Precompensation Tab
The Precompensation tab relates to the HDAWGPC Realtime Precompensation option and is only available if this option is installed on the instrument (see Information section in the Device tab).
Features

Realtime waveform processing for insitu tunability

Highpass compensation

Overshoot/undershoot compensation with 8 exponential filters

Bounce compensation for reflections and standing waves

Programmable FIR filter with convolution length 30 ns

Precompensation Simulator

Latency calculation

Filter reset by AWG sequence instruction
Description
The Precompensation tab provides access to the realtime precompensation filter configuration and the Precompensation Simulator. Whenever the tab is closed, clicking the following icon will open a new instance of the tab.
Control/Tool  Option/Range  Description 

Precompensation 
Configure the Realtime Precompensation filters. 
The Precompensation tab shown in Figure 1 consists of a configuration section on the left and a display section on the right. The configuration section is further divided into a subsection for device parameters on the left, and for simulation parameters on the right.
There is one precompensation unit for each Wave output of the HDAWG. The individual units are represented as numbered sidetabs. Each unit consists of a chain of filters represented in the Device Filter tab. The filter chain is shown as a flow diagram in Figure 2 and consists of 8 exponential compensation filters, 1 highpass compensation filter, 1 bounce compensation filter, and 1 FIR filter. All configurable filter parameters are represented as rows in the Device Filter tab, with the exception of the FIR filter parameters which are located in the separate Device FIR subtab. The entire filter chain can be bypassed or enabled with the Enable Precompensation Filter button, and each filter can be enabled using a button on the left side of the corresponding filter settings. The filter parameters are explained in the following section.
The right side of the configuration section as well as the display section represent the Precompensation Simulator. The Precompensation Simulator is a softwarebased tool to accurately model the hardware filter and thus allows one to find suitable filter parameters based on a step response measurement of the external circuit. The display section shows three simulated step response curves based on the currently set parameters in the Sim Filter subtab:

Input signal : this curve represents an ideal step signal for reference

Precompensated signal : this curve represents the simulated output of the precompensation unit, when the AWG generates a step signal as an input to the precompensation unit, e.g. using a sequencer instruction
playWave(ones(10000))
. The precompensated signal approximately equals the signal expected at the Wave output of the HDAWG with enabled precompensation filter, apart from distortions introduced by the analog output stage of the HDAWG. 
Signal path response: this curve represents the step response of the inverse of the precompensation filter. This is the step response of a hypothetical signal path that the currently simulated filter would ideally compensate.
The precompensated signal generated by the hardware is limited by the DAC full scale. If the Simulator Input Signal is chosen to be equal to the AWG output (including digital AWG gain factors) and the Simulator Precompensated Signal absolute value exceeds 1, an overflow condition is to be expected at the DAC. The overflow condition is then indicated by a red filter status LEDs when enabling the precompensation filter. This condition is to be avoided by either adjusting the filter parameters, or rescaling the AWG output signal. 
In a typical workflow, the user would start by a measurement of the step response of the full signal path from the AWG to the reference point at the device under test. Often this can be achieved by an averaged oscilloscope measurement at the reference point while generating a step signal with the AWG with disabled precompensation filter, e.g. using the following sequence program:
const f_sam = 2.4e9; // sampling rate const f_seq = fs/8; // sequencer clock rate const plateau_width = 10e6; const wait_time = 10e3; const amplitude = 0.5; while (true) { playWave(amplitude*ones(plateau_width*f_sam)); setPrecompClear(1); playWave(zeros(16)); wait(wait_time*f_seq); }
Consider the following points when using this sequence program:

The step signal amplitude (here 0.5) should be less than 1.0 in order to provides the digitaltoanalog converter with a certain headroom to avoid overflow in case the filter generates positive corrections. This is always the case for the highpass compensation filter.

The timescale of the plateau width (here 10 µs) should correspond the timescale of the final signals, and should notably be shorter than a possible highpass filter time constant in the signal path.

The waiting time between pulses (here 10 ms) should be much larger than a possible highpass filter time constant in the signal path, such that there are no history effects.

