content/10MHz_ref.rst

Wed, 13 Mar 2024 11:03:13 +0100

author
David Douard <david.douard@sdf3.org>
date
Wed, 13 Mar 2024 11:03:13 +0100
changeset 152
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permissions
-rw-r--r--

Update URLs for pcb & firmware to sourcehut

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1 ==========================================
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2 Building a bench 10MHz reference: part 1
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3 ==========================================
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4
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5 :author: David Douard
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6 :Category: Electronics
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7 :Tags: test equipment, 10MHz, Rubidium
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8 :series: 10MHz Bench Reference Standard
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9 :series_index: 1
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11 Many electronics test equipment need an accurate time base. Most
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12 often, it's build on a 10MHz base.
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14 For quite a while now, I have a used rubidium frequency and time
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15 reference (Efratom Model LPRO-101) waiting for a nice enclosure. I
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16 also have a small Extron Electronics video signal amplifier I thought
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17 I could use to make my freq reference able to provide its signal to
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18 several test equipments at once.
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20 The Rubidium Frequency Reference
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21 ================================
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22
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23 The device is a small enclosure with only a 2x5 pins connector:
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24
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25 .. image:: {static}/images/freq_ref/efratom.jpg
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26 :alt: The Efratom LPRO-101 frequency standard.
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28 The exact model is an Efratom Rubidium Frequency Standard Model LPRO-101:
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30 .. image:: {static}/images/freq_ref/efratom_sticker.jpg
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31 :alt: The Efratom LPRO-101 frequency standard.
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33 Inside the enclosure, there is a single PCB with SMD and through hole
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34 components. It's quite dense.
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36 .. image:: {static}/images/freq_ref/efratom_inside.jpg
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37 :alt: The PCB of the Efratom LPRO-101 frequency standard.
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38
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39 The rubidium lamp with the resonant cavity occupy almost a quarter of the
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40 total area. The cover of the enclosure is compartimented, mainly to
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41 shield the lamp+cavity device from the remaining of the board:
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42
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43 .. image:: {static}/images/freq_ref/efratom_enclosure.jpg
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44 :alt: The enclosure of the Efratom LPRO-101 frequency standard.
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47 The Rubidium Freq Ref seems to workjust fine. I've powered it a few
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48 times, and it take a few minutes to lock the lamp and stabilize the
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49 output frequency. I've made a quick measurement of the power
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50 consumption; at startup time, the current goes up to 1.5A, but after a
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51 while, it stabilize around 0.5 or 0.6A. According a voltage 24VDC, it
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52 starts with 36W then goes down to 15 or 16W.
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53
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54 Now, I have to embed it in a nice enclosure (preferably an aluminium
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55 one so I can use is as a heatsink, since the frequency reference
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56 produces a bit of heat.)
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58 The Extron Video Amplifier
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59 ==========================
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60
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61 The video amplifer on the other hand has a bit of a trouble:
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63 .. image:: {static}/images/freq_ref/extron.jpg
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64 :alt: the Extron Electronics video amplifier
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66 As you can see on the picture above, the inductor is broken, and there
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67 is no visible mark on it. This inductor is used to produce the
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68 symetric voltages required to poser the opamp used to dispatch the
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69 signal.
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71 The switching power modules used on the board is a LT1616_ (marked as
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72 LTNB on the S6 package). So I reversed engineered a bit the schematic
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73 of the power to try to guess what kind of value this double inductor
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74 could be, since the reference schematic in the datasheet does not use
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75 such a dual inductor(/transformer).
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77 Fro the schematic I could extract, it appeared that the design is in
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78 fact very close to the reference schematic proposed by LT in the
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79 datasheet_, in the Bipolar Output DC/DC Converter configuration page
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80 16:
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81
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82 .. image:: {static}/images/freq_ref/lt1616_bipolar_output_DC2DC.svg
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83 :alt: LT1616 Bipolar Output DC/DC converter.
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84 :align: center
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86 It produces +/-5V from the 12V input. But still, I did not know what
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87 kind of inductor/xformer I should use. I did not notice at first sight
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88 the two references for the inductor that are proposed in the reference
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89 design. So I first searched using the nice faceted search tool of
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90 digikey (reducing the selection according the informations I have on
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91 the broken device and then looking at the pictures), I finally could
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92 find the reference of the inductor. By the way, it's a Sumida CLS62
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93 Series inductor (as indicated in the datasheet). And according to the
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94 reference design, it should be a 22µH one. So let's make a purchase
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95 for this inductor as well as some 20k resistors for my HP8904A_.
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97 .. _HP8904A: {filename}/hp8904a.rst
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101
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102 .. _LT1616: http://www.linear.com/product/LT1616
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103 .. _datasheet: http://cds.linear.com/docs/en/datasheet/lt1616fs.pdf

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