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+++ b/content/10MHz_ref.rst Thu Mar 10 22:18:00 2016 +0100
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+==========================================
+ Building a bench 10MHz reference: part 1
+==========================================
+
+:author: David Douard
+:date: 2016-02-21
+:Category: Electronics
+:Tags: test equipment, 10MHz, Rubidium
+
+
+Many electronics test equipment need an accurate time base. Most
+often, it's build on a 10MHz base.
+
+For quite a while now, I have a used rubidium frequency and time
+reference (Efratom Model LPRO-101) waiting for a nice enclosure. I
+also have a small Extron Electronics video signal amplifier I thought
+I could use to make my freq reference able to provide its signal to
+several test equipments at once.
+
+The Rubidium Frequency Reference
+================================
+
+The device is a small enclosure with only a 2x5 pins connector:
+
+.. image:: {filename}images/freq_ref/efratom.jpg
+ :alt: The Efratom LPRO-101 frequency standard.
+
+The exact model is an Efratom Rubidium Frequency Standard Model LPRO-101:
+
+.. image:: {filename}images/freq_ref/efratom_sticker.jpg
+ :alt: The Efratom LPRO-101 frequency standard.
+
+Inside the enclosure, there is a single PCB with SMD and trough hole
+components. It's quite dense.
+
+.. image:: {filename}images/freq_ref/efratom_inside.jpg
+ :alt: The PCB of the Efratom LPRO-101 frequency standard.
+
+The rubidium lamp with the resonant cavity occupy almost 1/4 of the
+total area. The cover of the enclosure is compartimented, mainly to
+shield the lamp+cavity device from the remaining of the board:
+
+.. image:: {filename}images/freq_ref/efratom_enclosure.jpg
+ :alt: The enclosure of the Efratom LPRO-101 frequency standard.
+
+
+The Rubidium Freq Ref seems to workjust fine. I've powered it a few
+times, and it take a few minutes to lock the lamp and stabilize the
+output frequency. I've made a quick measurement of the power
+consumption; at startup time, the current goes up to 1.5A, but after a
+while, it stabilize around 0.5 or 0.6A. According a voltage 24VDC, it
+starts with 36W then goes down to 15 or 16W.
+
+Now, I have to embed it in a nice enclosure (preferably an aluminium
+one so I can use is as a heatsink, since the frequency reference
+produces a bit of heat.)
+
+The Extron Video Amplifier
+==========================
+
+The video amplifer on the other hand has a bit of a trouble:
+
+.. image:: {filename}images/freq_ref/extron.jpg
+ :alt: the Extron Electronics video amplifier
+
+As you can see on the picture above, the inductor is broken, and there
+is no visible mark on it. This inductor is used to produce the
+symetric voltages required to poser the opamp used to dispatch the
+signal.
+
+The switching power modules used on the board is a LT1616_ (marked as
+LTNB on the S6 package). So I reversed engineered a bit the schematic
+of the power to try to guess what kind of value this double inductor
+could be, since the reference schematic in the datasheet does not use
+such a dual inductor(/transformer).
+
+Fro the schematic I could extract, it appeared that the design is in
+fact very close to the reference schematic proposed by LT in the
+datasheet_, in the Bipolar Output DC/DC Converter configuration page
+16:
+
+.. image:: {filename}images/freq_ref/lt1616_bipolar_output_DC2DC.svg
+ :alt: LT1616 Bipolr Output DC/DC converter.
+
+It produces +/-5V from the 12V input. But still, I did not know what
+kind of inductor/xformer I should use. I did not notice at first sight
+the two references for the inductor that are proposed in the reference
+design. So I first searched using the nice faceted search tool of
+digikey (reducing the selection according the informations I have on
+the broken device and then looking at the pictures), I finally could
+find the reference of the inductor. By the way, it's a Sumida CLS62
+Series inductor (as indicated in the datasheet). And according to the
+reference design, it should be a 22µH one. So let's make a purchase
+for this inductor as well as some 20k resistors for my HP8904A_.
+
+.. _HP8904A: {filename}hp8904a.rst
+
+
+
+
+.. _LT1616: http://www.linear.com/product/LT1616
+.. _datasheet: http://cds.linear.com/docs/en/datasheet/lt1616fs.pdf