content/eip545b_3.rst

Thu, 10 Nov 2022 15:31:26 +0100

author
David Douard <david.douard@sdf3.org>
date
Thu, 10 Nov 2022 15:31:26 +0100
changeset 139
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Add a mastodon back url in the credits footer

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1 ========================================
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2 EIP 545B RF Frequency Counter - Part 3
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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, 545B, counter
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8 :series: EIP545B Frequency Counter
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9 :series_index: 3
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10
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11
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12 Default settings
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13 ================
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14
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15 One of the annoying thing with this 'SPECIAL' edition of the firmware are the
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16 default settings:
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17
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18 - Frequency Offset -160MHz
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19 - Resolution 5 digits
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20
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21 One of my goals while disassembling the formware was to 'fix' these defaults
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22 settings. I was wondering wether they where located in an undocumented part of
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23 the EEPROM or directly as hardcoded values in the firmware itself.
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24
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25 I have partially understood the setup routine, the one called from the RST
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26 vector; the interrupt vector is located at addresses 0xFFF2-0xFFFF, with the
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27 RST routine address being at 0xFFFE. In my case, the RST handler is located at
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28 0x5F19:
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29
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30 ::
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31
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32 FFF2 svec_SWI3 FDB hdlr_SWI3 ;08 20
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33 FFF4 svec_SWI2 FDB hdlr_SWI2 ;9E 90
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34 FFF6 svec_FIRQ FDB hdlr_FIRQ ;48 45
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35 FFF8 svec_IRQ FDB hdlr_IRQ ;70 F1
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36 FFFA svec_SWI FDB hdlr_NMI ;00 00
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37 FFFC svec_NMI FDB hdlr_NMI ;00 00
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38 FFFE svec_RST FDB hdlr_RST ;5F 19
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39
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40
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41 But completely disassembling the whole RST routine is slow and difficult. At
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42 some point, I found that a smarter approach would be to compare the content of
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43 the RAM after a fresh start and after having modified some configuration
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44 values, like the Frequency Offset.
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45
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46 The RAM addresses where the offset values are documented in the Service Manual,
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47 unfortunately, each model as its how address map, and no available Service
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48 Manual covers the EIP 545B. So I first needed to find where the frequency
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49 offset is stored in RAM so I can then look where this area is modified in the
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50 firmware.
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51
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52 Thanks to the TEST10 allowing to read any address in the address space (be it
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53 RAM, EEPROM, EPROM or even I/O registers), I wrote a simple Python script to
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54 read the whole content of the RAM via GPIB.
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55
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56 It's very slow, since I did not find a way using GPIB commands to read 2 memory
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57 addresses without quitting the TEST10 mode between two reads. And I had to add
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58 several sleep() since the EIP is pretty slow to respond to GPIB commands. A
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59 complete dump of the 2ko RAM took somethin like 15 or 20 minutes.
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60
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61 I did not even wrote a script, in fact, just throw some code in an IPython
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62 session. However, the crude code looks like:
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63
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64
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65 .. code-block:: python
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66
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67 import serial
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68 cnx = serial('/dev/ttyUSB0', baudrate='115200')
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69 cnx.write('++addr 17\r\n')
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70
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71 def rd(x):
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72 cnx.write('TA10\r\n')
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73 time.sleep(0.1)
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74 cnx.write('%04X\r\n' % x)
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75 time.sleep(0.1)
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76 cnx.write('++read\r\n')
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77 time.sleep(0.05)
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78 r = cnx.read_all()
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79 cnx.write('TP\r\n')
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80 return r
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81
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82 def getmem(n=0x800):
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83 out = []
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84 for x in range(n):
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85 v = rd(x).strip()
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86 v = v.split('-')
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87 out.append(v)
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88 time.sleep(0.5)
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89 return out
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90
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91 ramcontent = getmem()
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92 # [...] write ramcontent in a file
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93
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94
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95 This allowed me to quickly find where the Frequency Offset is stored:
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96 0x0276-0x027C (using the format described in the service manual).
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97
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98 For the number of digits, I'm not sure yet, but it looks to be located at
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99 0x0045 or 0x0046.
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100
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101 So I could then look for this Freq. Offset address (OxO276) in the firmware:
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102 It's used directly only a couple of times in the code, one of them lloking
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103 like:
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104
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105 ::
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106
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107 6118 LDX #M0276 ;8E 02 76
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108 611B LDA #$01 ;86 01
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109 611D STA ,X ;A7 84
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110 611F STA $02,X ;A7 02
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111 6121 LDA #$60 ;86 60
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112 6123 STA $03,X ;A7 03
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113
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114
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115 in which we can see that the value 0x01 (from register A) is stored at address
