Stanford DG535 Pulse Generator

Delays and Triggers

The DG535 has four delay outputs and two pulse outputs. Each delay can be set from 0 to 1000 seconds relative to the trigger with 5 ps resolution. The two pulse outputs are defined by pairs of delay outputs. The first delay specifies the leading edge of the pulse, while the second delay defines the trailing edge or pulse width.

You can trigger the DG535 internally from 1 mHz to 1 MHz with four-digit frequency resolution. External, single-shot and burst mode triggers are also supported. For power control applications, the DG535 can be synchronized to the line voltage.

Flexible Outputs

Each output amplitude can be independently adjusted from -3 to 4 V with 10 mV resolution. For convenience, several preset output levels are also provided: TTL, ECL, and NIM. For applications requiring higher output voltages, optional rear-panel outputs provide the same precise timing at output amplitudes up to ±35

Simple to Use

The DG535 is easy to use. All delay values can be entered numerically, or by convenient cursor keys. A GPIB interface is standard, allowing complete instrument control by an external computer. For quick configuration, the instrument stores up to nine complete setups in non-volatile memory.

DG535 Features

The DG535 Digital Delay and Pulse Generator can provide four precisely timed logic transitions, or two precisely controlled pulses. The four digitally controlled time intervals may be programmed from the front panel or via the GPIB Interface. Front panel BNCs provide high slew rate outputs for TTL, ECL, NIM or continuously adjustable levels. The outputs may be set to drive either 50 ohm or high impedance loads. The high accuracy (1 ppm), precision (5 ps), wide range (0 to 1000 s), and low jitter (50 ps) make the DG535 the ideal solution to many difficult timing problems.

Delay Outputs

There are four delay output BNCs: A, B, C and D. The logic transitions at the outputs of A, B, C and D can be delayed by up to 1000 seconds in 5 ps increments relative to the T0 pulse. The T0 pulse, which marks the beginning of a timing cycle, is generated by the internal rate generator or in response to an external trigger. Insertion delay between the external trigger and the T0 pulse is 85 ns. Delays for each channel may also be "linked" to another channel. For instance, you can specify the delays of the 4 channels as:

A=T0 + .00125000
B=A+.00000005
C=T0+.10000000
D=C+.00100000

In this case, when the A delay is changed, the B output will move with it. This is useful, for instance, when A and B specify a pulse, and you want the pulse width to remain constant as the delay of the pulse is changed. Regardless of how the delay is specified each delay output will stay asserted until 800 ns after all delays have timed out. The delays will then become unasserted, and the unit will be ready to begin a new timing cycle.

In this case, when the A delay is changed, the B output will move with it. This is useful, for instance, when A and B specify a pulse, and you want the pulse width to remain constant as the delay of the pulse is changed. Regardless of how the delay is specified each delay output will stay asserted until 800 ns after all delays have timed out. The delays will then become unasserted, and the unit will be ready to begin a new timing cycle.

Pulse Outputs

In addition to the four delay outputs there are four pulse output BNCs: AB, -AB, CD and -CD. The AB pulse output is asserted when the A delay times out and unasserted when the B delay times out. For instance, in the previous example, a 50 ns pulse would appear at the AB output and a 1 ms pulse at CD. Pulses as short as 4 ns (FWHM) can be generated in this manner. The complementary outputs (-AB and -CD) provide a pulse with identical timing and inverted amplitude.

Flexible Output Control

Each delay and pulse output has an independently adjustable offset and amplitude which can be set between -3 V and 4 V with 10 mV resolution. The maximum transition for each output is limited to 4 V. In addition, you can also separately select 50 W or High Impedance termination for each output. For convenience, preset levels corresponding to standard logic families can also be selected. TTL, NIM, and ECL levels can all be selected with a single keypress.

Optional Outputs

For applications requiring higher voltages, a rear-panel high voltage option is available. This option provides five rear-panel BNCs which output an amplified 1 ms pulse at the transition time of the front-panel T0, A, B, C, and D outputs. The high voltage option does not affect the function or the timing of the front panel outputs. The amplitude of the rear-panel outputs is 10x the corresponding front-panel output, and the outputs are designed to drive 50 ohm loads. Since these outputs can only drive an average current of 0.8 mA, charging and discharging the cable capacitance may be the most important current limiting factor to consider when using them (assuming a high impedance load). In this case, the average current is: I = 2Vtf / Z, where V is the pulse step size, t is the length of the cable in time (5 ns/meter for RG-58), f is the pulse repetition rate, and Z is the cable's characteristic impedance (50 W for RG-58)

Internal and External Timebases

Both internal and external references may be used as the timebase for the DG535. The internal timebase can be either the standard 25 ppm crystal oscillator timebase, or the optional 1 ppm Temperature Compensated Crystal Oscillator (TCXO). The internal timebase is available as a 1 Vpp square wave on the rear panel BNC. This output is capable of driving a 50 ohm load, and can be used to provide a master timebase to other delay generators. Any external 10.0 MHz (±1%) reference signal with a 1 Vpp amplitude can also be used as an external timebase. For instance, the FS700 LORAN-C Frequency Standard can be used with the DG535 to provide a timebase with 1 part in 1012 long term stability.

