Model3490A

3-162. Sample/HoldTrigger(TTL). Compatibility with TTL logic circuits requiresthis signalgo be from HIGH (>+ 2.4 V) to LOW ((+ 0.4 V) for a minimum of 30 nanosecondsin order to initiate a Hold mode. The signal must be HIGH for at least600 ps prior to going LOW. The external triggeringcircuit must be capableof sinking I mA at (+ 0.4 V.

3-163.AC Trigger. The leadingedgeof a negative-going pulse at least 30 nanosecondswide and 2 V to 200 V in amplitude initiates a Hold mode. The sigral must be stable for at least 2 ps before the negative-goingtransi- tion.

3-164. Termination of UnusedS/H Trigger Input Con- nections. When operatingin a Sample/Holdmode,some precautionsarenecessaryto preventunwantedtriggering of the SampleiHold circuits, which can be causedby crosstalk within an external cableattached to the trigger input connector.Only one of the two triggerinputs will be used at any one time in a given situation, and spurioustriggeringat the other input may be prevented in one of two ways:

l.The unusedinput may be left open; that is, with no wire in the cable connected to this pin on the connector.

2.The unusedinput may be terminatedat the other end of the cable. The TTL Sample/Hold Trigger (dc coupled) input line shouldbe connectedto a HIGH logic level ()+ 2.4V). The AC Trigger input line should be connectedto a LOW logiclevel(<+ 0.4 V).

3-165Initiating.Sample/HoldMeasurement.

3-166. The Sample/Hold option is availablein three different configurations which employ various methods of initiating a Sample/HoldmeasurementSampleiHold. is aväilable with the BCD/Remote Expand Option 020. (Either Data Output Option 021 or Remote Control Option 022 or both may also be installedwith Option 020.) Sample/Hold is also availablewith the General Purpose Interface Bus I/O Option 030. In the third con{iguration, Sample/Holdis availableas Option 045, without any of the other options. No data output or remote control, except Sample/HoldTrigger,is available in Option 045. The following paragraphsdiscussmethods of initiating a measurementin instrumentshaving the various options. Table 3-9 lists the methods of externallytriggeringa Sample/Holdmeasurement.

3-167. Internal SampleControl, Option 045 and Option

040with Option 020. When the iront panel Sample Rate control is set to any position except Hold and no Sample/Hold Trigger command is applied, the instru- ment will automaticallyhold and readthe input signalat the sample rate selected.However, this method of operation providesonly random sampling,sinceit is not possibleto s1'nchronizethe measurementto any specific point in the input signal.

SectionIII

Table3-9. Initiating S/H Measurementsby

ExternalTriggering.

Sample/Hold

OtherOptions

Measurement

Option

in the 34904

Initiated By:

o40

o20

Separate External Encode

 

 

command or Stretched

 

 

PulseOutput connected

 

 

to External Encode input.

040

030

Sample/Hold Trigger

045neither 020 Sample/Hold Trigger or O30

3-168. Internal SampleControl, Option 040 with Option 030. The 3490A with the GPIB Option 030 may be programmed for internal sample control. If so, the instrument will automaticallyhold and read the input signal at the sample rate selected. Operating with internal samplecontrol providesonly random sampling, since the measurement is not synchronized to any specificpoint in the input signal.

3-169. ExternalTriggering,Option 045. When Sample/ Hold is present in an instrument with neither BCD Remote Expand Option 020 or GPIB Option 030, it is designatedas Option 045. In this case,either Sample/ Hold Trigger command automatically initiates a mea- surement.

3-170. External Triggering, Option 040 with Option

020.With this combination of options, a Sample/Hold measurementmay be initiated by external triggeringin

two ways. For either method, the 34904 measurement circuits must be in the "Hold" state.That is, the Sample

Rate control must be in the Hold position, or if the instrument has Remote Control Option 022, the lnter- face Hold line may be held LOW. One method of

initiating a Sample/Hold measurementrequires two 'separate

signalinputs, a Sample/HoldTriggercommand ," to initiate a Hold mode, and an External Encode command to initiate a measurement.Both commands may be appliedat the sametime, or the ExternalEncode command may be applied up to ll2 secondafter the Sample/Hold Trigger. In the second method, the StretchedPulse Output from the Sample/Holdtrigger circuits (J7 pin 10) is connected externally to the External Encode input (J6 pin 46 or J7 pin 24 or 28).

This connection providesan External Encodesignalto initiate a measurementimmediately upon receipt of a Sample/HoldTriggercommand.Neither signalwill have any effect if appliedwhile a measurementis in progress. If a Sample/HoldTriggercommandis givenwithout an External Encode command, the Sample/Hold circuits will be locked in the Hold mode. This condition canbe terminatedby applyingan ExternalEncodecommandor by settingthe Sample/Holdswitchto Off.

3-171. External Triggering,Option 040 with Option

030. When operating Sample/Hold with General Pur-

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