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{{Instrument Sidebar | |||
The '''Tektronix Type 280 Trigger Countdown Unit''' takes in a signal with repetition rate between 50 MHz and 5 GHz, | |manufacturer= | ||
and puts out a pulse train with repetition rate in the 15 to 45 MHz range | |model=280 | ||
whose leading edges are synchronized with a particular trigger point in the input signal. | |class=Accessory | ||
|series= | |||
|summary=Trigger Countdown Unit | |||
|image=Tek_type_280_1.jpg | |||
|caption=Tek Type 280 | |||
|introduced=1963 | |||
|discontinued=(?) | |||
|designers= | |||
|manuals= | |||
* [http://bama.edebris.com/download/tek/280/Tek%20280.pdf Tektronix 280 Manual (PDF)] | |||
}} | |||
The '''Tektronix Type 280 Trigger Countdown Unit''' takes in a signal with repetition rate between 50 MHz and 5 GHz, and puts out a pulse train with repetition rate in the 15 to 45 MHz range whose leading edges are synchronized with a particular trigger point in the input signal. | |||
The main purpose of the 280 is to extend the frequency range of the trigger circuitry in sampling scopes. | The main purpose of the 280 is to extend the frequency range of the trigger circuitry in sampling scopes. | ||
For example, consider a scope with a [[3T77]] and a [[3S2]] with [[S-2]] sampling heads. | For example, consider a scope with a [[3T77]] and a [[3S2]] with [[S-2]] sampling heads. | ||
The S-2 has response to at least 4 GHz but the [[3T77]] is only rated to trigger up to 300 MHz, | The S-2 has response to at least 4 GHz but the [[3T77]] is only rated to trigger up to 300 MHz, so the setup cannot, for example, be used to observe a 2 GHz sinewave. | ||
so the setup cannot, for example, be used to observe a 2 GHz sinewave. | |||
By using a Type 280, the 2 GHz sinewave can be fed to the Type 280, | By using a Type 280, the 2 GHz sinewave can be fed to the Type 280, which will produce 15 to 45 MHz pulses that can be fed to the 3T77, which will easily trigger. | ||
which will produce 15 to 45 MHz pulses that can be fed to the 3T77, which will easily trigger. | |||
The main concern with a setup like this is jitter. The 3T77 is rated at 50 ps jitter. | The main concern with a setup like this is jitter. The 3T77 is rated at 50 ps jitter. | ||
The presence of the 280 does nothing to help the jitter of 3T77, so that becomes the limiting factor. | The presence of the 280 does nothing to help the jitter of 3T77, so that becomes the limiting factor. | ||
Assuming that for a clear display, the triggering jitter must be less than 10% of the period of the observed signal, | Assuming that for a clear display, the triggering jitter must be less than 10% of the period of the observed signal, this gives a minimum signal period of 500 ps (2 GHz). | ||
this gives a minimum signal period of 500 ps (2 GHz). | |||
The 280 thus extends the top of frequency range of the system from 300 MHz to 2 GHz. | The 280 thus extends the top of frequency range of the system from 300 MHz to 2 GHz. | ||
This was very useful with a [[661]] with a [[4S2|4S2A]] and a [[5T1]] as the 5T1's maximum triggering frequency is 1 GHz and it has 30 ps of jitter | This was very useful with a [[661]] with a [[4S2|4S2A]] and a [[5T1]] as the 5T1's maximum triggering frequency is 1 GHz and it has 30 ps of jitter but the 4S2A's frequency response extends to 3.5 GHz. | ||
but the 4S2A's frequency response extends to 3.5 GHz. | Without the 280, the maximum sinewave frequency that can be observed is 1 GHz; with the 280 it is 3.5 GHz. | ||
Without the 280, the maximum sinewave frequency that can be observed is 1 GHz | |||
Later timing plug-ins like the [[5T3]] include built-in synchronizing circuits that eliminate the need for the countdown units such as the 280. | Later timing plug-ins like the [[5T3]] include built-in synchronizing circuits that eliminate the need for the countdown units such as the 280. | ||
Signal connections on the 280 use [[GR-874 connector]]s. | Signal connections on the 280 use [[GR-874 connector]]s. The trigger circuit is built around a [[152-0099-00|50 mA (152-099)]] [[tunnel diode]]. | ||
The trigger circuit is built around a [[152-0099-00|50 mA (152-099)]] [[tunnel diode]]. | |||
==Pictures== | ==Pictures== | ||
| Line 45: | Line 46: | ||
</gallery> | </gallery> | ||
[[Category:GR874]] | [[Category:GR874]] | ||