Multi-channel Boxcar Averager¶
Note
This tutorial is applicable to UHF Instruments with the UHF-BOX Boxcar Averager option installed.
Goals and Requirements¶
This tutorial explains how to extract the envelope of an amplitude-modulated pulse waveform with the Output PWA tool or multi-channel boxcar averager. More generally, the Output PWA enables measurements of signals that are modulated with two time bases: the fast time base produces the pulses as measured by the boxcar averager, and the slow time base corresponds to a change of the pulse envelope. A typical application would be an amplitude modulated narrow laser pulse waveform.
To follow this tutorial, you need an external arbitrary waveform generator with an external AM modulation capability.
Preparation¶
Connect the cables as illustrated below. Make sure that the UHF unit is powered on and connected by USB to your host computer or by Ethernet to your local area network (LAN) where the host computer resides. After starting LabOne the default web browser opens with the LabOne graphical user interface.
The tutorial can be started with the default instrument configuration (e.g. after a power cycle) and the default user interface settings (e.g. as is after pressing F5 in the browser).
Amplitude-Modulated Pulse Test Signal Generation¶
Using the external arbitrary waveform generator, a pulse waveform with the following specification should be generated.
Pulse Specification | Value |
---|---|
Pulse Type | Square |
Amplitude | 100 mVpp |
Frequency | 9.7 MHz |
Duty Cycle | < 16% |
Note
An Agilent 33500B Truefrom waveform generator is used in this example. The minimum duty cycle for a 10 MHz signal for this instrument is about 16%. An external amplitude modulation scheme is activated with 100% AM depth.
Furthermore, a sine wave should be generated from the UHF instrument to amplitude modulate the AWG output. The output settings of the UHF instrument are given below.
Tab | Sub-tab | Section | # | Label | Setting / Value / State |
---|---|---|---|---|---|
Lock-in | All | Oscillators | Frequency (Hz) | 10.0 kHz | |
Lock-in | All | Signal Outputs | 2 | Amp (Vpk) | 1.5 V |
Lock-in | All | Signal Outputs | 2 | On | ON |
Scope | Control | Horizontal | Sampling Rate | 28.1 MHz | |
Scope | Trig | Trigger | Signal | Signal Input 1/ON | |
Scope | Trig | Trigger | Enable | ON | |
Scope | Trig | Trigger | Run/Stop | ON |
Now, one should be able to see a waveform in Scope that is similar to the one shown below.
Envelope Recovery with Output PWA¶
Just like the previous tutorial in PWA Averager, the PWA can be used to observe the pulse train. Although the measured result is similar to the previous tutorial, one can see in the PWA screen shot below that the peak-to-peak amplitude is no longer 100 mV peak but rather around 50 mV. One has to remember that we have now an amplitude modulated pulse, and the PWA is showing the average amplitude of these pulses over time. If one decreases the number of averages in PWA then the pulse amplitude will start fluctuating.
As shown previously, the Boxcar averager can be used to obtain the integrated pulse energy over a pre-defined gate width. This integrated value will of course be amplitude modulated as well. The Output PWA is able to recover this envelope of the integrated value. To do this, one now has to place an instance of the Out PWA tab on the LabOne user interface. The settings of the Output PWA are given below.
Tab | Sub-tab | Section | # | Label | Setting / Value / State |
---|---|---|---|---|---|
Out PWA | Settings | Signal Input | 2 | Input Signal | Boxcar 1 |
Out PWA | Settings | Signal Input | 2 | Osc Select | 2 |
Out PWA | 2 | Run / Stop | ON |
One should be able to observe a sine wave similar to the one shown below. The V magnitude is proportional to the AM modulation depth. One can verify this by changing the AM depth to 50% (see second screen shot). The envelope magnitude indeed decreased by a factor of 2. The Output PWA acts like a multi-channel boxcar. In combination with the UHF-MF option, the Output PWA enables analysis at multiple modulation frequencies.