This page describes the Siemens R218 Impedance and Admittance Bridge type Scheinwid.-Messbr./IMPEDANCE BRIDGE Rel 3 R 218 a F-Nr.6/802923 |
by ralph klimek Vk3ZZC April 2018 |
This
article is copyleft.. You are free to use my material for all honorable
purposes with the exception of corporate trademarks |
abstract Siemens R218 impedance bridge, user manual, schematic, owner's impressions, images |
Introduction
The
Siemens R218 impedance bridge is a precision instrument for
measuring complex impedance and admittance in two separate bridge
mechanisms covering 60khz to 30Mhz.
This
remarkable unit was purchased by me at the recent EMDRC Hamfest.
I have seen this unit on sale at other hamfests but there were no
takers. It looks expensive, once upton a time it actually
was expensive, very expensive indeed. I raised the courage to ask
the price. Only $20 ! I bit the bait. Maybe I should
have been more circumspect and checked whether or not I could even lift
this object. No, I could barely budge it, much less heave it back
to the car. Adam, VK3FAFP generously helped
me haul it to my car.
I was intrigued by this device.
It looked like it belonged inside a German U-Boat. Given
that it was manufactured in the early 1960s it actually may have been
designed by the same people that had made U-Boats !
I
finally got this unit home and onto my outdoor bench which has the
space to open this thing up. It weighs in at an astonishing 38 kgs ! Why so much ? Its just a piece of electronic test equipment !
When
I opened up the case, I was greeted with the same smell that greeted my
when I first opened up my British Army Larkspur radios. It smelled
OFFICIAL ! I was 15 years old when I had first bought and opened
up a British Army Larkspur radio and gazed in awe and wonder at
the sheer mind boggling complexity. This same sense of awe
was upon me once again. The interior of the R218 Bridge is
a magnificence of German precision engineering and pride in
workmanship. It is an intricate maze of clockwork that brings the
front panel controls to the controlled variable devices and
switches. It is all heavy castings, brass gears, sealed
metal boxes that hide their trade secrets and high purpose.
I
have noted that this unit was expensive. It had been purchased by
the ancestor of Telstra, the Australian Post-Master-Generals Department
in the early 1960s for some unfathomable purpose. They had
purchased the Rolls-Royce version. I am sure that something
cheaper would have been appropriate. It is clear from the
condition of the device that it had only barely been used. Given its
eye watering cost they would not have let field personel near it. It
must have sat in their research lab for years pretty much unused
until disposal.
The best part of all was that this device
came with the BOOK ! I have the manual ! Just as well .
Operating the bridge is not intuitive. The real item of antiquarian
interest is that the BOOK contained usage notes dated 1970,
written by one of their engineers who was trying to come to terms with
using this unit. He recommends having a slide-rule handy !
It is impossible to deduce the devices purpose design or
circuit by examination due to its high component density and mechanical
complexity. The business parts of the bridge are contained inside
sealed metal containers. The unit is progressively assembled
making exploration or disassembly extremely challeging . It was
built with laboratory grade components, better than mil-spec, with
the clear intention that repair should never be required.
The
best way to open this unit is to stand it upright with ample bench
space in front. Remove the screws holding it into the case. Slide
the unit forward. It has internal slides on the base to permit
this to happen. Do not attempt to lift it out with the case lying
flat. Nearly all the weight ,38kgs, is in the unit, not in the case
which is only a couple kg. Make sure that all test terminal connectors
from the top of the unit have been removed. The case is notched
to permit the unit top terminals to simply slide out. Caution; they are
only 3mm brass tapped posts and are fragile and would be
extremely difficult to replace.
The generator and detector ports
use a funny Siemens proprietery coaxial connector. Unless you can find
these , you may have to improvise. This wount be a problem, this
part of the circuit is not sensibly sensitive to impedance, so that any
decent BNC connectors will do.
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My observations about the theory of operation.
