Multi-channel Boxcar Averager

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.

Figure 1. UHF connections to an external arbitrary function/waveform generator

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.

Table 1. Narrow pulse signal specifications
Pulse Specification	Value
Pulse Type	Square
Amplitude	100 mVpp
Frequency	9.7 MHz
Duty Cycle	< 16%

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.

Table 2. Settings: observe the pulse waveform
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.

Figure 2. Amplitude-modulated pulse waveform measured with the Scope

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.

Figure 3. Averaged carrier pulse waveform in PWA in the Boxcar tab

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.

Table 3. Settings: observe the pulse waveform
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.

Figure 4. AM envelope in Out PWA with 100% and 50% AM depth