Easy to Use, Easy to Program

All instrument functions can be accessed through a simple, intuitive, menu-based interface. Delays can be entered with the numeric keypad, in either fixed-point or exponential notation, or by using the cursor keys to select and change individual digits. The 20 character backlit LCD display makes it easy to view dela settings in all lighting conditions.

The DG535 comes standard with a GPIB (IEEE-488) interface. All instrument functions can be queried and set via the interface. You can even display the characters the DG535 has received over the interface on the front-panel LCD display. This can be enormously valuable when debugging programs which send commands to the instrument.

ATE Applications

The DG535's versatility, precision, and accuracy recommend it for a wide variety of test and measurement tasks. In this example, the DG535 is used to measure the setup times for the data, preset, and clear inputs to a flip-flop. The measurements may be made with picosecond resolution. The logic thresholds for the device under test may be measured using the DG535's adjustable output levels. All measurements may be controlled from the front panel or by a computer via the standard GPIB interface.

Precision Time Control

A single DG535 can provide four transitions for precise system timing. Several DG535's may be used if more channels are needed. The 10 MHz reference may be daisy-chained between units so that each DG535 in an experiment uses the same timebase. All of the units may be controlled over the same GPIB bus. The flexible output levels and simple architecture of the pulse/delay generators make it simple and easy to rapidly reconfigure test systems.

Laser Timing Applications

The DG535's 4 independent outputs make it ideal for laser timing applications. In the above example the T0 output of the DG535 fires the flashlamp of a pulsed laser. The DG535's internal rate generator controls the repetition rate of the laser and the overall experimental repetition rate. The A delay output controls the firing of the laser Q-Switch. The B delay output can be used to synchronize some aspect of the experiment to the laser pulse, e.g. the application of a voltage pulse, or the triggering of a discharge. Finally, the C delay is used in this example to trigger the gated integrator looking at the detector output. Note that both the B and C delays in this example can be specified relative to the A delay. In this way, as the laser pulse is moved by changing the A to T0 delay, the experimental trigger and the gated integrator trigger will stay fixed relative to the laser pulse.

Fast Rise Time and Fall Time Modules

External in-line modules are available to reduce the rise or fall time of the DG535 outputs to 100 ps. These modules use step recovery diodes to speed up the rise time (option 04A) or the fall time (option 04B). The bias tee (option 04C) allows these modules to be used with the optional rear-panel outputs to produce steps up to 15 V. Applications include time domain reflectometry measurements, pulse response measurements of fast amplifiers, testing high speed digital circuits, or use as a low jitter trigger source in high EMI environments.

The devices consist of a step recovery diode and matching network mounted in an in-line package. The units provide a fast, low distortion step into a 50 ohm line with adjustable amplitudes from 0.5 V to 2.0 V, and up to 15 V when used with high voltage rear panel outputs.

Operation

For step amplitudes of less than 2.0 V the fast transition time units should be attached directly to the front panel of the DG535.

The DC offset is critical to the operation of the device: the offset is used to forward bias the step recovery diode (SRD) prior to the pulse output from the DG535. When the pulse from the DG535 begins, the stored carriers in the SRD maintain the conduction in the diode, shunting the output pulse to ground. When the stored carriers are depleted (about 3 ns after the start of the pulse), the diode abruptly stops conduction, creating a very fast transition time step at the output.

The offset must be increased when the output amplitude is increased and adjusted for the best output pulse shape. If the offset is set too high, the output step will overshoot: if the offset is too small, the output step will undershoot the final value

Output Pulses Up To 15 Volts

The fast rise time and fast fall time units may be used with the high voltage rear panel outputs (Option 02) to generate pulses up to 15 V. A bias tee, Option 04C, is required for this mode of operation.

The high voltage rear panel outputs are AC coupled. Therefore some accommodation must be made to provide a DC current to forward bias the SRD prior to the output pulse. This current is applied via a "bias tee" (Option 04C) which passes the bias current through an inductor to the diode. The same inductor prevents the pulse from the rear panel output from passing to the bias source. A front panel output may be used as the bias source.