The manual has a brief writeup in English about how it works, and a detailed design description in German. The
bridge has two component bridges, a Maxwell bridge for
impedance measurements and a Wheatstone bridge for admitance. Both have
differant scopes. Impedance of low magnitude Z is best done
with a Maxwell Bridge. High magnitude impedance is best measured
with an admittance bridge. The test ports are brought out
to separate terminals for each bridge.
The bridge is first
zero calibrated with the test short circuits. The measurement is
then made on the device-under-test using an external signal
source and an radio external detector.
The bridge is brought to
balance internally with variable capacitance elements for the Complex Z
or Y. The Real part of Z or Y is balanced by a variable resistor.
Now this part is stunning. A variable resistor, a "pot"
is a mechanical component of very uncertain calibration and a
complex impedance that is not "canonical". The R218 uses a
"pure" resistor. Only it is not a resistor, it uses a
thermistor to balance the bridge. The thermistor is heated
to change its resistance is response to the front panel
controls. The user thinks he is turning a pot. Not so. The front
panel resistance arm controls are made from precision wire wound
resistors. There is an internal Wheatstone bridge based servo
that makes the thermistor track , exactly, the resistance
assembled from a switched network of wirewound references. This
is done by modulating a 50Hz supply that is transformer coupled to the
thermistor but is still isolated by chokes from the measurement
path. The thermistor has a range from low ohms to about a kilo
ohm. Its AC impedance reactance does not sensibly change over
this range and the measurement path does not have to traverse an
uncertain network of switches and pots. The design is
ingenious. Its the sort of "alien artifact" design that HP used
to do at this time in the 1960s.
The resistance standards are true standard resistors. They are all "wickles"
or wirewould resistors. Not the kind we are accustomed to.
When Siemens made a precision resistor they wound a calculated
length of fine wire on a spool and called it a resistor! The fact that it has a significant inductance is not an issue here because only the "dc" component of the thermistor resistance is
measured by an internal servo bridge to the actual thermistor
resistance which is made to track the front panel selection. Genius !
The
design philosophy is contrasted to HP or General Radio
(GenRad). With HP, the design philosophy of the 1960s was
that mechanical complexity should be replaced by electronic complexity
and as much as possible should be self calibrating through servo
loops. GenRad , by contrast, relied on mechanical precision and
complexity. The R218 is kind of a hybrid in this respect.
I think half of their design team distrusted electronics and put
their faith in precision and complex machinery. The thermistor
control must have been quite a leap of faith on their part.
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| | | 300mm ruler to scale | it once belonged to the PMG Scheinwid.-Messbr./IMPEDANCE BRIDGE Rel 3 R 218 a F-Nr.6/802923 | | | | |
| | | bridge selector | test terminals with official short-circuit | grossly oversized mains transformer | | | | | | | a Borgs' eye view sideways | thru the clockwork maze | resistance standards | | | | | | | | | measurement terminal | | | | the very simple electronics | resistance range selectors | big wirewound pot for manual balance | | | | the resistance range. | | | | | | | | | |
...and now.....the manual !
It
is type written. They cannot have sold many of these units.
I imagine that they would have built these only on receipt of an
order. Nothing about this device was cheap. |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | I will join these blueprint fragments into a coherent whole, at a later day. Download the high res images for yourself. Notice
how deceptively simple the circuit looks. Pity that there isnt a
mechanical drawing, because the complexity is in the clockwork is
quite overwhelming. | | | | These
written notes dated 1970 was written by a PMG engineer who
must have been tasked with making this extremely expensive purchase do
something usefull. They had no calculators , save for a slide
rule and manual arithmetic. It includes a worked problem.
Maybe this is why later generation communications engineers
prefered the HP direct reading impedance bridges that just told
you what you wanted to know. | | | | | | | | | blank | | | | LHS bridge | middle portion bridge | RHS bridge
| | I will join these into a single sheet at some later stage | |
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Thu Apr 12 19:24:10 EST 2018 initial posting Mon Apr 16 12:22:39 AEST 2018 spelling, grammer, notes
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