Analog Computing

I recently had the pleasure to visit Prof. Dr. Bernd Ulmann, leading Guru of Analog computing, an avid collector of aged computing devices and electronics paraphernalia. I had an older HP Spectrum analyser which  I wanted to donate and I was invited to see the beautiful collection of analog computers.

Unlike the recent devices that all of us use, an analog computer does not execute algorithms in a step-by step fashion but uses precise analog circuits that are wired to solve the problem in question. Interestingly, for many kinds of problems this approach is more suitable than digital, sequential processing. Calculations happen in real time by definition and parameters can be varied by the turn of a potentiometer, lending a tactile element to IT. One area of interest is the simulation of dynamic processes such as the control of a self propelled projectile or the suspension of a car. In fact, the A4 rocket was controlled by an analog computer called the “Mischgerät”.

Pictured below is a RA770 computer by Telefunken

There are some youtube videos detailing the systems operation:

Bernd also owns plenty of more recent digital computers, such as the lovely HP9820 (aka Model 20) on which I learned programming.

In case you are interested in these areas, check out his websites at: (more on the digital side of things)

and (the analog systems)

I can definitely confirm, the man has the knack! It’s time to get him into ham radio.

RF Field Strength Indicator

Here is the latest toy that I finished. It is an active RF field strength meter. So far, I had a passive one (a detector which feeds into a microamperemeter). This one here has an op-amp stage plus a nifty VCO that outputs a sound that varies in pitch with the strength of the signal that was received.

The circuit follows the one published in Funkamateur 10/2010 by DK6UU. The circuit contains a bug as that is is published with the wrong type of chip (74HC4046 should be: CD4046).


Hamfest Score

I recently was so lucky to run into a gentleman selling two vintage Fluke differential voltmeters, a Fluke 891 (DC only) and 893 (AC and DC) for 10 Euro each. What could go wrong?

Both instruments were in a fairly questionable state so I took them through a thorough cleaning cycle and re-calibrated them. All knobs were rinsed ultrasonically and one of the meters had to be disassembled and oiled to bring it back to life. I could not find a manual for the 891 but since the 893 is practically the same instrument, all the information is there.

Luckily I had seen W2AEWs excellent Video review, so I knew what to expect.

What a fabulous instrument! How amazing is it to have an instrument which is almost as old as myself that can measure a voltage to the nth decimal digit?

What had to be done was:

  • Thorough cleaning
  • Fixing the meter on the 891
  • Recalibrating both instruments
  • Adding a power cord plug to the 893