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Looking for low-frequency PIN diode

Discussion in 'Electronic Design' started by [email protected], Jun 29, 2013.

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  1. Guest

    I’m developing a simple, five-frequency superhetrodyne receiver to operate between 2.5 MHz to 20 MHz in which the local oscillator crystals and whipantenna tuning circuits are selected (switched) using diodes. Here’s theproblem:

    The whip tuning circuit is comprised of a single variable capacitor with five fixed-value inductors, each in series with a BA277 "bandswtiching" diode.. A single diode/inductor combination draws about 6 mA when ON (switched in). This works fine, but at and above 15 MHz, the BA277 diode significantly contributes to the series resistance of the inductor, inducing a 3dB "insertion loss" (as compared to the same circuit with the diode short circuited)..

    The BA277 datasheet indicates that at 6 mA and 100 MHz, the series resistance of the diode should be much less than 1 Ohm. No performance data is given for any other frequencies. My bench measurements indicate that at 15 MHz,the diode presents much more resistance than that (probably tens of Ohms).No surprise there.

    QUESTION: The BA277 works OK for what I’m doing, but is there a bandswitching or PIN diode out there better suited for operation below 30 MHz?

    -Dave
     
  2. Dave, I don't know much about what you are doing.
    But isn't the dynamcial resistance of the diode the thermal voltage dividedby the current?
    so ~25mV/6ma ~ 4 ohms
    At 20 MHz you could use a swtich or a relay.

    George H.
     
  3. Looks like standard 1n914's were used in SSB radios for the Upper
    lower and AM inductors in single crystal PLL radios. It was actually a
    1S2075K, similar to the 1n914.
    The PLL crystals were ~11Mhz or so. Not sure of the diode current,
    doesn't look like more than 3ma.

    Cheers
     
  4. Phil Allison

    Phil Allison Guest

  5. Phil Allison

    Phil Allison Guest

    "Jeff Liebermann = trolling FUCKHEAD "


    ** Lose the FUCKING ASSHOLE ATTITUDE !!!

    You LAZY, STUPID, PIG IGNORANT

    RADIO HAM CUNTHEAD !!

    And fucking stop posting stupid damn lectures

    And then FOAD.




    ..... Phil
     
  6. Robert Baer

    Robert Baer Guest

    Try a transistor,lower drop in inverse ("up-side-down") configuration.
     
  7. Guest

    IIRC only the 1N4007 (the 1000 V version) has some kind of PIN
    action), the lower voltage versions don't.
     
  8. Guest

    How long is the whip ? It is going to have a very high capacitive
    reactance if it is less than 3 m (1/4 wave) at 25 MHz and much worse
    at lower frequencies.

    Is there a risk that a real outdoor antenna is to be connected to the
    receiver or is it just whip only ?

    There are several ways to connect an electrically short antenna into
    the receiver.

    One way is to put a equally large inductive reactance (loading coil)
    in series with the actual whip and there is a pure resistance between
    the antenna and the receiver input. However, the radiation resistance
    for such short antenna might only be a few ohms, so still some
    matching might be needed.

    Considering the atmospheric noise levels, it might be a good idea to
    have a resonant antenna (with loading coils) at 20 MHz, but at lower
    frequencies, there are still going to be sufficient signals available
    despite mismatch.

    An other approach is to use some kind of source follower as a voltage
    probe. The antenna capacitance of the whip (perhaps 10-100 pF) form a
    voltage divider with the gate/source capacitance of the FET.

    The voltage at the gate is then transferred to the resistor (or
    emitter follower) at the source electrode, which can be loaded with a
    typical 50 ohm receiver input impedance.

    Of course, the source follower should operate with a sufficient
    operating voltage (5-24 V), so that combined waveform from the antenna
    does not clip.

    This kind of construction is common in semiportable receivers with
    antenna input for a real outdoor antenna as well as equipped with a
    whip for portable operation. The outdoor antenna is connected directly
    to 50/75 ohm antenna input, but when whip antenna is selected, the
    whip and source follower drive the receiver input, instead of the
    outdoor antenna.

    What is the capacitance range of this capacitor ? If it is wide, then
    at Cmax and large inductor at 2.5 MHz or alternatively Cmin and
    smallest inductor at 20 MHz, the required inductance range is
    reasonable. However, with a narrow capacitance range, a 1:8 frequency
    range requires nearly 64:1 inductance range, which can cause problems
    with strays and impedance levels.

    Is this a mechanical variable capacitor or some varicap ?

    If this is a mechanical capacitor, I can understand using only one,
    but still I would consider switching in serial/parallel capacitors
    (which of course reduces tuning range), instead of simply inductor
    switching.

    With varicaps, why not simply use 5 varicaps and have 5 independent
    resonant circuits and select between these resonant circuits with PIN
    diodes. In this way, there are no lossy switching element _within_ the
    resonant circuit, which would dissipate the resonant current and hence
    lower the LC circuit Q and hence increase bandwidth.

    With separate resonant circuits, you can also tailor the impedance
    values as needed for each band, considering also the (short) antenna
    reactance.
     
  9. Guest

    Thank you for all the constructive suggestions. A few comments on your replies:

    Regarding the BA277, the manufacturer (NXP Semiconductors) bills the deviceas suitable for “Low loss band switching in VHF television tuners” anddoesn’t explicitly call it a PIN diode. If it is one, then compared to most PIN diodes, its carrier lifetime must be fairly long if it can be usedat VHF frequencies (30 to 300 MHz).

    I substituted the BA277 with 1N914 diodes in my prototype circuits and they’re definitely more lossy in the frequency range I’m working with (2.5 to 20 MHz). Therefore, the 1N914 and its equivalent, the 1N4148, are unsuitable.

    In my application, the capacitance is constant at all frequencies. Consequently, as someone correctly pointed out, the reactance of the tuned circuitis low at 20 MHz and so the BA277 diode in series with the inductor noticeably contributed to the loss (lowers the quality factor Q).

    Based on your posts, here are the two options I’ll consider:

    1) Keep the whip antenna tuning circuit as is (single tuning capacitor, multiple inductors switched with BA277 diodes) and live with the loss. This keeps the circuit as simple as possible.

    2) Redesign the tuning network to minimize the BA277’s contribution to the loss. This can be done with a capacitor (varactor) paired with each inductor, and switching these L/C pairs with BA277’s. This keeps the diode out of the L/C parallel combination.

    Thanks again!

    -Dave
     
  10. josephkk

    josephkk Guest

    Indeed. In a test lab we had no problem making 4-digit accurate and
    repeatable (4-terminal) measurements multiplexing scores of devices in
    thermal chambers with to-5 class latching relays and careful layouts,
    wiring and fixturing at 10 MHz. Did have to be careful to use consistent
    cable lengths (80 coaxial conductors and 196 pin connectors).

    ?-)
     
  11. --------------------

    You're asking for very low resistance.

    I've use the SMP1352 (Skyworks) before, but it only approaches 1 ohm. (1.5μs CL, 50 μm I-region)

    You can arrange things at those frequencies such that by paralleling them (as far as AC goes), you get the benefit of "resistors in parallel."

    I/O
    O
    |
    ---
    ---
    |
    +----+
    | |
    D^ Dv
    | |
    Ib->+ +->Ib
    | |
    --- ---
    --- ---
    | |
    +----+
    |
    O
    I/O
     
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