Baseline distortion after filtering

Discussion in 'General Electronics Chat' started by Peter Pan, May 15, 2009.

  1. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Hello All,I have designed a highpass filter with 6.5 MHz cut-off frequency and this has brought severe distortions of baseline as shown on picture attached, where blue trace is input and red trace is signal after amplification and filtering. Picture of schematics and frequency response modelled in LTspice is also attached.My question is whether filter was poorly designed or this distortions is normal and has to be corrected further?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello Peter,

    Is the blue line also distorted when you shift it up?
    At the moment you can see that the blue line is cut off on the botom.
    You will not see what is below the bottom line.

    Greetings,
    Bertus
     
  3. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Hello Bertus,Perhaps blue trace is cut off on the bottom indeed but this is how my PC scope shows it. (Vertical axis's was set to 'Auto'.)The both traces were acquired using the same scope, in the same mode.Maybe some phasing has to be applied to the red trace in order to get it properly processed?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Is the shown trace a FT of a FID ?
    If so, what is the amplitude of the FID in comparison with full screenscale?

    Greetings,
    Bertus
     
  5. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Yes, red trace is Fourier Transform of a signal whose peak-to peak voltage in time domain is about 10 mV - much smaller than full screenscale.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What is the most sensitive setting of your scope in manual mode?
    If it is 2mV/Div the signal will be higher.

    Greetings,
    Bertus
     
  7. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    For time domain the most sensitive setting for Y-axis is 10 mV and in frequency domain it is 5 V. I have attached picture of signal acquired with these settings.Actually, I am interested whether filter is capable to produce such phase distortion upon signal as I described?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello Peter,

    That is hardly a signal.
    What happens when you measure it behind the amplifier (before the filter)?
    The resolution of the FT spectrum will increase when you increase the aquisition time.(more datapoints).

    Greetings,
    Bertus
     
  9. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Hello Bertus,I agree, on volt-scale it looks nothing. Anyway I have got a picture where blue trace is a signal after amplifier and before my filter; red trace is after the filter. Red trace is weaker than blue one because passive LC filter is used and this time red trace got some cut-off at bottom. It looks like weaker signals get cut-off at bottom on my oscilloscope.Thank you for your time! :)P.S. I will also play a bit with acquisition time to get better resolution.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello Peter,

    What circuit are you using as amplifier?
    Is the amplifier adding extra noise?

    Greetings,
    Bertus
     
  11. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
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    Hi Bertus,I use 3 stage amplifier as shown on the schematics attached. I am not sure how much noise this adds to signal - not much, perhaps, looking at its input and output. P.S. I am going to try a different schematics using MAR-6+ but at the moment not sure whether it will work much better or not.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Are you realy using 2N3904?
    Those are switching transistors and not very good for RF amplifiers.
    For RF amplifiers you can use BFY90 , BFR91 , BFR 96 or 2N3571.

    Greetings,
    Bertus
     
  13. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Okay.To be precise, 2N3904 is designed as a general purpose amplifier and switch. The useful dynamic range extends to 100 mA as a switch and to 100 MHz as an amplifier as http://www.fairchildsemi.com/pf/2N/2N3904.html says.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello Peter,

    For your information, a part of the list of transistors (including those for transmitters) from a local supplier specialized in RF parts.

