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I want to build an LO

Discussion in 'Electronic Design' started by M. Hamed, Jun 8, 2013.

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  1. M. Hamed

    M. Hamed Guest

    Hello the most knowledgeable electronics group on earth!

    I am trying to build a few LC oscillators for experimenting with mixers. I do not want to use Crystals, Frequency Synthesis, DDS, etc. Let's say 2 MHz and 100 MHz. I don't know where to start!

    I have a few RF books and the treatment of oscillator can be one of the following:

    1- Too much theory on the analysis of oscillator circuits, phase noise, IMD, etc. No circuits to build. Just demo circuits for illustration, sometimes only the small signal model.
    2- Circuits to build but designed at very specific frequencies with very specific components. Very little on how to generalize things into different frequencies. (The ARRL handbook 2010 and Experimental Methods fall under this).
    3- Books that give me a design procedure with S parameters but I can't find any S parameter files for the devices I'm interested in using.
    4- Cookbooks that have the same problem as (2).

    Why is it so hard to build an oscillator at an arbitrary frequency?
  2. Andrew Holme

    Andrew Holme Guest

    "M. Hamed" wrote in message
    Idea 1: find a DIP meter schematic. They typically cover a fairly wide
    range using plug-in coils i.e. you just need to change the inductor. And
    it's often a single FET circuit.

    Idea 2: You could cover quite a wide range using an unbuffered CMOS inverter
    (4069UB) with: cap from input to ground; cap from output to ground; inductor
    from input to output; maybe a resistor in series with the output. You
    should get a sinusoid at the inverter input. It'll need buffering to drive
    your mixer.

    In both cases you should be able to cover quite a wide frequency range with
    suitable choices of L and C. You can lash it up on a breadboard.

    But the upper frequency limit for the above circuits will be well below 100
    MHz. You will need a different circuit for VHF.
  3. Guest

    With current components, it is no big deal to make a free running
    oscillator in the 10-100 GHz range.

    However, in order to be usable in communication systems, some
    frequency accuracy is needed.

    At MF at 1 MHz a frequency accuracy of 1 % would correspond to 10 kHz
    (2x5 kHz audio passband). The same accuracy at 10 GHz TV satellite
    band would correspond to 2-4 channels.

    The real question is that it is no problem making a scanner receiver
    handling the 0-5 GHz frequency range, however making a good receiver
    or that frequency range is nearly impossible.
  4. tm

    tm Guest

    Just try to build an amplifier and most times you will get an oscillator.

  5. About 20 years ago I had fun with this:


    Air-core coil wound on a pencil, can't remember the fet but nothing

    No ebay in those days so I just had a DFM. I should hook one up again
    and see how good/awful it is now I have lots of test kit.
  6. Phil Allison

    Phil Allison Guest

    ** A free running oscillator is as accurate as YOU set it.

    The real issue is *stability over time* or drift and that is nearly all due
    to temperature variations.

    ** With most AM receivers, one tunes the LO to the desired station by ear or
    the uses of an optical indicator. The allowed error is in the hundreds of
    Hz. Over time the LO may drift, then one retunes it.

    ** Communications on VHF, UHF and above pretty much depends on the use of
    crystal locked oscillators at both ends.

    Luckily they were invented long ago.

    ** Why is that the "real" question ?

    Seems like a smelly red fish to me.

    .... Phil
  7. Tauno Voipio

    Tauno Voipio Guest

    Maybe I repeat myself: It seems that you need a good book on
    basic radio design, with the emphasis on practice.

    Please get yourself an ARRL handbook <>.
    It contains plenty of information for your project and
    excellent links for more information.
  8. Guest

    Operating in that frequency range was a challenge 50 years ago with
    germanium transistors. The parameter spread was very large, so making
    a reliable oscillator was hard. To sustain oscillation, the _power_
    gain of the active element(s) must be larger than the losses in the
    resonator and other passive components. This gets hard, when the
    typical fT for an RF germanium transistor was 40-200 MHz.

    These days transistors with much higher fT are available. Even some
    cheap MMICs have considerable gain up to 5-10 GHz. If the gain is not
    enough, cascading an other MMIC does not cost much. Of course, at
    frequencies at UHF and above, lumped LC resonators can not be used,
    stripline and microstrip constructions are often used.
  9. Guest

    If you want to handle the 2-100 MHz range in a single sweep in a
    single oscillator, that is not going to happen.

    Of course, you could make a 2-100 MHz sweep with a VFO running between
    122-220 MHz and mix it down with a 120 MHz overtone crystal, but the
    frequency stability will be quite bad, due to the VFO in the VHF

    If you want 2-100 MHz from a single oscillator, that is a frequency
    range of 1:50. If only one component is tunable, typically the
    capacitor, this requires a 2500:1 capacitance range. This is clearly
    impractical due to stray capacitances. Clearly both the capacitance
    and inductance must be tuned, but getting 50:1 or similar ratios for
    both the inductance and capacitance is hard.

