF-MD Multi-demodulator option.

Features

  • Phase coherently add and subtract oscillator frequencies and their multiples

  • Control for AM and FM demodulation

  • Control for AM and narrow-band FM generation

  • Direct analysis of higher order carrier frequencies and sidebands

Description

The MOD tab offers control in order to phase coherently add and subtract the frequencies of multiple numerical oscillators. Whenever the tab is closed or an additional one of the same type is needed, clicking the following icon will open a new instance of the tab.

Table 1. App icon and short description
Control/Tool Option/Range Description

MOD

btn mnu mod um

Control panel to enable (de)modulation at linear combinations of oscillator frequencies.

The MOD tab (see Figure 1) is divided into two horizontal sections, one for each modulation unit.

functional mod
Figure 1. LabOne UI: MOD tab

The modulation units are designed for experiments involving multiple frequencies. For many of such experiments the associated spectrum reveals a dominant center frequency, often called the carrier, and one or multiple sidebands symmetrically placed around the carrier. Typical examples are amplitude modulated (AM) signals with one carrier and two sidebands separated from the carrier by the AM modulation frequency. Another example is frequency modulation (FM) where multiple sidebands to the left and right of the carrier can appear. The relative amplitude of the sideband for both AM and FM depends on the modulation depth, which is often expressed by the modulation index.

The classical approach of analyzing such signals (in particular when only analog instruments are available) is to use a configuration called tandem demodulation. This is essentially the serial cascading of lock-in amplifiers. The first device is referenced to the carrier frequency and outputs the in-phase component. This is then fed into the subsequent lock-in amplifiers in order to extract the different sideband components. There are several downsides to this scheme:

  • The quadrature component of the first lock-in tuned to the carrier has to be continuously zeroed out by adjusting the reference phase. Otherwise a serious part of the signal power is lost for the analysis which usually leads to a drop in SNR.

  • The scheme scales badly in terms of the hardware resources needed, in particular if multiple sideband frequencies need to be extracted.

  • Every time a signal enters or exits an instrument the SNR gets smaller (e.g. due to the instrument inputs noise). Multiple such steps can deteriorate signal quality significantly.

All these shortcomings are nicely overcome by providing the ability to generate linear combinations of oscillator frequencies and use these combinations as demodulation references.

The MOD unit is linked to demodulators 1, 2, and 3. It can make use of up to 3 oscillators, which can be even referenced to an external source by using ExtRef or a PLL. Figure 2 gives an overview of the different components involved and their interconnections.

functional mod block diagram
Figure 2. Modulation Option block diagram

For convenience the UI provides access to presets for AM and FM in the Mode column. In Manual Mode all settings can be chosen freely.

Whenever a MOD unit is enabled, all the settings in the Lock-in tab that are controlled by this unit will be set to read-only.

On top of signal analysis the MF-MOD AM/FM Modulation option can also be utilized for signal generation. The Generation section provides all the necessary controls to adjust the carrier and sideband amplitudes.

FM signals are generated by coherent superposition of the carrier signal with two sideband frequencies on either side that have the same amplitudes but opposite phases. The phase shift is achieved by using negative amplitudes as displayed in the Lock-in tab. This FM generation method approximates true FM as long as the modulation index is well below 1, i.e. higher-order sidebands can be neglected. For a modulation index of 1 true FM provides more than 13% of signal power in the second and higher order sidebands.

More details regarding AM and FM signal analysis and generation can be found on the Zurich Instruments web page, e.g. https://blogs.zhinst.com/sadik/2014/02/sideband-analysis/.

Functional Elements

Table 2. MOD tab
Control/Tool Option/Range Description

SG Channels

Select SG Channel to display corresponding set of oscillator frequencies.

Frequency (Hz)

Oscillator frequency.

Oscillator Select

Selection of the oscillator used for the generated sine signal.

Harmonic

Multiplies the oscillator’s reference frequency with the integer factor defined by this field.

Frequency (Hz)

Frequency of the selected oscillator.

Phase Shift (deg)

Sets the phase of the sine signal.

I Sin Amplitude

Sets the amplitude of the sine signal sent to the I input of the digital mixer.

I Cos Amplitude

Sets the amplitude of the cosine signal sent to the I input of the digital mixer.

I Enable

ON / OFF

Enables the I input of the digital mixer.

Q Sin Amplitude

Sets the amplitude of the sine signal sent to the Q input of the digital mixer.

Q Cos Amplitude

Sets the amplitude of the cosine signal sent to the Q input of the digital mixer.

Q Enable

ON / OFF

Enables the Q input of the digital mixer.

Run/Stop

btn uielements runstop um

Runs the AWG sequencer.

Sequencer Status

grey/green/red

Displays the status of the sequencer on the instrument. Off: Ready, not running. Green: Running, not waiting for any trigger event. Yellow: Running, waiting for a trigger event. Red: Not ready (e.g., pending elf download, no elf download).

Modulation Enable

ON / OFF

Enables digital modulation of the waveforms generated by the AWG.

AWG Output Amplitude

Sets the amplitude of the first AWG output.

AWG Output Amplitude

Sets the amplitude of the second AWG output.

Hold

ON / OFF

Keep the last sample (constant) on the outputs even after the waveform program finishes. It is recommended to use only AWG waveforms with lengths equal to a multiple of 16 together with this functionality. Waveforms with other lengths are automatically padded with zeros at the end by the AWG compiler.

AWG Output Gain Amplitude

Sets the amplitude scaling factor of the given AWG channel. The amplitude is a dimensionless scaling factor applied to the digital signal.

AWG Output Gain Enable

ON / OFF

Indicates the routing of the AWG signal (row) to the digital mixer inputs (column).