[HBR] Re: GB> RSGB Keyer

[Mike McCarty]

Chris Trask wrote:
>> It isn't a matter of comprehension. I comprehend what you wrote. It's
>> just that what you wrote is incorrect. A multivibrator is two inverting
>> gain stages with the output of each connected to the input of the other.
>> A timer is not an MV (necessarily) nor is an MV a timer (necessarily).
>> Even if the first stage were a timer[*], it is not a multivibrator.
>>
> 
>     Your's thinking of a bistable multivibrator, which usually does require
> two stages, each one being comparable to a one-shot multivibrator or timer
> which trigger each other.

I'm thinking of any sort of multivibrator, whether astable, monostable,
or bistable. It was originated by Abraham and Bloch in 1918[*]. Eccles
and Jordan developed some modifications a year later. Schmitt developed
a cathode coupled variation. I first heard of them and studied them in
about 1966 or so. I've never seen the term used to refer to any circuit
which did not use two amplifying stages. You may look here

http://en.wikipedia.org/wiki/Multivibrator

(not that Wikipedia is all encompassingly authoritative) and find this
wording:

[QUOTE MODE ON]

A multivibrator is an electronic circuit used to implement a variety of 
simple two-state systems such as oscillators, timers and flip-flops. It 
is characterized by two amplifying devices (transistors, electron tubes 
or other devices) cross-coupled by resistors and capacitors. The most 
common form is the astable or oscillating type, which generates a square 
wave - the high level of harmonics in its output is what gives the 
multivibrator its common name. The mulitvibrator originated as a vacuum 
tube (valve) circuit described by William Eccles and F.W. Jordan in 1919.

[QUOTE MODE OFF]

Note that Abraham and Bloch reported on such a circuit before Eccles and
Jordan did, so this entry is not entirely correct, I believe. You may
also look here

http://www.answers.com/topic/multivibrator?cat=technology

and see

[QUOTE MODE ON]

Sci-Tech Encyclopedia: Multivibrator

A form of electronic circuit that employs positive feedback to 
cross-couple two devices so that two distinct states are possible, for 
example, one device ON and the other device OFF, and in which the states 
of the two devices can be interchanged either by use of external pulses 
or by internal capacitance coupling. When the circuit is switched 
between states, transition times are normally very short compared to the 
ON and OFF periods. Hence, the output waveforms are essentially 
rectangular in form.

[QUOTE MODE OFF]

If you know of any examples in the literature where one single stage
of amplification is called a multivibrator, I'd be glad to see it,
as I have never encountered such a use.

>> I'm all for generalizing and seeing correspondences between different
>> kinds of circuits for purposes of understanding their behavior in
>> multiple ways, but I can't quite stretch a single stage like that
>> into being a multivibrator.
>>
> 
>     The term one-shot multivibrator" is often used in place of "timer", and
> it does not require two stages.  A one-shot multivibrator uses an external
> trigger, in this case the key.

I've designed one-shot multivibrators, both with discrete and
integrated components. A one shot is not necessarily a MV. I've
implemented what is sometimes called a "half one-shot" using
just a couple of DTL gates with a cap to ground between them, and no
feedback at all. Those were more popular with the DTL we used to use,
which had resistive pull ups, though I've seen them on occasion with
CMOS using an external resistor in series from the output of the
first gate/inverter to the cap and perhaps a diode. Sometimes one-shots
are called pulse stretchers.

So, I'm fairly familiar with both the terminology and the circuitry.

"Timer" refers to a fairly broad class of circuits which provide
timing reference to other circuitry. The 555, for example, is a
"timer" which may operate in either astable or monostable modes.
The 74121 and LM131 are other old timer ICs I've used. I'm also
somewhat familiar with the LM2905, though I've never designed it
into a circuit. None of these is particularly good, and I've heard from
friends of mine who still design such circuits that much better ones
have been developed, particularly in the realm of temperature stability.
When we needed precise timing and pulse widths, we always used
a crystal oscillator and a counter along with a decoder and gate, but I
hear that now reasonable pulse timing can be achieved using modern
one-shots and caps with reasonable tempcos. Specifically I've heard that
the 74HC4538 achieves reasonable predictability. But we're sorta
straying from tubes, now, I think.

I reiterate that I've never seen the term "multivibrator" used in
reference to a circuit with only one gain stage, and would appreciate
seeing such, if you know of any.

[*]Publication 27 of the French Ministère de la Guerre, and in Annales 
de Physique 12, 252 (1919)

Mike
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