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116 0x0276 and 0x278 (thanks to the STA $02,X instruction, with X being 0x0276 when
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117 this former instruction is executed, it stores the content of A (0x01) at
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118 addres 0x0276+2=0x278), and the value 0x60 is stored at 0x0279 (STA $03,X).
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119
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120 It worth noting that this snip of code is executed shortly after intializing the
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121 RAM content to zero.
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122
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123 So the result of this piece of code is that the content of the RAM at this
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124 0x0276 address is:
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125
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126 ..
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127
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128 01 00 01 60 00 00 00
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129
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130 ie., conververted in frequency: -160MHz, the initial value I want get rid of!.
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131
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132 So I just have to replace these code by NOP instructions and it should be fine
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133 for the Frequency Offset, which I haven't done yet (I was waiting for a cheap
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134 EPROM eraser from eb, which just arrived).
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135
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136 Concerning the resolution, some clue can be found just after the previous
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137 routine::
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138
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139 6125 LDX #M005D ;8E 00 5D
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140 6128 LDA #$0D ;86 0D
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141 612A STA $01,X ;A7 01
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142 612C LDA #$05 ;86 05
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143 612E STA >M0045 ;B7 00 45
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144
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145 In which the value 0x0D is written at RAM address 0x005E (0x005D + 0x01 from
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146 the opcode ``STA $01,X``), and the value 0x05 is written at RAM address 0x0045,
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147 which we identified previously as a candidate for the digits resolution
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148 setting; pretty consistant with the 0x05 value written there. Not sure what the
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149 0x0D is used for at address 0x005E.
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150
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151
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152 .. _sensitivity-problem:
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153
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154 The sensitivity problem
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155 =======================
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156
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157 I have also investigated a bit the sensitivity problem I have on band 3: I can
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158 hardly detect signals lower than -10dBm at 1 or 2GHz, where it should be able
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159 to catch a -30dBm.
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160
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161 I've tried to recalibrate (on that frequency range) the YIG filter, it did not
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162 help.
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163
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164 Following the advise of HighPrecision on the eevblog forum, I did give a check
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165 at the SMD capacitors in the IF amplifier/detector detector part of A201. I
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166 still need to investigate more, but I haven't found an obvious culprit in
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167 there.
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168
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169 But since I had disassembled the A203 unit, I also gave a closer look at the
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170 YIG filter itself, taking pictures with my small USB microscope.
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171
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172 On one side is the ceramic support for connections from the output YIG coupling
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173 loop:
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174
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175 .. image:: {static}/images/eip545b/yig_top.jpg
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176 :alt: Top view of the YIG filter
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177
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178 On the bottom side, we can see the YIG spheres and the coupling loops:
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179
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180 .. image:: {static}/images/eip545b/yig_bottom.jpg
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181 :alt: Top view of the YIG filter
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182
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183 So this filter consists in a 2 stage bandpass filter (with input and output
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184 couplig loops at 90° each other).
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185
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186 Having a closer look, the problem appears:
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187
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188 .. image:: {static}/images/eip545b/yig_broken.jpg
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189 :alt: Broken stage 1 of the YIG filter
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190
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191 As can been seen there, the YIG sphere took off the holding rod. Tha cage made
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192 of the 2 coupling loops is small enough the sphere stayed there, but it is not
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193 at the center of the 2 loops any more, nor it is correctly orientated. No
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194 surprise the input signal is so low (in fact the suprise is that is still works
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195 somehow).
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196
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197 I tried to move the sphere a bit, using a sharp (as sharp as possible) wooden
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198 stick under the USB microscope, but it is really hard to orientate the sphere
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199 (which diameter is around 0.1 or 0.2mm).
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200
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201 I'll try again, just in case, since I can see the mark of the rod on the
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202 sphere, I think I may be able to roughly reorientate the sphere, but it is
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203 really hard to do without micrometric actuators.
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204
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205 Meanwhile, I've bought on eb a cheap (20$) A203 assembly. It's not exactly the
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206 same model (it's probably one for the 26.5GHz version, according to
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207 HighPrecision), we'll see if it works. I hope it's a 127MHz version, otherwise
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208 I'll have to replace the YIG filter (A201) on my A203 unit with this one.
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209
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210 By the way, for thoses interested, I found these 2 simple documents interesting
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211 to begin to understand how a YIG oscillator and a YIG filter works:
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212
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213 - `YIG TUNED OSCILLATORS`_
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214 - `YIG TUNED FILTERS`_
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215
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216 .. _`YIG TUNED FILTERS`: http://www.microlambdawireless.com/uploads/files/pdfs/ytfdefinitions2.pdf
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217 .. _`YIG TUNED OSCILLATORS`: http://www.microlambdawireless.com/uploads/files/pdfs/ytodefinitions2.pdf

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