    Code ( (Unknown Language)):
    1.  
    2. BIPOLAIRE TRANSISTOREN:
    3.  
    4. 2N2219A     Medium power NPN                                                0.54
    5. 2N2905A     idem PNP                                                        0.68
    6. 2N2907A     pnp hf/vhf                                                      1.25
    7. 2N2369      =BSX20  video/switching                                         0.36
    8. 2N3439      video                                                           0.79
    9. 2N3553      175 MHz 2.5W tx driver/output                                   2.72
    10. 2N3632      28V 14W VHF                                                    10.66
    11. 2N3866      2W VHF origineel Motorola!                                      3.50
    12. 2N4031      pnp 0.8W                                                        0.68
    13. 2N4045      Precisie dual NPN metal can                                    19.90
    14. 2N4427      VHF med.power metalcan                                          9.90
    15. 2N4031      pnp 0.8W                                                        0.68
    16. 2N5108      TO39 1W @ >1GHz                                                 3.50
    17. 2N5109      =BFW16A breedband tot 5 GHz                                     4.75
    18. 2N5179      VHF metalcan                                                    1.36
    19. 2N5322      TO39 PNP =BSS17 100V 2A 10W                                     0.45
    20. 2N5401      HF Transistor                                                   0.90
    21. 2N5551      Video/RF med.power 160V 0.6A 0.625W >100MHz TO92                0.45
    22. 2N5589      3W 12V VHF =2N6080 keramisch                                   15.88
    23. 2N5590      10W 12V VHF =BLY88C                                            20.42
    24. 2N5643      40W 28V VHF =2N6199                                            26.77
    25. 2N5944      2W UHF 12V zie ook BLW89                                       11.34
    26. 2N6081      VHF 15W 12V = BLY88C                                           22.81
    27. 2N6083      VHF 30W 12V                                                    32.00
    28. 2SC1678     3W PEP to 50MHz TO220                                           6.33
    29. 2SC1945     =2SC2509=MRF479=2SC3133 RF Power tot 30 MHz, 15W, TO220        12.95
    30. 2SC1946A    (MRF1946) verbeterde 2SC1946 VHF 12V 40W                       22.24
    31. 2SC1947     Mitsubishi 4W 175MHz 11dB gain TO5 metalcan als MRF237         11.90
    32. 2SC1969     RF power TO220 = MRF475 orig. Mitsubishi                       15.90
    33. 2SC1970     TO220 RF power orig. Mitsubishi                                 8.90
    34. 2SC1971     Origineel Mitsubishi                                           13.90
    35. 2SC2166     6W TO220                                                        9.90
    36. 2SC2290     Power                                                          39.90
    37. 2SC2547     Transistor                                                      0.25
    38. 2SC2879     Power output orig.Toshiba                                      52.90
    39. 2SC3240     Toshiba RF power transistor                                       59.90
    40. 2SD666      Transistor                                                      0.25
    41. BF199       NPN universeel HF                                               0.16
    42. BF324       PNP HF                                                          0.32
    43. BF370       NPN HF voor SAW filters                                         1.59
    44. BF420       NPN 300V 50mA 0.39W 60 MHz                                      1.34
    45. BF820       smd sot23 300V 50mA 80MHz npn                                   0.86
    46. BF821       idem pnp                                                        0.86
    47. BF979       PNP Ft=1.3 GHz 20V                                              1.02
    48. BFG11       Po=0.5W @ 1.9 GHz werkt al vanaf 3.6V                          11.34
    49. BFG65-3     Ft=8 GHz 3-pin                                                  1.82
    50. BFG91A      UHF/SHF TO50                                                    5.67
    51. BFG93A      VHF/UHF/SHF SOT143                                              3.90
    52. BFG97       Ft=5 GHz SOT223 npn                                             2.68
    53. BFG135      Ft=7 GHz SMD SOT223 (met koelvin)                               2.95
    54. BFG195      Extra low noise UHF                                             5.86
    55. BFG591      Breedband SOT223 20V-200mA-Ptot=2,00W FT=7 GHz Gp=13dB          5.90
    56. BFP196      Ft=8.5 GHz smd SOT143 C-E-B-E vervangt vele Philips en
    57.             Siemens typen. B.v. als driver voor BFG196                      4.49
    58. BFP405      Ft=20 GHz speciaal voor X-band (10GHz) DRO osc. (toepassings-
    59.             schema aanwezig)                                                5.67
    60. BFP420      Ft=25 GHz SOT343                                                5.90
    61. BFP450      Ft=15 GHz SOT343 Max.100mA                                      6.13
    62. BFQ19S      FT=5 GHz SOT89 (smd-met-vin) Pt=1W                              3.86
    63. BFQ24       pnp Ft=5 GHz                                                    5.40
    64. BFQ34       Final UHF 2.5W see also MRF587                                 18.90
    65. BFQ43       zie SD1127
    66. BFQ65       =BFG65 in 3-pin X-case     zie BFG65
    67. BFQ68       UHF 5 W (high power BFQ34) Output 1.5W @ 13cm                  39.90
    68. BFQ69       Ft=5.5 GHz 15 mA                                                2.95
    69. BFQ234      Hi-voltage 300mA SOT172 Ft=1GHz                                 4.50
    70. BFR34A      UHF                                                             4.49
    71. BFR90A      =BFR90 Lownoise                                                 1.13
