Yaesu 857d IF tap receive issue

Discussion in 'Radio Circuits, Repair & Performance' started by 2E0AJX, May 2, 2018.

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  1. 2E0AJX

    2E0AJX Ham Member QRZ Page

    Hello guys/gals, I hope someone can help me.

    I've used an SMA pigtail to tap the IF on my 857d so I can use it with an SDR dongle. I soldered the coax directly to the board, and am coupling to it with a capacitor and resistor outside the unit so I can tweak the values with out reopening the radio.

    With the SDR and coupling box disconnected the radio functions perfectly. With it connected, the SDR works great, and I can see the band the 857 is tuned to in HDSDR, but the signal into the radio drops considerably.

    Without, I get S9 or S9+ on my local repeater output, with it connected I get S3. Other than that it all seems ok with no other issues.

    It's coupled via a 100pf capacitor, then a 150ohm resistor as per the instructions I followed. I've switched the circuit for two trimmer pots in parallel so I can tweak the values, but I'm looking for some suggestions.

    Thanks in advance!!
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Is the resistor in series with the feed?

    If so, try bigger values (I'd start at 1k ) until it doesn't load the IF (and reduce the signals on the rig).

    You should still get enough signal to drive your SDR Dongle.

    Roger G3YRO
    2E0AJX likes this.
  3. 2E0AJX

    2E0AJX Ham Member QRZ Page

    Thanks for this, will try it later on. I've got a 10k trimmer pot that I can play with to vary the resistance :)

    Will post back with results later!
  4. AF7XT

    AF7XT Ham Member QRZ Page

    I will explain in detail if needed.
    The 857 IF will be loaded by any passive attempt to connect an SDR.
    A far better method is an Hi-Z in Low-Z out buffer amp.
    Even with G4HUP's passing it seems the board is available.
    On the Huprf site is a link to a youtube example of setup with HDSDR.

    Should be enough to get you going. Again, there is no way I'm aware of that will get enough isolation with passive components.
    CX3CP and 2E0AJX like this.
  5. 2E0AJX

    2E0AJX Ham Member QRZ Page

    Thanks for this. I've tried my setup with the potentiometer between 0 and 10k ohms, and it doesn't seem to make any difference to the receive on the radio, although you can visibly see it decreasing the signal on the SDR. Should I try the resistor before the capacitor rather than after? Or am I just in need of something different as you suggest.

    I don't quite understand that you mean by Hi-Z/Lo-Z, if you wouldn't mind explaining further that would be most helpful :)
  6. AF7XT

    AF7XT Ham Member QRZ Page

    High impedance input will not load the circuit under inspection. In this case the intermediate frequency amplifier chain. The symptom of "loading" the amplifier chain is seen as desensitization of the over all receive.

    An amplifier of some sort is necessary in this case an operational amplifier having a Hi-Z input and a Low-Z output. The amplifier needs little if any gain over unity and is there only to insulate the intermediate frequency chain from a Low impedance tap. Low impedance output for the SDR input is then sufficient to drive the SDR.

    It can not be done with passive components.
    2E0AJX likes this.
  7. 2E0AJX

    2E0AJX Ham Member QRZ Page

    Thank you again :). Sorry for my ignorance, I understand now that by Hi-Z you mean high impedance. I've been out of this for a while and was never really that experienced to begin with! Lots to learn.

    I've ordered a PAT 70M bard for my Yaesu after checking the link you provided. I'm hoping that one day I'll understand all this well enough to produce all these circuits myself. I can understand capacitance, resistance, inductance etc but impedance always eludes me.

    Will post back here after the new board arrives, but for the moment my little homebrew circuit is working, although limited.
  8. G3YRO

    G3YRO Ham Member QRZ Page

    Firstly - the resistor and capacitor you mention are both in series with the inner of the coax, right? (and not connected to chassis) If so, it doesn't matter which is inline first.

    But the whole point is that once you have a 10k resistor in series, that IS providing a fairly high impedance from the rig. So I am surprised that it is loading it at all !

    Just as a test, try a 47k fixed resistor, and see if that still loads the receiver signals.

    Obviously, that resistor will reduce the signal fed to your SDR dongle . . . but as long as you still have ENOUGH signal, it shouldn't be a problem.

    The point is that your SDR dongle is designed to receive tiny signals from an aerial . . . not an amplified output from the IF ! So you really shouldn't need a buffer amplifier to create a high impedance . . . a series resistor should be all you need to NOT load the receiver IF.

    Understanding Impedance used to be the kind of basic stuff you had to learn to get a licence !
    Basically, a resistor has the same resistance to AC as DC. An inductor has varying resistance to AC, which depends on the frequency of the signal, which is called Inductive Reactance. A Capacitor also has varying resistance to AC, which again depends on the frequency of the signal - this is called Capacitive Reactance. Impedance is just the combination of all three factors in a circuit - Resistance, and both types of Reactance.

    But your series resistor I am suggesting will have a high impedance to an AC (ie RF) signal, almost equal to its resistance value. (slight capacitance across the ends will reduce the impedance slightly at very high frequencies)

    Roger G3YRO
    Last edited: May 3, 2018
    2E0AJX likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would try a 10 pf cap. Tapping into the IF can be tricky.

    The cap needs to be at the board, Because the capacitance of the coax will make a difference.

    Have Fun.
    KD2ACO likes this.
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Don's suggestion is critical here.
    In order to not load the IF circuit down, you need to put an isolation cap right at the board. Even a short piece of coax (and then the cap/resistor combo) will soak up your IF signal.

    When doing this, I've used a tiny cathode follower (boat anchorage) or source follower with a FET and mount that right next to where I want to tap the signal.

    The follower doesn't load down the IF circuit, and it drives the cable. Another way to think about this is that the follower isolates the cable and what's after it from the IF node on the board.

    for older tube radios...

    for modern radios.


    Some people like to keep it simple and not use an active device. As stated by others above, you do want some isolation and the best place to do that is right at the node on the board where you want to tap the signal from, with a very small value capacitor.

    Going the passive route, you may loose a couple dB of signal and if this is acceptable, then go that route because it is easy! :)

    Using an SMD cap... heat up the lead on the board and drop this right in there. Solder the coax to the free end.

    Last edited: May 3, 2018
    W2VW and KA9JLM like this.

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