Connect with us

IF Amplifier input impedance value ???

Discussion in 'Electronic Design' started by sparks, Nov 15, 2008.

Scroll to continue with content
  1. sparks

    sparks Guest

    Hi,
    I wonder if anyone can work out the input impedance of the 1st IF Amp
    in the circuit Ive linked to below.
    The frequency of the IF is 1.4MHz and Im trying to work out what the
    termination impedance is of the crystal filters. The filters are SEI
    brand made in the 1960's. Ive also been trying to locate some data on
    the filters used, but its not getting to any facts.
    I have stored a jpeg of the circuit here ....
    http://www.box.net/shared/8zrb3embil

    Any comments on this would be greatly apreciated. Ive tried to use
    Thevenins theorem on this, but the combination of AC and DC feedback
    from collector to base is beyond me in terms of what it does to poor
    old Thevenin.
    regards,
    JEFF
    ZL3JK
    New Zealand
     
  2. Phil Allison

    Phil Allison Guest

    "sparks"

    ** I reckon there is very little IF signal appearing across R57 (150 ohms)
    as the primary Z of T13 is much higher. So, the input Z is simply R54 plus
    R57 ( ie 300ohms) in parallel with the input Z at the base of VT13.

    ( VT13 = RCA 40235, Hfe =170, Ft = 1GHz )

    But VT13 has an unbypassed emitter resistor of R58 (27 ohms), so its base Z
    is many times higher.

    So, the load as seen by the Xtal filters is fairly close to 300 ohms, say
    250 ohms.

    Seems a not unlikely value for Xtal filters.



    ...... Phil
     
  3. Andrew Holme

    Andrew Holme Guest

    View in a fixed pitch font:

    150 150
    Vin Iin ___ V2 I2 ___
    -------->---|___|----o--->--|___|----.
    | |
    | |
    / \ |
    ( | ) Ic=Vin/27 |
    \V/ |
    | |
    | |
    === ===
    GND GND


    Iin = Ic + I2 = Vin/27 + I2 = Vin/27 + V2/150

    I2 = V2/150 = (Vin - 150*In)/150

    2*Iin = Vin/27 + Vin/150

    Vin/In ~ 50 Ohms

    The LTSpice simulation below seems to agree with this.

    Do those filters contain internal matching transformers?

    Version 4
    SHEET 1 880 680
    WIRE -80 -32 -224 -32
    WIRE 112 -32 -16 -32
    WIRE 288 -32 112 -32
    WIRE 448 -32 288 -32
    WIRE 688 -32 528 -32
    WIRE 288 0 288 -32
    WIRE -224 16 -224 -32
    WIRE 112 16 112 -32
    WIRE 688 16 688 -32
    WIRE 288 112 288 80
    WIRE -224 160 -224 96
    WIRE -96 160 -224 160
    WIRE 48 160 -96 160
    WIRE 112 160 112 96
    WIRE 112 160 48 160
    WIRE 224 160 112 160
    WIRE 688 160 688 96
    WIRE 288 240 288 208
    WIRE 48 288 48 160
    WIRE 656 336 496 336
    WIRE -224 352 -224 160
    WIRE 288 352 288 320
    WIRE 288 352 160 352
    WIRE 496 368 496 336
    WIRE 656 368 656 336
    WIRE 288 400 288 352
    WIRE 160 416 160 352
    WIRE 496 480 496 448
    WIRE 576 480 496 480
    WIRE 656 480 656 448
    WIRE 656 480 576 480
    WIRE -224 528 -224 432
    WIRE 48 528 48 368
    WIRE 160 528 160 480
    WIRE 288 528 288 480
    WIRE 576 528 576 480
    FLAG -224 528 0
    FLAG 48 528 0
    FLAG 288 528 0
    FLAG 688 160 0
    FLAG 160 528 0
    FLAG 576 528 0
    FLAG -96 160 Vin
    SYMBOL res 640 352 R0
    SYMATTR InstName R1
    SYMATTR Value 4k7
    SYMBOL res 544 -48 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R2
    SYMATTR Value 150
    SYMBOL res 272 224 R0
    SYMATTR InstName R3
    SYMATTR Value 27
    SYMBOL res 272 384 R0
    SYMATTR InstName R4
    SYMATTR Value 220
    SYMBOL res 96 0 R0
    SYMATTR InstName R5
    SYMATTR Value 3k3
    SYMBOL res -240 0 R0
    SYMATTR InstName R6
    SYMATTR Value 150
    SYMBOL res 32 272 R0
    SYMATTR InstName R7
    SYMATTR Value 3k3
    SYMBOL cap 144 416 R0
    SYMATTR InstName C1
    SYMATTR Value 100n
    SYMBOL cap -16 -48 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C2
    SYMATTR Value 100n
    SYMBOL current -224 352 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName I1
    SYMATTR Value SINE(0 1u 1.4e6)
    SYMBOL voltage 688 0 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 12
    SYMBOL ind2 272 -16 R0
    SYMATTR InstName L1
    SYMATTR Value 5m
    SYMATTR Type ind
    SYMBOL ind2 480 352 R0
    SYMATTR InstName L2
    SYMATTR Value 5m
    SYMATTR Type ind
    SYMBOL npn 224 112 R0
    SYMATTR InstName Q1
    TEXT 406 160 Left 0 !.tran 5u
    TEXT 408 88 Left 0 !K L1 L2 1
     
