comparison: content/eip545b_6.rst
content/eip545b_6.rst
- changeset 110
- 64b852fb7470
- child 115
- 6b6e13653348
equal
deleted
inserted
replaced
| 109:96e9f64b86ed | 110:64b852fb7470 |
|---|---|
| 1 ======================================== | |
| 2 EIP 545B RF Frequency Counter - Part 6 | |
| 3 ======================================== | |
| 4 | |
| 5 :author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: test equipment, RF, EIP, 545, 545A, 545B, 575A, 578A, counter | |
| 8 :series: EIP545B Frequency Counter | |
| 9 :series_index: 6 | |
| 10 | |
| 11 | |
| 12 Band 3 Sensitivity | |
| 13 ================== | |
| 14 | |
| 15 I have been trying to figure out a bit more about the `sensitivity problem | |
| 16 <{filename}/eip545b_3.rst#sensitivity-problem>`_ described in `part 3 | |
| 17 <{filename}eip545b_3.rst>`_. | |
| 18 | |
| 19 The main problem remains the first YIG sphere that has fall off the holding | |
| 20 rod. I have tried to manipulate it with thin wood stick, under my cheap USB | |
| 21 microscope, but as expected, this operation is *very* difficult. | |
| 22 | |
| 23 So for now, this is not fixed at all. | |
| 24 | |
| 25 However, I was wondering if this YIG sphere was the only problem involved in | |
| 26 this lack of sensitiy. | |
| 27 | |
| 28 As far as I can understand how the unit works, there are 2 level-related output | |
| 29 signals from the A203 assembly: the Band 3 RF level signal and the IF threshold | |
| 30 signal. | |
| 31 | |
| 32 The overall process of the Band 3 is as follow: | |
| 33 | |
| 34 .. image:: {filename}images/eip545b/band_3_operations.png | |
| 35 :alt: Simplified Band 3 Operations | |
| 36 | |
| 37 For which the first step is the search for signal: | |
| 38 | |
| 39 .. image:: {filename}images/eip545b/band_3_search.png | |
| 40 :alt: Band 3 Search for Signal | |
| 41 | |
| 42 I seems to me that the RF level is used for two purposes: in the search for | |
| 43 signal operation first, to measure the RF level then. | |
| 44 | |
| 45 The RF level is measured from the so called "Video Amplifier", which can has a | |
| 46 gain of around one, or can be set to +15dB. | |
| 47 | |
| 48 So another possible culprit could be the this section of the Video Amplifier. | |
| 49 | |
| 50 Unfortunately, the schematics of the A203 assembly is not available in the | |
| 51 service manual: | |
| 52 | |
| 53 The assembly drawing and schematic for both the VCO and IF circuits are not | |
| 54 available. The entire A203 assembly must be tested as a complete unit to | |
| 55 ensure proper performance of the counter. Repair of the Microwave (YIG) | |
| 56 module can only be done at the factory. The VCO and IF Amplifier boards | |
| 57 require special test equipment, therefore field repair is not recommended. | |
| 58 | |
| 59 RF and video amplifier | |
| 60 ====================== | |
| 61 | |
| 62 In order to understand, I have drawn the schematics of the board on which are | |
| 63 the IF amplifier and the video amplifier: | |
| 64 | |
| 65 .. image:: {filename}images/eip545b/A201_2020303.jpg | |
| 66 :alt: A201 RF amplifier board | |
| 67 | |
| 68 Fortunately, the board is quite simple, and despite being made of SMD | |
| 69 components, single sided. | |
| 70 | |
| 71 The schematics looks like: | |
| 72 | |
| 73 .. image:: {filename}images/eip545b/A201.svg | |
| 74 :alt: Schematic of the A201 RF amplifier board | |
| 75 | |
| 76 The video amplifier consist of a simple MC1458 opamp (mainly a dual 741). | |
| 77 | |
| 78 The 15dB gain is selected by an input pin, leveled by the first half of the | |
| 79 MC1458 (used as a comparator) driving of transistor. The marking on the SOT-3 | |
| 80 package is 6A Y, which is not easy to find. I finally figured it might be | |
| 81 2SJ106, a JFET transistor. | |
| 82 | |
| 83 Before finding the kind of transistor it is, I though it was faulty, because a | |
| 84 simple measurement of the DS with an ohmmeter gives a low value (around 200 | |
| 85 ohms). It took me a little while to rememeber it's a normal behaviour for this | |
| 86 kind of transistor (the Drain and the Source being two sides of a single piece | |
| 87 of silicon). | |
| 88 | |
| 89 I removed it from the board, but a quick check using a 20$ LCR/transistor meter | |
| 90 showed it looks fine. | |
| 91 | |
| 92 However, I also need to have a rough understanding of the YIG filter and mixer | |
| 93 assembly. | |
| 94 | |
| 95 | |
| 96 YIG filter filter and mixer assembly | |
| 97 ==================================== | |
| 98 | |
| 99 The YIG tuned filter and mixer assembly looks like: | |
| 100 | |
| 101 .. image:: {filename}images/eip545b/YIG-mixer.jpg | |
| 102 :alt: YIG filter output and mixer assembly | |
| 103 | |
| 104 Once again, I am trying to understand what this whole thing works, but I really | |
| 105 don't have the knowledge to do so, really. | |
| 106 | |
| 107 Looking at this macro photo of the output stage of the YIG filter, it looks | |
| 108 like a single diode balanced mixer. The LO signal, provided by the VCO, is | |
| 109 coupled with the input signal comig from the YIG filter. | |
| 110 | |
| 111 It looks to me that the first square block after the coupler could be a mixer | |
| 112 diode (otherwise I really don't understand how this could act as a mixer). |
