this page describes my HP Model 411 RF Millivoltmeter
by ralph klimek VK3ZZC  Dec-2014
I acquired this boat anchor  at the recent southern peninsula  ham fest.  the SPARC  runs a most excellent hamfest every year.  For some reason it attracts sellers of most interesting boat anchors  and I have left behind far too  much pocket monery  there  in prior years. This year was no exception.  I like to collect Hewlett Packard equipment and bring it back to life. I  do not run a museum, so my boat anchors must perform for me some usefully function in the ham shack.

Traditionally it is easy to measure RF power  in WATTS.   A diode peak detector is good for measuring rf power from about 0.5 Watts up.  Not so easy to measure meaningfully RF power  between 1 milli-watt and one watt.  Diode peak detectors transition from a square law response to "linear" response somewhere at the 10milliwatt level.  Do you have a rational basis  for saying that your detector  measures  1 volt,  are you entitled thereby to conclude that the RF power  is  1^2 / 50 = 20mW ?  Nope, not at all.  The diode equation  is NOT linear  , its exponential !  Do you know and understand where all the stray capacitances are in your probe ?  Nope.  Do you understand the non linear diode junction capacitance effects as a function of reverse bias ?  Nope.  Didnt think so.

A good way of calibrating a diode RF power detector is to feed a known, measurable, high power signal  and compare the detector reading using a calibrated resistive attenuator.  This is quite a sensible way of doing it, at least through to low VHF  where we still have some faith in the behavior of resistors and stray reactances can still be managed or ignored.  We have two knowns, input reference power and attenuation.

The HP411 RF millivoltmeter uses this principle to automatically  measure  milliwatt level signals using the comparison method.  The comparison is done with a servo loop  that steps down a reference internal RF source until it matches with what the "business" detector diode in the probe body is measuring.  The value of the control voltage signal to the reference gain control is now  " the reading".    This system is self-calibrating and sensibly linear based on a collection of mostly true principles being :

  • that the detector diode in the probe head and the comparison diode have similar DC and AC  forward conductance curves
  • that the bevaviour of a diode detector  is similar at the 100kHz reference frequency  and whatever it is that you are measuring.  This is actually mostly true up to almost 1 Ghz !
  • that the behaviour of the variable voltage controlled attenuator is sensibly monotonic,  which is also mostly true !

was it possible using a design begun in the late 1950s  to achieve this using the technologies and electronic components at hand ?  Yes,  barely.   The HP411 design is required to compare and amplify  sub millivolt DC signals, without offset and without drift.   In an era before monolithic  integrated OP AMPs,  this was all but impossible with vacuum tube technology. It is possible using solid state devices to contrive a DC coupled amplifier. Due to the nature of the control grid of a thermionic valve it is generally not possible construct a DC coupled amplifier.  It has been done, but these are heroic efforts required many differant finely controlled power supplies to provide many levels of controlled bias, and generally not commercially viable.  The pre sixties technique for DC amplification was to use a chopper amplifier. This is still the technique of choice when the ultimate  offset and drift performance is required.  It is relatively easy to construct a fixed gain , low drift AC amplifier.  The AC amplifier does not have any DC offset or drift  BY DEFINITION !  The input DC signal is chopped at some arbitrary frequency, the resultant AC amplified, then synchronously rectified by the same chopper mechanism. The chopper output is low pass filtered or integrated to give a resultant amplified  DC signal.  The traditional DC chopper was a noisy buzzer with a number of commutating contacts, very prone to wear and accoustically noisy.

In the HP411  the chopper used for DC amplification  is done optically using a rotary vane optical chopper .  The chopped light  activates  a photocell  which chops  the DC signal coming from the detector heads' diodes.  The very tiny AC component is then amplified, rectified by the same chopper assembly using differant photocells and this resultant DC signal now  provides the servo control signal for the variable gain attenuation.  In this device I have encountered the very first implementation of fibre optic light guides.  There are a pair of plastic light pipes that take a sample of chopped light to the output chopper detector head.  This was absolute genius, the alternative was to have the machine full of expensive mechanical rotating axles.  This is possibly the very first instance of the commercial use of solid state light guides  (pipes) to achieve a control/signalling  function.  There was also used, possibly for the first time, a pair of new fangled transistors. The 2N404 transistor must have  cost a fortune in 1961.  It was used for the 100khz reference oscillator and variable gain amplfier.

My unit thankfully came with the BNC detector head  which was intact.  It is a little intermittant and this problem will be addressed in the fullness of time.  It consists of two diodes in an isothermal enclosure. One diode rectifies the business end of the probe, the other rectifies the 100Khz  reference signal  where the resultant differance signal is applied to the input chopper.

My HP411 did not work initially, but was otherwise in very fine condition with zero internal dust and grime.  It must have been in good dust proof storage for a long time.  There were only two faults.  One of the chopper  light sources was dead. Just a 6.3AC  dialight. Quickly replaced.  The other fault was that the  paint on the meter movement had peeled. The peeled paint had prevented the meter needle from moving.  This was also quickly fixed.  The diode detector head only required to be reassembled.  What happened then, after nearly 60 years, when I powered it on ?  It is original golden age HewletPackard, so no surprises here. Off  course, it just worked  !!!

I have made only one modification, and this time it is not to the electronics.  The case has ventillation holes on top.  In my experience, they are rarely  required and only permit ingress of dust, debris and fluids.  If things get so appalingly hot then I fit a ventillation fan to my boatanchors. A great deal of internal corrosion in boatanchors  is due to the corrosive action of moisture and dust  that has entered the equipment from top ventillation holes

underchassis viewlight pipes linking the chopper drive to the detector head.  Is this the very first commercial use of light pipes ?The HP-411A  rf millivoltmeter
the big can style electrolytics are still perfectly seviceable after 53 years. Tell that to the young people of today and they wont believe you !there are two transistors here. possibly one of HP's first use of these new fangled  transistor thingiespower zener diodes for bias regulation
the chopper motor. the little window tells you if its rotatingI had to replace just one chopper light bulbold style light source. These might get replaced by white leds if the spectral response of the photocells is suitable
the chopper used a vane system to do the light choppingthe detector head. light pipes go to the chopper unitI have added a sheet of metal at the top of the case to prevent ingress of junk and dust from the top. ventillation is not compromised.
this once belonged to our nations' weapons research establishment and had been extremly well looked after.
thumb.imgp7467-hp411.jpgthumb.imgp7468-hp411.jpgthe paint had peeled inside the meter movement and the peelings prevented the meter from moving. I think it is funny that the electronics used by HP all those years ago lasted longer than the paint ! Apart from the burned out chopper lamp this was the only fault i had to fix.
extract from the original  HP manual

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this page was created on Thu Dec  4 14:22:09 EST 2014