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4S1 (view source)

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The 4S1 was developed in 1961 and [[introduced in 1962]].
The 4S1 was developed in 1961 and [[introduced in 1962]].


Each channel has a trigger pickoff, a [[delay line]], and a sampler.  Specified rise time is 350 ps.
Each channel has a trigger pick-off, a [[delay line]], and a sampler.  Specified rise time is 350 ps.
The sampling pulses for each channel are generated by a step recovery diode.  
The sampling pulses for each channel are generated by a step recovery diode.  


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=== Sampling Bridge ===
=== Sampling Bridge ===
The sampling pulses are fed to a sampling bridge made of four [[sampling diodes|GaAs diodes]].   
The sampling pulses are fed to a sampling bridge made of four [[sampling diodes|GaAs diodes]].   
These diodes are fragile and exact replacements are hard to find.   
These diodes are fragile and exact replacements are hard to find.  They have however been
People have [[Sampling diodes|improvised replacements using modern Schottky diodes]]
successfully [[Sampling diodes|replaced by modern Schottky diodes]], e.g. Agilent HSMS-28x series.
such as those in HP/Agilent's HSMS-28x series.


=== Dual-trace and X-Y Modes ===
=== Dual-trace and X-Y Modes ===
The 4S1 has a dual-trace mode, where the A waveform and the B waveform  
The 4S1 has a dual-trace mode, where the A waveform and the B waveform are both displayed in the
are both displayed in the time domain at the same time with a single  
time domain at the same time with a single trigger source and a single equivalent-time sweep rate.   
trigger source and a single equivalent-time sweep rate.  Another mode  
Another mode is called "A vert/B horiz".  This mode is used in conjunction with an X-Y mode switch  
is called "A vert/B horiz".  This mode is used in conjunction with  
on the panel of the 661. The signals enter the 4S1 through [[GR-874 connector]]s on the front panel.
an X-Y mode switch on the panel of the 661. The signals enter the 4S1
through [[GR-874 connector]]s on the front panel.


=== Trigger Amplifier ===
=== Trigger Amplifier ===
The 4S1 uses an unusual trigger amplifier with two trigger signal paths.  One path is for low frequencies, the other for high frequencies.
The 4S1 uses an unusual trigger amplifier with two trigger signal paths.  One path is for low frequencies,  
(A similar approach was also used in the main vertical signal path of the [[7A29]] several years later.)
the other for high frequencies. (A similar approach was also used in the main vertical signal path of the
[[7A29]] several years later.)


From serial number 101 to 1349, the low frequency path starts with a 25 kΩ resistor that DC-couples  
From serial number 101 to 1349, the low frequency path starts with a 25 kΩ resistor that DC-couples  
the input signal to a [[2N1429]] PNP emitter-follower stage.   
the input signal to a [[2N1429]] PNP emitter-follower stage.   
After that is a [[151-103]] NPN common-emitter.  The high frequency path starts with a trigger pickoff
After that is a [[151-103]] NPN common-emitter.  The high frequency path starts with a trigger pick-off
transformer on the input.  The pickoff signal feeds a common-base amplifier that uses a [[2N700]] PNP transistor.
transformer on the input.  The pick-off signal feeds a common-base amplifier that uses a [[2N700]] PNP transistor.
The low frequency and high frequency trigger paths meet at the trigger output of the 4S1.   
The low frequency and high frequency trigger paths meet at the trigger output of the 4S1.   
From there, the trigger signal passes through the 661, and into a [[5T1]] or [[5T3]].   
From there, the trigger signal passes through the 661, and into a [[5T1]] or [[5T3]].   
4S1 units with serial number 1350 and up use a slightly less complex trigger amplifier circuit. 
 
The later circuit uses a [[TA2333]] silicon NPN transistor to amplify the entire frequency range.
4S1 units with serial number 1350 and up use a slightly less complex trigger amplifier circuit,
with a [[TA2333]] silicon NPN transistor amplifying the entire frequency range.


=== Comparison with S-1 Sampling Heads ===
=== Comparison with S-1 Sampling Heads ===
The sampler board in the 4S1 is roughly equivalent to a pair of [[S-1|S-1sampling heads]].
The sampler board in the 4S1 is roughly equivalent to a pair of [[S-1|S-1 sampling heads]].
A short piece of RG/174 coax connects the 661 timing pulse coaxial connector to the
A short piece of RG-174 coax connects the 661 timing pulse coaxial connector to the
[[SMB connector]] timing pulse input on the 4S1 sampler board.  This signal triggers
[[SMB connector]] timing pulse input on the 4S1 sampler board.  This signal triggers
a blocking oscillator which drives a [[snap-off diode]], D2004, which generates the fast
a blocking oscillator which drives a [[snap-off diode]], D2004, which generates the fast
falling edge for the channel A and channel B sampling bridges.  The 4S1 uses [[7586]]
falling edge for the channel A and B sampling bridges.  The 4S1 uses [[7586|7586 nuvistor triodes]]
triodes as high-impedance buffers between the sampling bridge and the amplification that
as high-impedance buffers between the sampling bridge and the amplification that follows.
follows.


The 4S1 has a main chassis and sub-chassis that plug into it.
The 4S1 has a main chassis and sub-chassis that plug into it.
There is a sub-chassis extension, part number [[012-069]], that allows maintenance of the
A sub-chassis extension, part number [[012-069]], allows maintenance of the sub-chassis.
sub-chassis.


==Pictures==
==Pictures==
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