The
setPrecompClear(1)
instruction together with the subsequent playback of a zerovalued waveformplayWave(zeros(16))
ensures that the precompensation filters are reset after the measurement, to prevent history effects.
In the next step, the user would adjust the simulator filter parameters such that the displayed signal path response curve matches the measured step response as closely as possible. The measurement graphs shown in the following section may help to visually identify which type of filter applies to a given signal characteristic observed in the step response measurement.
Once satisfied with the result, the simulation parameters can be transferred to the hardware filters by clicking on . The step response measurement described above can then be repeated, this time with enabled precompensation filter. At this stage, fine adjustments of the enabled filters can be done while observing their effect on the signal.
After having eliminated the largest signal distortions, smaller exponential overshoots or undershoots may now become measurable. Iteratively, one can now enable additional exponential filters in order to suppress these.
In case that the GUIbased procedure described here does not yield sufficiently accurate results, consider using a systematic numeric optimization using the one of LabOne APIs in order to exploit the full potential of the HDAWGPC Realtime Precompensation. Please refer to the documentation of the Precompensation Module in the LabOne Programming Manual. The LabOne Python API comes with an example implementation of a numeric filter optimization.
Latency and Status
Enabling the full filter chain leads to a signal processing latency of 9 filter clock cycles, where the filter clock runs at 1/8 of the sampling clock. With a sampling clock rate of 2.4 GSa/s, the filter clock frequency is 2400/8 MHz = 300 MHz and the minimum latency is 9/(300 MHz) = 30 ns. Each filter of the chain adds an additional latency when it is enabled, as specified by the red numbers in Figure 2. The total latency caused by the current filter configuration is displayed at the bottom of the Device Filter subtab. Each filter has a Status LED on the right side of the settings. A red LED indicates that an arithmetic overflow currently occurs, while a yellow LED indicates that an overflow occurred in the past. The status of all filters can be cleared with the button. An overflow occurs if the output of the precompensation filter reaches the maximum or minimum of the digitaltoanalog converter range. The reason for an overflow can often be diagnosed by inspecting the precompensated signal in the simulator, and can often be eliminated by adjusting the AWG signal. Be aware that an overflow may also hint at the fact that a desired precompensation is not physically realizable. For example, it is not possible to apply a highpass compensation to a square signal which is already at the maximum output voltage of the instrument, as this would require a signal voltage increasing indefinitely over time beyond the maximum output voltage.
Filter Chain Specification
IIR Filter Theory
The highpass, exponential, and bounce compensation filters are infinite impulse response (IIR) filters. IIR filters are a special case of linear digital signal processing filters. We specify the operation of the IIR filters by means of the socalled timedomain difference equation:
y[n]=b[0]x[n]b[1]x[n1]+b[2]x[n2]…+b[N]x[nN] a[1]y[n1]a[2]y[n2]…a[M]y[nM]
where the variables x[n] and y[n] represent the digital input and output at discrete time n, respectively. Each coefficient a[k] and b[k] with index k is the weight of a feedback and feedforward stage with k clock cycles delay, respectively. In this definition, the weight a[0] of the most recent output value y[n] is set to one, i.e. a[0] = 1. The operation of the IIR filter is completely determined by the coefficients a and b. The table below lists the definitions of a and b for all precompensation filters.
Name  Applications  Model step response \(y\): uncorrected \(y_c\):corrected  Parameters  Coefficients in the ideal difference equation 