  4. Andrew Holme

    Andrew Holme Guest

    The above ignores re which is about 2 ohms, since Ie ~ 14mA.

    This gives Ic = Vin/29 which gives Vin/Iin even closer to 50 ohms.

    A surprising result for matching crystal filters.
     
  5. sparks

    sparks Guest

    Thanks for the help on this guys.
    In the interim, Ive had some advice from someone who used to work at
    Redifon (the radio's manufacturer) and he says the filters they used
    were sometimes (but not always, depending on radio model) 50
    ohms.These filters used small internal matching transformers inside
    the case. There were other options however for filter value such as
    280 and 680 ohms.
    So it looks like the filters are 50 ohms by your calculations....
    Thats great confirmation.
    regards,
    JEFF
     
  6. sparks

    sparks Guest

    Guys,
    I did chop the circuit off a it too cropped. The line going down off
    screen via C37 is going to an AGC diode attenuator network, so
    basically the diodes only conduct when a reasonanble signal is present
    to reduce the gain. Otherwise C37 is floating.
    JEFF
     
  7. sparks

    sparks Guest

    Hi Guys,
    Ive placed another circuit of the IF Amp on box.net, which contains
    the missing diode atten network. Its here...
    http://www.box.net/shared/hvem3ip2zx

    I have another 2 related questions on the design I wonder if you could
    help me with please ...

    What do I need to change in the circuit so to increase the input
    impedance to 910 ohms (so I can use another more modern brand of
    filter) ?
    Would this alteration to 910 ohms still be consistent if the IF freq
    was moved up to 10.7Mhz, as this is the preferred filter freq Id like
    to use.

    regards,
    JEFF
     
  8. Andrew Holme

    Andrew Holme Guest

    Remove R54. Reduce R55 and R56 to 1K8.
    Should be. T13 might be less efficient at 10.7 MHz.

    Are there any tuned circuits in the later IF stages? What about the BFO?
    Changing IF frequency sounds a bit drastic.
     
  9. Andrew Holme

    Andrew Holme Guest

    CORRECTION:
    Just tried this in simulation and it doesn't work. This is better:

    Leave R55 and R56 alone.
    Remove R54.
    Short the cold end of C34 to ground and/or short-out R57
     
  10. sparks

    sparks Guest

    Thanks Andrew. Im wanting to use a wider filter and hence the change
    to 10.7Mhz IF where I can obtain suitable (free) 8 pole filters of
    various bandwidths.
    These filters are 910 ohms, hence the question re changes required to
    accommodate the 910 termination impedance.
    Its probably likely that the transistors used in the IF wont be too
    good at 10.7Mhz, but I havent checked their spec yet.
    Yes, there is a couple of tuned circuits in the last part of the IF.
    Noise shaping the manual says they are for. The first is balanced
    output to accommodate a balanced crystal filter config for CW
    reception, but I plan to gut it, and use only one tapped tuned circuit
    to match into the detector. BFO is present but not required.
    How easy / expensive is LTSpice? Ive heard of it, but never seen it in
    action.
    JEFF
     
  11. Andrew Holme

    Andrew Holme Guest

    LTSpice is easy to use and free!

    40235, 2N3904 and 2N3906 should be fine at 10.7 MHz
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-