Wideband Vertical Amplifier And Samplers

Everything to the right of the colored line in the block diagram of Fig. 4 is identical in both the 1-GHz vertical amplifier and the wideband vertical amplifier. Sensitivities, display modes, recorder outputs, and internal operation are the same for both. The wideband unit, however, has no built-in delay lines and, instead of probes, uses one of the three dual-channel feed-through samplers. Fig. 5 is a photograph of the wideband unit installed in the main frame, and Fig. 6 is a block diagram of the samplers.3

Depending upon which of the three wideband samplers is used, the bandwidth of the wideband vertical amplifier plug-in can be either 12.4 GHz or 4 GHz. Two of the samplers are ultrawideband units. One has a rise time of less than 28 ps and optimum pulse response (overshoot <5%) but a VSWR that increases with frequency (3 at 12.4 GHz); the other has a bandwidth of 12.4 GHz and a VSWR typically less

3 See article, p. 12, for a description of the wideband sampling devices used in these samplers.

Fig. 6. Block diagram of dualchannel remote samplers used with Model 1411A Sampling Vertical Amplifier. Ultra-wide-hand feedthrough samplers are described beginning on p. 12.

Fig. 6. Block diagram of dualchannel remote samplers used with Model 1411A Sampling Vertical Amplifier. Ultra-wide-hand feedthrough samplers are described beginning on p. 12.

To Channel A ac Amplifier

Sampling Trigger from Pulse Generator

To Channel 8 ac Amplifier

Wideband Sampling - Vertical Amplifier Plug-in

To Channel A ac Amplifier

Sampling Trigger from Pulse Generator

To Channel 8 ac Amplifier than 1.8 at 12.4 GHz, but has 5% to 10% more overshoot in the pulse response. The third sampler is a 4-GHz, 90-ps unit for applications where the widest bandwidths are not needed and lower cost is attractive. A five-foot cable (10-foot cable optional) connects the plug-in and the sampler so that measurements can be made at remote locations. Input signals are not terminated by the feedthrough samplers, so time domain reflectometry and signal monitoring are straightforward.

Oscillograms of 8-GHz and 18-GHz sine waves, taken using the low-VSWR (CW-optimized) sampler, are shown in Figs. 7 and 1(a) respectively. Note that time jitter is less than 10 ps, even at the highest frequencies.

Step response of the pulse-optimized sampler is shown in Figs. 1 (b) and 8 for two different time scales. The flat top and absence of excessive overshoot are evident in Fig. 8. Fig. 9 shows the reflection from the pulse-optimized sampler for an incident step having a rise time of 20 ps. The vertical scale is calibrated to read reflection coefficient with a scale factor of 0.1 /cm, and the sampler reflection is only 6%.

TRIGGERING Triggering of both the single-sweep time base4 and the dual-sweep time base and delay generator5 can be either automatic or manually adjustable and, when used with the 1-GHz vertical plug-in. either internal or external. Except for an extra UHF countdown in the single-sweep unit, all trigger circuits are identical circuits based on a newly designed tunnel-diode thresh-

4 See Fig. 10. 5 See Figs. 2 and 5.

Fig. 7. Wideband sampling oscilloscope display of 8-GHz sine wave. See Fig. 1(a) for display of 18-GHz sine wave. Vertical: 20 mV/cm. Horizontal: 50 ps/cm.

Fig. 8. Response of pulse-optimized Model 1430A Sampler to step with 20-ps rise time has 30-ps rise time, small overshoot, flat top. Vertical: 50 mV/cm; Horizontal: 100 ps/cm. See Fig. 1 (b).

Fig. 7. Wideband sampling oscilloscope display of 8-GHz sine wave. See Fig. 1(a) for display of 18-GHz sine wave. Vertical: 20 mV/cm. Horizontal: 50 ps/cm.

Fig. 8. Response of pulse-optimized Model 1430A Sampler to step with 20-ps rise time has 30-ps rise time, small overshoot, flat top. Vertical: 50 mV/cm; Horizontal: 100 ps/cm. See Fig. 1 (b).

Fig. 9. Reflection from wideband, sampler in 40-ps TDR system is only 6%. Vertical: reflection coefficient = 0.1/ cm; Horizontal: 100 ps/cm.

old detector. The detector produces an output to start the sampling process when the incoming trigger signal crosses the level set by the level control. A slope switch determines whether triggering will occur on the positive or negative slope of the trigger signal.

The threshold detector operates in one of three modes, depending upon the setting of the mode control which varies the supply current to the detector. Turning the control clockwise increases the supply current. For low supply currents the detector is bistable, that is, the incoming signal must both trigger and reset the detector. As the current is increased the trigger circuit becomes monostable; that is, it is triggered by the incoming signal, but it resets itself. For still higher currents the circuit becomes astable and oscillates.

The bistable mode is used to trigger on the trailing edges of pulses or on pulses that are so long that the trigger circuit would normally re-arm and trigger again before the end of the pulse. The monostable mode is used to 1 # = ^

Fig. 10. Single-sweep Model 1424A Sampling Time Base Plug-in has calibrated marker position control and direct readout of both magnified and un-magnifled sweep rates. Another new horizontal plug-in is dual-sweep time base and delay generator, shown in Figs. 2 and 5.

trigger on short pulses and on sine waves up to about 100 MHz. This mode is more sensitive than the bistable mode, especially on pulses shorter than about 30 ns. In the astable mode the detector oscillates at 10 to 40 MHz, depending upon the mode control setting, and any incoming sine wave alters this frequency so that it is a sub-harmonic of the incoming frequency. This type of circuit is called a 'countdown,' because for triggering to occur the incoming frequency must be greater than the oscillation frequency. At about 100 MHz the astable mode is as sensitive as the monostable mode, but at 1000 MHz the astable mode is about twenty times more sensitive.

To prevent double triggering on complex waveforms in which the desired trigger level and slope appear more than once each cycle, both horizontal plug-ins have a variable holdoff control which can be used to increase the minimum time between samples.

Continue reading here: Sweep Delay And Sweep Expansion

Was this article helpful?

0 0