Highpass 
ACcoupling, 
\(y(t) = e^{t/\tau}\) 
\(\tau\): Time constant 
\(b_0 = \frac{k+1}{k}\) , \(b_1 = \frac{k1}{k}\) with 
Exponential under and overshoot compensation 
Distortions due to LCR elements 
\(y(t) = g(1+A e^{t/\tau})\) 
\(\tau\): Time constant \(A\): Amplitude 
\(b_0 = 1kk\alpha\) with 
Bounce correction 
Reflections due to impedance mismatches 
\(y(t) = x(t) + A y(t\tau)\) 
\(\tau\): Delay \(A\): Amplitude 
\(b_0 = 1\) , \(b_d = A\) with 
Finite impulse response (FIR filter) 
Shorttimescale distortions 
\(y_c(t) = k(t) * x(t)\) (convolution) 
\(k(t)\): FIR filter kernel 
\(b_0,b_1,...,b_{71}\), where \(b_i = b_i+1\) for \(i \geq 8\) 
The IIR difference equation is available in MATLAB as filter(b,a,x)
, or in the SciPy Python library as lfilter(b,a,sig)
. This can be used to predict the outcome of the realtime precompensation listed in the table above. Note, however, that the implementation of realtime digital filters in hardware requires certain simplifications to approximate the ideal filter operation. These simplifications result in slight deviations of the operation of the realtime filters from the ideal operation of the IIR filter. The Precompensation Module of the LabOne APIs contains a method to accurately model the hardware behavior of the filter. Please refer to the LabOne Programming Manual for details.
Highpass Compensation
The purpose of the highpass compensation is to correct for firstorder highpass filters such as DC blocks or biastees. An example of its application is shown in Figure 3.The only parameter of the highpass compensation filter is the time constant in units of seconds. When enabled, the highpass compensation filter introduces a delay of 12 clock cycles, which corresponds to 12/(300 MHz) = 40 ns. Since the highpass compensation filter integrates the signal over time, it may be necessary to clear the filter regularly using the AWG sequencer instruction setPrecompClear(1)
. Whenever this command is placed in front of a playWave
or playWaveDIO
command, a clearing signal is sent to the highpass compensation filter synchronous to the playWave
or playWaveDIO
command. The Clearing dropdown selector defines when the clearing signal is sent relative to the waveform playback: during the entire waveform playback (Level), at the start (Rise), at the end (Fall), or both at the start and at the end (Both).
Exponential Compensation
The exponential overshoot and undershoot compensation filters can correct for exponential signal components typically introduced by spurious capacitances or inductances combined with resistors. An example of their application is shown in Figure 4. The parameters are the time constant of the exponential decay and the amplitude of the exponential relative to the input step size. A positive amplitude corrects for overshoots, while a negative amplitude corrects for undershoots. When enabled, each exponential compensation filter introduces a delay of 11 clock cycles, corresponding to 11/(300 MHz) = 36.67 ns.
Bounce Compensation
The bounce compensation can be used to suppress unwanted reflections in the cables of the experimental setup. An example of its application is shown in Figure 5. The bounce compensation adds to the original input signal the input signal multiplied with a configurable relative amplitude and delay. When enabled, the bounce compensation filter introduces a delay of 4 clock cycles, corresponding to 13.33 ns.
Finite Impulse Response Filter
The realtime finite impulse response (FIR) filter is suitable for correcting signal transients on the nanosecond time scale. An example of its application is shown in Figure 6. The user can set 40 coefficients of the FIR filter kernel represented in the Device FIR subtab. The operation of the FIR filter corresponds to a convolution of the input signal with the FIR filter kernel. The first 8 coefficients are directly applied to the first eight taps of the FIR filter at the full time resolution. The remaining 32 coefficients are applied to pairs of taps. Therefore, the total number of taps is 8+232 and the user can choose 8+32 = 40 coefficients. The Device FIR subtab gives access to the coefficients from the graphical UI, which is suitable for quick manual tests. For systematic configuration of this filter, the use of the API is recommended.
Specification Table
Parameter  Value 

Number of precompensation units 
1 per AWG channel (4 or 8 total depending on HDAWG model) 
Compensation filters per unit 
1× highpass, 8× exponential, 1× bounce, 1× FIR 
Highpass compensation filter type 
IIR, configurable time constant 
Highpass compensation time constant range 
208 ps to 166 ms 
Exponential compensation filter type 
IIR, configurable time constant and amplitude 
Exponential compensation time constant range 
15 ns to 1 ms (available range depends on amplitude setting) 
Bounce compensation filter type 
FIR, configurable delay and amplitude 
Bounce compensation delay range 
0 to 100 ns 
Bounce compensation delay resolution 
1 sample period (417 ps at 2.4 GSa/s) 
Bounce compensation amplitude range 
1 to +1 (dimensionless) 
FIR filter type 
Causal FIR filter, 72 coefficients (corresponding to 30 ns at 2.4 GSa/s), first 8 coefficients freely configurable,remaining 64 coefficients pairwise equal 
FIR filter coefficients range 
4 to +4 (dimensionless) 
FIR filter coefficients resolution 
18 bit 
Precompensation Simulator display options 
Forward filter, inverse filter, uncompensated step response 
Functional Elements
Control/Tool  Option/Range  Description 

