This page describes the HP 8405A Vector Voltmeter
by VK3ZZC    June 2016
This is one of HP's  classic golden age instruments.  The manual shows design and production dates from 1966. There are no integrated circuits.  The chassis layout has each signal processing card in its very own fully screened enclosure.  The previous owners had thoughtfully  left the HP supplied  extender card inside as well. This make life easier. All the components are Mil-Spec.  The meters  are precision taut-band suspension movements, a sticker on the level meter says "individually calibrated".  Despite the adjustment hole, there is not actually an adjustment. I had mistakely opened up the movement looking to find the adjustment , thinking that it has been broken off. Nope, do not do this.  It is hard up against its left hand stop for a reason.  The display is logarithmic,  there is no logarithm that is zero !

The electronic design is amazing is the way that the signal being measured flitters between  the time and frequency domains.  The instrument  measures both amplitude and phase  by down sampling  (or mixing )  to 20KHZ (where it is easy to amplify and process) a specially crafted  pulse train that varies from 1Mhz to 2Mhz whose source is a voltage controlled oscillator.  The pulse train is very rich in sidebands  and soon, one of these mixing (down sampling) sidebands will be in the passband of the 20Khz IF chain and phase detectors.
One of these detectors serves to phase lock the voltage controlled oscillator to a sub-divisor of the input signal. Once locked, the 20kHz signal in the IF amplifyer is  a faithfull replica of the input signal.  The phase of the down sampled signal is also a faithfull replica of the input signal from the B channel.   It still surprises me that this is the case, even though I should have known this from my University Maths.
(my feeble excuse was that my Math is 35 years in the past)   .  The downsampled phase pulses are integrated in a simple low pass filter to directly drive the phase meter.    This design is brilliant !  How else could you measure phase  at 1Ghz  in 1966 ? with the active devices in existence in those days.  I still remember how silicon transistors were regarded as "exotic"  in those days,  when a germanium device with a 2Mhz fT  was regarded as an "RF" device !

This device requires some practice to use effectively.
The A chanel must be used to create the absolute phase reference.  Only the A chanel can be used for doing standalone  level measurements.
The little lock light must be OUT  before any measument is meaningfull.  Only once we have a stable  A CHANEL  reading can you probe with the B chanel.   Otherwise the phase meter  displays  jerky random looking movements. You might think ( as I did) that the B chanel is faulty.  This is not the case.  You cannot use the B chanel as an RF millivolt meter  standalone.  A valid A chanel signal must be present first.  Ind the first instance, short circuit the A and B chanel probes on a valid RF source.  This establishes  the ZERO phase reading, adjust the phase controls to achieve this.  Now insert the B probe  on the circuit to be measured.  The phase meter will read the B probe's phase with respect to the A Chanel.  It may lag or lead as the laws of physics and mathematics decree.  When the instrument is locked to a valid A chanel  then and only then can you select the level on the B chanel.  Now you have both an A chanel reading with 0 degrees phase and a B chanel amplitude and phase differance.  You have just measured a Vector !

Coming up.  There are two manuals currently on the web..   One from the BAMA which is quite good and another from a MIL site which has  notes added by the military to help  GIs  repair this instruments in the field  ... in places where HP did not do field repairs !  I am touching up these schematic images with the GIMP to make them more printable and readable.  I will release these soon.  I will also write an essay about how this instrument works, when I understand it better myself !

unretouched manuals
8405_service_6.pdf   (from BAMA)
HP 8405A Operating and Service Manual.pdf   see  (seriously ! I dont know how urls like this get generated, other wise put this title into Googly as is)

my retouched schematics images
(to be supplied)

The previous owners had kindly left the extender card inside !
thumb.imgp8299-vector.jpgHere we are measuring the phase delay of a length of coax and we even get the sensible result  above. This length of belden 9907 gave a 90 degree phase delay  at about 9 Mhz.
the work centre does not do much wood cutting these days. oh well

this page was created  Wed Jun 15 18:48:30 EST 2016