content/eip545b_6.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
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Update URLs for pcb & firmware to sourcehut

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1 ========================================
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2 EIP 545B RF Frequency Counter - Part 6
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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, RF, EIP, 545, 545A, 545B, 575A, 578A, counter
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8 :series: EIP545B Frequency Counter
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9 :series_index: 6
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12 Band 3 Sensitivity
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13 ==================
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14
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15 I have been trying to figure out a bit more about the `sensitivity problem
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16 <{filename}/eip545b_3.rst#sensitivity-problem>`_ described in `part 3
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17 <{filename}/eip545b_3.rst>`_.
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19 The main problem remains the first YIG sphere that has fall off the holding
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20 rod. I have tried to manipulate it with thin wood stick, under my cheap USB
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21 microscope, but as expected, this operation is *very* difficult.
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22
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23 So for now, this is not fixed at all.
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24
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25 However, I was wondering if this YIG sphere was the only problem involved in
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26 this lack of sensitiy.
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27
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28 As far as I can understand how the unit works, there are 2 level-related output
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29 signals from the A203 assembly: the Band 3 RF level signal and the IF threshold
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30 signal.
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31
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32 The overall process of the Band 3 is as follow:
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33
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34 .. image:: {static}/images/eip545b/band_3_operations.png
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35 :alt: Simplified Band 3 Operations
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37 For which the first step is the search for signal:
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39 .. image:: {static}/images/eip545b/band_3_search.png
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40 :alt: Band 3 Search for Signal
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41
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42 I seems to me that the RF level is used for two purposes: in the search for
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43 signal operation first, to measure the RF level then.
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44
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45 The RF level is measured from the so called "Video Amplifier", which can has a
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46 gain of around one, or can be set to +15dB.
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47
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48 So another possible culprit could be the this section of the Video Amplifier.
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49
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50 Unfortunately, the schematics of the A203 assembly is not available in the
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51 service manual:
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52
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53 The assembly drawing and schematic for both the VCO and IF circuits are not
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54 available. The entire A203 assembly must be tested as a complete unit to
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55 ensure proper performance of the counter. Repair of the Microwave (YIG)
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56 module can only be done at the factory. The VCO and IF Amplifier boards
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57 require special test equipment, therefore field repair is not recommended.
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58
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59 RF and video amplifier
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60 ======================
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61
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62 In order to understand, I have drawn the schematics of the board on which are
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63 the IF amplifier and the video amplifier:
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65 .. image:: {static}/images/eip545b/A201_2020303.jpg
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66 :alt: A201 RF amplifier board
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68 Fortunately, the board is quite simple, and despite being made of SMD
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69 components, single sided.
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71 The schematics looks like:
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72
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73 .. image:: {static}/images/eip545b/A201.svg
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74 :alt: Schematic of the A201 RF amplifier board
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76 The video amplifier consist of a simple MC1458 opamp (mainly a dual 741).
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78 The 15dB gain is selected by an input pin, leveled by the first half of the
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79 MC1458 (used as a comparator) driving of transistor. The marking on the SOT-3
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80 package is 6A Y, which is not easy to find. I finally figured it might be
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81 2SJ106, a JFET transistor.
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83 Before finding the kind of transistor it is, I though it was faulty, because a
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84 simple measurement of the DS with an ohmmeter gives a low value (around 200
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85 ohms). It took me a little while to rememeber it's a normal behaviour for this
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86 kind of transistor (the Drain and the Source being two sides of a single piece
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87 of silicon).
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88
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89 I removed it from the board, but a quick check using a 20$ LCR/transistor meter
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90 showed it looks fine.
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91
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92 However, I also need to have a rough understanding of the YIG filter and mixer
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93 assembly.
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96 YIG filter filter and mixer assembly
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97 ====================================
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98
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99 The YIG tuned filter and mixer assembly looks like:
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100
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101 .. image:: {static}/images/eip545b/YIG-mixer.jpg
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102 :alt: YIG filter output and mixer assembly
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103
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104 Once again, I am trying to understand what this whole thing works, but I really
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105 don't have the knowledge to do so, really.
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106
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107 Looking at this macro photo of the output stage of the YIG filter, it looks
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108 like a single diode balanced mixer. The LO signal, provided by the VCO, is
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109 coupled with the input signal comig from the YIG filter.
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110
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111 It looks to me that the first square block after the coupler could be a mixer
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112 diode (otherwise I really don't understand how this could act as a mixer).

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