    72. BFR91A      Lownoise                                                        1.13
    73. BFR92A      =BFR90A in SMD                                                  1.13
    74. BFR93A      =BFR91A in SMD                                                  1.13
    75. BFR94       zie BFT98
    76. BFR96S      Medium UHF power (NPN complementair BFT96)                      1.13
    77. BFR181      Ft=8 GHz, SOT23                                                 1.13
    78. BFR182W     Ft=8 GHz, SOT23                                                 2.90
    79. BFR193      see BFR181
    80. BFR520      Ft=9 GHz SOT23 Gain 15dB                                        5.00
    81. BFS17       SMD versie van BFW92                                            0.79
    82. BFS20       HF SMD                                                          0.90
    83. BFT65       NPN T-case Ft=4.5GHz zie ook BFQ65 or BFG65 (dual emitter)      1.59
    84. BFT66       Low noise VHF metalcan                                         17.20
    85. BFT77       metalcan VHF                                                    0.45
    86. BFT92       PNP 5 GHz SMD                                                   1.36
    87. BFW30       VHF/UHF metalcan 1.6GHz, vervangt ook 2N5179                    1.59
    88. BFW92A      UHF universeel T-case                                           0.82
    89. BFX34       120V 4A 100MHz                                                  3.86
    90. BFY90       VHF metalcan =BFX89                                             1.36
    91. BM30-12     30W VHF 12V 2 gats                                             19.00
    92. BSP19       smd NPN 400V 1A Ft=70 MHz                                       1.32
    93. BSS61       p darlington + diode                                            1.47
    94. BSV64       npn                                                             1.02
    95. BSV76       npn                                                             1.02
    96. BSW66A      npn                                                             1.59
    97. BSX20       zie ook 2N2369                                                  1.13
    98. BUS12       850/400V 12A =BUX48                                             3.40
    99. BUW13A      =BUS13A 1000/450V 175W                                          2.95
    100. BUZ54A      zie FETs
    101. KT920B      =BLY89C 22W VHF 12V keramisch                                  20.42
    102. KT922A      300 MHz 28V 5W stud                                             6.90
    103. KT922B      =BLY92 200 MHz 28V 15W stud                                    22.00
    104. MPSA29      N-Darlington TO92 100V 0.5A 0.625W Beta > 10000                 0.34
    105. MPSH10      Motorola NPN TO92 VHF/UHF                                       1.13
    106. MPSH81      Motorola PNP TO92 VHF/UHF                                       1.13
    107. MRF433      13W 12V 30 MHz                                                 13.57
    108. MRF450A     50W tot 30 MHz                                                 32.00
    109. MRF454      80W zie ook 2SC2290                                            69.00
    110. MRF455      =SD1446 60/75 W @ 30 MHz 12V                                   59.00
    111. MRF466      40W PEP 30MHz                                                  69.00
    112. MRF475-MOT  Origineel                                                      13.57
    113. MRF559      0.5W 870MHz X-can                                               5.59
    114. MRF581(A)   1W 1 GHz TO50                                                   6.58
    115. MRF587      3W UHF but usable from 5 MHz, High Gain Ft=5.5GHz
    116.             Gp=17dB@500MHz Lownoise met stud en moer                       12.90
    117. MRF603      10W VHF 12V                                                    15.88
    118. MRF604      1W VHF 12V kleine 3-pin metal can                               6.58
    119. MRF619      =BLU20-12 20W 470MHz flange-6 style                            25.00
    120. MRF654      18W @ 512MHz @ 12.5V > Po=19W @ Pi=2.5W                        17.90
    121. MRF901      UHF x-case NF=2.5dB @ 1GHz IP3=23dBm                            4.50
    122. NE288A      Ceramic-TO50 UHF transistor NEC                                 1.95
    123. NE476       Ceramic-TO50 UHF transistor NEC                                 1.95
    124. NE85635     Ft=7 GHz NF=1.1dB I=100mA micro-X case (maar lange pins) vervangt
    125.             nu vervallen Philips en Siemens BFG typen                       5.40
    126.  
    Here you will also find numbers for low noise transistors.

    Greetings,
    Bertus
     
    Last edited: May 16, 2009
  15. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Thank you, Bertus.What the numbers at far right column represent?
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello Vladimir,

    The numbers on the right side are the prices in euro's in holland.
    The list is taken from the guys website.
    http://www.xs4all.nl/~barendh/Indexned.htm
    Text is all in dutch.

    Greetings,
    Bertus
     
    Last edited: May 16, 2009
  17. bertus

    Administrator

    Apr 5, 2008
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  18. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Hello Bertus,Thanks a lot. :)Anyway I am going to try amplifier MAR-6+ (already have booked some of these in shop). It's low noise and cascadeble. I am going to use several stages of MAR-6+ together with passive LC filter in order to get decent SNR for useful signal and get rid of most of unwanted features in a spectrum.
     
  19. DickCappels

    Moderator

    Aug 21, 2008
    2,648
    632
    Awesome! I had been surfing the web, collecting numbers such as these in hopes of finding some of them at my local distribution.

    Thank you very much!!!!!:):):):):)
    (And I am not the kind of person to use exclamation marks freely).
     
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