    With only a tunable capacitor (capacitance diode) trying to reach
    larger frequency ranges larger than 1:3 is asking for troubles, as
    this already required a 9:1 capacitance range, including stray

    In the old days, when tubes and transistors were expensive, elaborate
    mechanical switching was used to switch in various inductances and
    capacitances in series/parallel. These days, when the active component
    costs practically nothing, it makes much more sense to build a
    complete oscillator for each subband and only connect power to the
    oscillator to be used at a particular time.

    You might for instance build an oscillator for 2-6 MHz with a big
    varactor intended for MW (AM) receivers.

    The next oscillator could cover 5-15 MHz, next 12-36 MHz and finally
    33-100 MHz, all these with low capacitance varactors, several in
    parallel at the lower bands. There are plenty of designs for these HF
    and VHF frequencies, which should be easily scalable to those
    frequency ranges.

    One other alternative would be to build a 50-100 MHz VFO followed by
    five divide-by-two frequency dividers followed by fixed low pass
    filters at each tap. Since the square wave output from the divider
    contains a lot of 3rd harmonic, to simplify the low pass filters, the
    VFO tuning range should be less than 1:3, in this case 1:2.
    I do not like the recent ARRL handbooks, the older ones have better
    technical contents. Try to locate a 10-20 years old ARRL handbook from
    a library or eBay etc.
  10. Phil Allison

    Phil Allison Guest

    ** The OP did not ask for anything like that and you are being

    FUCKING tedious WANKER !!!!

    by repeatedly inventing *straw men* for your self aggrandisement.

    Piss OFF !!!!!!!!

    ..... Phil

  11. If someone is not already credited on this, you should be.

    Many times the result is a very poor, inefficient one at that, as
    compared to dedicated (intended to be such at design time) oscillator
  12. tm

    tm Guest

    LOL. Yes, it was meant to be tongue in cheek. Another one is "the better you
    make the amplifier, the more it will oscillate".

  13. Fred Abse

    Fred Abse Guest

    As long as it doesn't give you the Clapp.
  14. Robert Baer

    Robert Baer Guest

    Oh, it is VERY easy; just build an amplifier!
  15. Robert Baer

    Robert Baer Guest

    What happens when you cross a Colpitts oscillator with an Armstrong
    You get an ArmPitts oscillator.
  16. John S

    John S Guest

    I must be doing something wrong. I simulated it in LTSpice and I get
    5MHz oscillation with a 10MHz tank. And it is a long way from a sine wave.

    Version 4
    SHEET 1 880 680
    WIRE -160 -256 -288 -256
    WIRE 16 -256 -160 -256
    WIRE 128 -256 16 -256
    WIRE 400 -256 128 -256
    WIRE -288 -208 -288 -256
    WIRE 16 -160 16 -256
    WIRE 128 -160 128 -256
    WIRE -288 -112 -288 -128
    WIRE 16 -16 16 -96
    WIRE 128 -16 128 -80
    WIRE 128 -16 16 -16
    WIRE 400 -16 400 -256
    WIRE 128 32 128 -16
    WIRE 336 32 128 32
    WIRE 128 192 128 32
    WIRE -160 240 -160 -256
    WIRE 64 240 -160 240
    WIRE 128 336 128 288
    WIRE 400 336 400 80
    WIRE 400 336 128 336
    WIRE 128 384 128 336
    WIRE 128 480 128 464
    FLAG 128 480 0
    FLAG -288 -112 0
    SYMBOL npn 64 192 R0
    SYMATTR InstName Q1
    SYMATTR Value 2N2369
    SYMBOL npn 336 -16 R0
    SYMATTR InstName Q2
    SYMATTR Value 2N2369
    SYMBOL res 112 368 R0
    SYMATTR InstName R1
    SYMATTR Value 4.3k
    SYMBOL voltage -288 -224 R0
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V1
    SYMATTR Value 5
    SYMBOL ind 112 -176 R0
    SYMATTR InstName L1
    SYMATTR Value 16u
    SYMATTR SpiceLine Rser=20
    SYMBOL cap 0 -160 R0
    SYMATTR InstName C1
    SYMATTR Value 15.8p
    TEXT -322 504 Left 2 !.tran 0 10u 5u
  17. John S

    John S Guest

    Oh. I thought he wanted the Xc and Xl to be 1000 ohms each at 10MHz,
    hence the Zo of 1000 ohms. What do you think he meant about Zo?
  18. John S

    John S Guest

    He has me kill filed.

  19. Have a few swigs of Ambrosia and examine the problem again.
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