Enable 
ON / OFF 
Enables the entire precompensation filter chain. 
Total Latency 
The total latency introduced by the entire precompensation filter chain. 

Status Flag Reset 
Resets the status flags of all precompensation filters of this output channel. 

Enable 
ON / OFF 
Enables the exponential compensation filter. 
Time Constant 
Sets the characteristic time constant of the exponential compensation filter. 

Amplitude 
Sets the amplitude of the exponential compensation filter relative to the signal amplitude. 

Status 
Indicates the status of the exponential compensation filter: Green: normal, Red: overflow during the last update period (~100 ms), yellow: overflow occurred in the past. 

Enable 
ON / OFF 
Enables the highpass compensation filter. 
Time Constant 
Sets the characteristic time constant of the highpass compensation filter. 

Clearing Slope 
Select when to react to a clearing pulse generated after the AWG Sequencer setPrecompClear instruction. 

Level 
During the entire clearing pulse. 

Rise 
At the beginning of the clearing pulse. 

Fall 
At the end of the clearing pulse. 

Both 
Both at the beginning and at the end of the clearing pulse. 

Status 
Indicates the status of the highpass compensation filter. Green: normal, Red: overflow during the last update period (~100 ms), yellow: overflow occurred in the past. 

Enable 
ON / OFF 
Enables the bounce correction filter. 
Delay 
Sets the delay of the bounce correction filter. 

Amplitude 
Sets the amplitude of the bounce correction filter relative to the signal amplitude. 

Status 
Indicates the status of the bounce correction filter: Green: normal, Red: overflow during the last update period (~100 ms), yellow: overflow occurred in the past. 

Enable 
ON / OFF 
Enables the finite impulse response (FIR) precompensation filter. 
FIR Coefficient 
FIR (finite impulse response) filter coefficients. The first 8 coefficients are applied to 8 individual samples, whereas the following 32 Coefficients are applied to two consecutive samples each. 

Status 
Indicates the status of the finite impulse response (FIR) precompensation filter: Green: normal, Red: overflow during the last update period (~100 ms), yellow: overflowed in the past. 

Latency Simulation 
Enables the simulation of the latency of the precompensated signal. 

Number of Points 
Length of the simulated wave in number of sample points. 

Input Wave 
Wave input source for the simulation. 

Step 
Step function. 

Pulse 
Single pulse. 

AWG Loading 
Load an AWG sequencer wave from the AWG as specified with the AWG wave index. 

File 
Load a wave from a CSV file as specified by the file selector. 

Open Directory 
Opens the directory where the CSV files are expected to be located. This is OS specific and could open additional windows. 

Columns 
The number of columns determined from the CSV file. 

Timestamp Column 
The index of the column in the CSV file containing the timestamp for each sample. 

Data Column 
The index of the column in the CSV file containing the samples. 

Sampling Frequency 
The sampling frequency determined by the timestamps from the CSV file. 

Sample Delay 
Artificial time delay of the simulation input. 

Gain 
Artificial gain with which to scale the samples of the simulation input. 

Offset 
Artificial vertical offset added to the simulation input. 

Status 
The status of loading the CSV file. 

To Device 
Copy the Simulation Filter parameters onto the respective Device Filter parameters. 

AWG Wave Index 
Determines which AWG wave is loaded from the the AWG output. Internally, all AWG sequencer waves are indexed and stored. With this specifier, the respective AWG wave is loaded into the Simulation. 

To Simulator 
Copy the Device Filter parameters onto the respective Simulation Filter parameters. 