Harmonics in signal generator higher bands. How to reduce them?

Discussion in 'General Technical Questions and Answers' started by SV3ORA, Jul 4, 2018.

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  1. SV3ORA

    SV3ORA Ham Member QRZ Page

    Hello, I am building this signal generator http://qrp.gr/rfgenerator/ and I notice that the harmonics are somehow increased in the higher bands (10-30MHz or so) from -40dBc to about -30dBc.
    I wonder what is the reason for that and what can I do to correct the problem?

    I have tried a fixed 30MHz LPF at the output which reduced the harmonics at 15MHz and above indeed but also affected the output frequency range of the generator, so I removed it. I may bring it back at the future but after a preceding attenuator, so as to hopefully isolate it from the buffer.

    I have also noticed that the switched attenuator I have used at the output, affects the output frequency a bit differently, dependent on the switching attenuation value (more attenuation, more pulling). Any guess what to do with this? Maybe adding a fixed attenuator prior to the switched one?
     
  2. VK3YE

    VK3YE Ham Member QRZ Page

  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would say that is about normal for any filter that you use.

    Just re-calibrate the generator for the amount of loss that you get.

    Have Fun.
     
    1 person likes this.
  4. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Sounds like you may need a "decoupling capacitor" on the output from the signal generator between it and the next stage (attenuator/filter). Perhaps 0.01mfd or smaller.
     
    1 person likes this.
  5. N1OOQ

    N1OOQ Ham Member QRZ Page

    Does the amplitude change with freq? (And maybe gets closer to clipping?)

    other than that, I don't have much to offer except that any amplifier adds some harmonic content. I noticed on the schematic that it used a FET final amp. Those supposedly have more even-order content (2nd, 4th, 6th) than bipolars.
     
  6. KM1H

    KM1H Ham Member QRZ Page

    You could switch in BPF's between the driver and output stage just for the bands you need to reduce complexity; they could even be plug-in.

    Carl
     
  7. KM3F

    KM3F Ham Member QRZ Page

    The last paragraph before the schematic tells it all.
    The photos of the circuit board layout is wide open for uncontrolled harmonics and reactions.
    You have to go the extremes of shielding and isolation for the clean outputs and stability your looking for.
     
    1 person likes this.
  8. SV3ORA

    SV3ORA Ham Member QRZ Page

    I haven't done inter-stage shielding but I have shielded the psu and rectifiers and the output switched attenuator (not shown in the schematic) from the rest of the circuit. And the whole circuit has been built inside a metal enclosure. However, I have not done separate shielding for each attenuator stage. But since the levels change depending on the switch and since it is only used on HF, I do not think that this does play a role in the frequency change or harmonics level.
     
  9. SV3ORA

    SV3ORA Ham Member QRZ Page

    There is already one, the 4.7uF tantalum.
    You say that I need a small one (e.g 100nF?). The generator goes down to 30KHz so this must not be too small.
     
  10. SV3ORA

    SV3ORA Ham Member QRZ Page

    No BPF. It will make the circuit more complex. I am trying to figure out why harmonics increase as frequency increase, what is the cause of it.
    Also I am trying to figure out why switching the attenuator in different attenuation valies, changes a bit the frequency at different amounts depended on the value.
     

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