Hartley oscillator: amplitude not stable

Discussion in 'Analog & Mixed-Signal Design' started by patpin, Jul 5, 2018.

  1. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    Hello, I made a Hartley oscill. at +- 513KHz;
    My problem is the 300mV amplitude instability on a Vpp of 13.5.
    The scope records 1 and 2 are made at the base of the Q2.
    The third record is even more unstable at c(Q1).
    Is there a way to make it better?
     
  2. danadak

    Well-Known Member

    Mar 10, 2018
    1,551
    321
    What does you power supply noise look like ? 60(50)or 120(100) Hz ripple ?

    Regards, Dana.
     
  3. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    Hello Dana, I have tested on a battery:same thing.
     
  4. danadak

    Well-Known Member

    Mar 10, 2018
    1,551
    321
    If you use a scope on infinite persistence and look at supply rail you
    can see pk-pk noise, will tell you if you have ineffective bypass solution
    for the power rail.

    Battery does not guarantee power rail is fixed, no noise. Depends on its
    internal ESR which rises as it discharges.

    Regards, Dana.
     
  5. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    I ment a car battery of 70AH, so I guess this will be stable source.
    In the mean time I found the cause. The coils were air coils.
    I have put 1/4.5 th of the windings on a toroid an here are the results.
    Greetings and thanks for interventions. By the way I baught a Psoc 4 in order to do some tests. I guess I cannot generate a 520KhZ wave with it?
     
  6. danadak

    Well-Known Member

    Mar 10, 2018
    1,551
    321
    What kind of wave are you trying to generate ?

    Regards, Dana.
     
  7. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    a sine wave
     
  8. danadak

    Well-Known Member

    Mar 10, 2018
    1,551
    321
    The PSOC 4 does not have the Wavedac in it because of no DMA unless
    you have the 4M family part.

    If you had the 4M part it does not allow use of WavedaC but not too difficult
    to set up DMA from a table feeding the DAC to get a user specified waveform.

    ThE PSOC 5LP has Wavedac.

    What freq do you need ?

    Regards, Dana.
     
    Last edited: Jul 6, 2018
  9. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    530KhZ
     
  10. danadak

    Well-Known Member

    Mar 10, 2018
    1,551
    321
    Max sample rate = 6 Msps / .53 Mhz = 11 samples which is too small
    a table to get reasonable sine accuracy and harmonic content.

    I have used 32 samples, recall I had > 40 db harmonic suppression.
    So that would equate to 187 Khz.

    Its DAC settling time the primary issue.

    Regards, Dana.
     
  11. Bordodynov

    Well-Known Member

    May 20, 2015
    1,847
    566
    I took the liberty and corrected your scheme. I reconnected the resistor R4. This significantly reduces the load and increases the quality factor of the oscillatory circuit. I also improved the transistor bias, and this will increase the temperature stability of the mode of this transistor. I also added an active oscillator amplitude adjustment. In this case, the transistor is not saturated, which also increases stability. In addition, the output spectrum is much better.
    2018-07-07_09-27-16.png 2018-07-07_09-28-02.png
     
  12. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    Thanks for yr help! I need an amplitude as high as possible but (with this 8V supply) with as small variations as possible in amplitude, say 1 - 2 %.
     
    Last edited: Jul 7, 2018
  13. Bordodynov

    Well-Known Member

    May 20, 2015
    1,847
    566
  14. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    Very nice result... apparently yr an expert in this domain!
     
  15. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    Do you think this circuit is robust enough to be functional (same amplitude at -10°C as at 85°C and with same amplitude variation of a few %) at -10°C as at 85°C ? (Is for automotive).
     
    Last edited: Jul 7, 2018
  16. Bordodynov

    Well-Known Member

    May 20, 2015
    1,847
    566
    amplitude variation 1 %.
    Plot.png cir.png
     
  17. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
  18. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    As power supply I have used a LDO volt.reg. ( LM2940T-8.0/NOPB ) and as C 0.47µF and 47µF. Is this enough for this circuit in an automotive environment or should I add 1 or more TVS's.
     
  19. Bordodynov

    Well-Known Member

    May 20, 2015
    1,847
    566
    I advise you to use an inductive choke and TVS.
     
  20. patpin

    Thread Starter Active Member

    Sep 15, 2012
    357
    18
    OK. Which one of each would you advise . The main power supply is car batt. & charging system up to 14.4V.
    And for the inductors L1 and L2: I do not have stable results with them. I have wound them on a toroid and L1 is less than one turn which is difficult to stabilise;L2 was 15 turns. Should I purchase one of the correct inductance or should there be a trimming possibility? for both? And in this case what would you make or buy to be robust and temp. resistant?
     
    Last edited: Jul 8, 2018
Loading...