Tuesday, September 5, 2023

Pulse Height Analyzer (PHA) option for Eberline ASP-1

 When it comes to those big, beige, analog metering system, heavy metal and clunky radiation detectors from the 80s... I am a huge fan of ... not Ludlum Model 3 but the Eberline ASP-1.  I have 3 such units and they are really versatile! 

For anyone interested I made a brief review and comparison between Ludlum Model 3 and ASP-1.

Eberline ASP-1 was and still is a fantastic meter for its time! In the name "Analog Smart Portable" (ASP), the "Smart" stands for "Microcontroller" - an 8-bit Intel 80C31 is in the heart of the meter, driving an 8-bit DAC (AD7524) which on the other hand drives the Analog Metering System. Two old-school 74HC157 multiplexers read the state of various config DIP-switches and the main rotary Range switch.

Reviewing the Service Manual for the meter years ago, I noticed that Eberline designed a Pulse Height Analyzer (PHA) module for it but for all these years I was never able to find, or even see a picture of one in existence. I am not even sure if this option was ever manufactured / sold for ASP-1 but I have seen dozens of ASP-1 units and not a single one had this module installed.

The Pulse Height Analyzer allows the meter to count pulses only when they have a specific energy range (of course, when the meter is equipped with an energy discriminating detector (scintillator)).

The PHA creates a "window" which has an adjustable width and can also be moved up and down the energy range so the counter can count only pulses produced due to gamma rays from a specific isotope - for example it can count only Cs-137 gamma at 662keV while ignoring any other energies, making it very selective at what activity is measured.

From a practical point, I can set it up for a specific U or Th decay chain isotope and being able to differentiate in the field if I am dealing with U or Th for example (that is if I don't have my Raysid with me)

The PHA module is interfaced via a "PHA Option" socket on the main board of ASP-1. This socket must be populated with a special shorting connector when the PHA Option is not installed and so was the case with all of my 3 ASP-1 units, until now...

After I was unable to procure the ASP-1 PHA Option for years, I got tired of looking for it and decided to make my own modules.

The PHA Option socket with the Shorting Connector removed.

The same PHA Option but for a different meter - the Eberline ESP-2. 
The schematics are virtually identical - only board outline and component topology appear to be different from the one, specific to ASP-1.

Fortunately, the Service Manual includes full documentation and schematics for this board.

This is the schematics for the PHA board published in the Eberline ASP-1 technical manual.
There are no exotic components and everything for this circuit is readily available on DigiKey. 
The circuit itself is fairly simple, using comparators and voltage dividers for reference to determine the energy "window". There is also a Dual BCD up-counter alongside a bunch of other passive components. 
The board outline is very specific as this board needs to fit between existing connectors and components located on the top side of the main board. 
The R32 Trimmer-potentiometer controls the width of the energy "window", and the slide switch allows to turn off PHA counting mode and to GROSS count all pulses. 

I started by re-creating the schematics in AutoCAD Eagle.

 I decided to keep the old-school DIP-14/DIP-16 versions for the ICs but replaced all other components with their SMD versions, using 1206 footprint for quick and easy soldering. 

Since it is a fixed size board there was no gain of going SMD for the ICs and their SMD versions were actually more expensive. 

I kept the original component names as listed in the technical manual - this makes component installation and troubleshooting very easy.

The PCB Layout created in AutoCAD Eagle.
Because of the location of the board, right behind the analog metering system, I realized that this board can also conveniently host LED backlighting for the metering system (the stock backlighting uses 2 incandescent light bulbs). 
I utilized the unused pin 2 on the Option header to provide the switched ground to the LED lighting and a small modification of the main board ties Pin 2 to the light switch. The LED Anodes are connected by jumper wire to the +VBAT

Top Layer of the completed PHA Option Board. Some additional milling of the board outline was needed to make it fit between the components.

Bottom PCB layer with the male DIP-16 header for interface with ASP-1

The PHA Board installed in the Option socket. The Blue Pot is the "Window" trimmer, and the red slide switch changes the mode between "GROSS" and "PHA" counting.

The PHA Option board sits nicely behind the Analog Metering System.

I have 3 completed PHA Option for ASP-1 modules left, available for purchase at $50/each + shipping.