Capacitor Selection for Capacitive Coupling of RF Signal

Discussion in 'Wireless & RF Design' started by mallred, Mar 2, 2015.

  1. mallred

    Thread Starter New Member

    Mar 2, 2015
    3
    0
    Hello,

    I want to couple the IF signal of an existing receiver to a secondary software defined receiver. I am trying to identify the correct capacitor to use for this coupling.

    I believe the impedance of the secondary receiver to be 50 ohms.

    The IF output is centered on 40.455Mhz.

    Can anyone help?

    Thanks,
    Mike
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Xc=1/(2∏fC)

    More importantly, is the primary receiver's IF buffered so that it can drive a low impedance cable going to the SDR? Does it have a jack labelled IF Out?

    If not, it is much more complicated than just sticking in a capacitor.
     
  3. mallred

    Thread Starter New Member

    Mar 2, 2015
    3
    0
    Mike,

    Thanks for the reply. I just found an online calculator using the same formula.

    There is no IF out, I will be tapping into the IF directly from a trace on the board.

    I cannot confirm that it is buffered. Others have used this method with success, however, their success is measured only by the fact that they are indeed getting signals to the SDR at a usable level. Furthermore, they have only been running it for a matter of days. So, long term success is unproven.

    The people that have done this seem to be selecting their coupling capacitors quite arbitrarily. That is what led me here. I was concerned about possibly damaging the IF circuitry due to too much load/impedance mis-match.

    I will say that other radios by the same manufacturer have been modified in this way by many and I haven't read reports of failures. Of course, that does not mean that this is the best way to do it.

    I certainly don't want to degrade the performance of the existing receiver or jeopardize any components therein.

    If the signal is strong enough to drive the SDR without the buffer amp, why else is one needed?



    Thanks Again,
    Mike
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    post a schematic of the mixer and 40MHz IF. There is likely a place where the signal can be brought out. It may not even need a coupling cap, depending on the dc level.

    To get the sdr to see as much bandwidth as possible, you should get as close to the mixer output as possible, likely come out ahead of any IF stages. To minimize loading, the cable to the SDR should be as short as possible.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    a very low capactance will do, just a few pf. he sdr is very sensative and requires little signal to recieve. are you going to use the sdr at the if frequency as more stages and as a detector? the old timers used what was called a q5'er a surpluss reciever tuned to the if frequency to add a bit more gain and selectivity to their recievers.
     
  6. mallred

    Thread Starter New Member

    Mar 2, 2015
    3
    0
    Thanks to both of you for your response.

    MikeML: I don't have a copy of the schematic, but I have requested a copy of the relevant sections from my colleagues. Hopefully I will receive a response soon. If not, I can purchase the service manual for ~$15. Someone that does have a copy has reported a suitable IF tapping point. I would certainly appreciate a second opinion from you though if I can produce the schematic.

    alfacliff: Thanks - Yes, my calculations show that somewhere between 3-5pf may be the best bet as research suggests that presenting a high impedance to the main receiver's IF stage is a good idea. I was hoping that the SDR would be sensitive enough to overcome the loss this higher impedance would present. My main priority is protecting the main receiver from ill effects of this project, so I'm willing to tolerate less than perfect performance from the SDR.

    All: I have found a couple of sources for low cost buffer amps that are advertized as suitable for this purpose. What is the advantage of adding one?


    Thanks Again,
    Mike
     
Loading...