Home Forum Support LND7807 noise problem.

This topic contains 10 replies, has 3 voices, and was last updated by  Radu Motisan 5 days, 21 hours ago.

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  • #6223

    R_A_Ghosh
    Participant

    I am making a uRadmonitor variant using LND7807 GM tube from LND. I am using anode signal biasing as per 7807 datasheet. R2 is 330k ohm, R1 anode resistor os 3.3M ohm. How to calculate and connect compensation capacitor in anode signal connection mode. Signal dc blocking capacitor is 47pF
    Basic wiring is as below

    +500V—–R2—–R1—–GM tube anode
    |
    C1
    GM tube cathode is connected to ground.

    I am getting spurious noise at high radiation rates. Though solution are there for cathode signal connection method. But I am using anode connection. Please guide.

    #6225

    Wolferl
    Moderator

    Hi,

    You have not written what you want to do and what you have so far…so it’s a bit of guessing, but anyway….
    So you are using a circuit like the KIT1 has (see attachment)?
    The KIT1 has a resistor/capacitor network connected to the cathode of the Geiger tube, so you can use 330kohm as R6 and 50pF as C9 to match your tube. It doesn’t change the tube’s performance if you have that R/C network on the anode as shown on your picture, or cathode wire, as in the KIT1, the tube “sees” the same voltage across it.
    You’ll need to eiter change the voltage divier network R3/R4 or modify the firmware to get the required 500V supply, since the SBM20 tube is using “only” 380V.
    You also may need to use a different inductor L1.

    Cheers,
    Wolferl

    #6259

    R_A_Ghosh
    Participant

    I am using anode signal model without the compensation capacitor, so after 3mR /he I am getting noise in GM reading and false high counts, why is it happening with lnd7807 tube? .as a solution I may need compensation capacitor…Is this how shall I connect compensation capacitor to avoid noise (find attached)?
    I am using 3.3M R1 as anode resistor, 330k as divider R3
    I am not using cathode signal model as your kit….

    #6261

    Wolferl
    Moderator

    Hi,

    That will never work. The HV supply comes from a switched power supply, so that has some serious ripple and spikes on it. These spikes are the false counts you are getting. The signal you want to measure is the voltage drop across R3 (on your picture), but unfortunately that signal is referred to HV which makes it difficult to measure correctly. If you connect that up as shown on your picture, you measure the tube impules /plus/ the HV spikes, which is not what you want 🙂

    It is much better to measure the current impulses /through/ the tube on a cathode resistor. That way the cathode impulses are referred to GND (and not to noisy HV), and the measuring amplifier is also referred to GND. You are not measuring any spikes from the HV.

    Please hook your tube up as shown in the KIT1 circuit, and it will work nicely.

    Cheers,
    Wolferl

    #6262

    R_A_Ghosh
    Participant

    Though my HVDC power supply is clean and well filtered, I did not observe any false count on my monitor. I have to impose the system on high radiation and see if it still works or gets GM tube noise again.
    My problem was that I had simple wiring earlier and everything was fine until 3mR/hr or 450 counts/s, over that it worked well for few minutes and then suddenly a high fixed false pulse count of 4000/s started to come. It fell back to normal when radiation was reduced. I have proper data upto 3mR/hr. I want to measure upto 10mR/hr but the GM tube noise is causing issues. The noise is not from HVDC power supply for sure.

    #6263

    Wolferl
    Moderator

    Ah, ok, you could have mentioned that important fact earlier 🙂
    Anyway, usual Geiger tubes saturate, they have a maximum impulse count they can deliver. I was not able to find that info on the LND7807, only a minimum dead time of 210µS is specified.
    However, if you want to measure up to 10mR/hr, that will be 1500 discharge impules on your HV. Your HV supply needs to be /very/ powerful to deliver that many impulses with a peak current of 150µA (500V into 3,3 MOhms). To measure higher radiation levels you need to keep the anode and cathode resistors as small as possible (2,7 MOhms).
    You also need to look into capacitance (parts and stray) to see if your anode voltage is really stable while counting heavily.
    BTW: How have you determined your HV supply is stable and does not produce any spikes? Did you use an oscilloscope? Which probes did you use then?

    Cheers,
    Wolferl

    #6264

    R_A_Ghosh
    Participant

    The power supply I am using has capacity of 500V, 1mA. I am using 100X probe to measure HVDC. As per LND7807 datasheet and some emails sent to me by LND support for 10mR/hr it consumes no more than 10uA current…attaching graph here sent to me by LND support.
    I have a switching frequency of 9KHz of the DC to HVDC supply. The link to make one is here HVDC Power supply

    #6266

    R_A_Ghosh
    Participant

    One more thing is that I have the both biasing resistors on the PCB board and the gm tube is then connected by a foot long wire and kept far. If as per suggested I can take the anode resistor to GM tube assembly close to the GM tube anode and from return wire of cathode i take cathode wiring connection as recommended by you and hence using a compensating capacitor on board, then can the problem be mitigated ?
    I have R anode=3.3M
    R drop at cathode =330K
    Capacitance of tube = 8pF
    then formula 3.3Mx8pF=Ccx330K
    Cc =80pF
    So if i do such that I change the wiring to cathode signal type and use 80pF compensation capacitor.

    #6267

    Wolferl
    Moderator

    Hi,

    Hm, I admit I don’t have that much experience with Geiger tube circuits, but in EE in general. Geiger tubes are high impedance devices. To avoid capacitance as good as possible, you’ll need to use special low-capacitance high voltage cables, which are available.
    Then I’d start using two 1,5 MOhm resistors in series (standard resistors usually are specified for 200V or 250V) to get 3 MOhm 500V as anode resistor, and a 330k as cathode resistor, from the tube’s cathode you go via a small capacitor (Cb) into the amplifier/pulse shaper. I have no idea what the “compensation capacitor” is good for, though. It just loads your measurement signal down, which is not what you want.
    It could be you need a higher anode resistor to reduce load on HV, if you still have a problem measuring at 10 mR/hr. You’ll have to try.
    The 4000 count you observed on high radiation levels could very well be the switching frequency!

    Cheers,
    Wolferl

    #6302

    R_A_Ghosh
    Participant

    So I finally did that. Used cathode wiring and put 82pF compensation capacitor across R2 and it worked like magic. Got good calibration and now basic instrument working. Rest will be done in due time.

    #6303

    Radu Motisan
    Keymaster

    @r_a_ghosh, would you post a few pics too?

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