40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  ATF eLog  Not logged in ELOG logo
Entry  Fri Feb 4 05:26:18 2011, Zach, Electronics, GYRO, PDH2 PCB layout PDH2.pdf
    Reply  Fri Feb 4 08:08:39 2011, Koji, Electronics, GYRO, PDH2 PCB layout 
       Reply  Fri Feb 4 14:56:38 2011, Zach, Electronics, GYRO, PDH2 PCB layout 
          Reply  Fri Feb 4 20:23:13 2011, Koji, Electronics, GYRO, PDH2 PCB layout 
             Reply  Mon Feb 7 13:37:31 2011, Zach, Electronics, GYRO, PDH2 PCB layout 
Message ID: 1285     Entry time: Fri Feb 4 14:56:38 2011     In reply to: 1284     Reply to this: 1288
Author: Zach 
Type: Electronics 
Category: GYRO 
Subject: PDH2 PCB layout 

Thanks for the comments. Here are my replies:


  • The lines are too thin. I used 10mil track, when I should probably use 20mil. I can change this for the signal pathway without much problem. Is it terrible if things like the comparator are still done in 10mil?
  • Why? The board is laid out in a sort of compartmentalized functional area kind of way. The signal enters at top left and progresses left through the filter chain. Once this is done, it is directed to the "output area" to be buffered and sent to the output ports. This area has its chips upside-down relative to the filter chain chips for the obvious reason that pin 6 faces the front panel. Every example circuit I am using in Altium has instances of multi-directional chipping. I am not arguing, but wondering if there is a deep reason why they should be in the same direction?
  • They are. The potentiometers for the gain adjust and the phase shift are on opposite sides of the front panel. All connectors we will need access to externally will be on the front panel. The phase shifter in/out is board-mounted for the following reason(s):
    • The output is board-mounted (and SMA) because it will go to a standalone mixer that will be on the board, as well (like in the universal box).
    • The input is board-mounted because we will have a flying coaxial cable from the board to the second level of our double-height NIM module. There is just not enough room to fit two SMA connectors on the edge of the board. In any case, we will need a double-height for our double stacked BNCs, so we will have plenty of room to fit the SMA connectors on the second level and patch to them via SMP or whatever.
    • While on this topic, I thought I would mention what the input/outputs will be.
      • The signal will come in via SMA from the RFPD. It will go into a throughput on the top of the front panel, where an SMA patch cable will take it to the mixer. There, it will meet the LO signal, which has come into the box in the same way and then passed through the phase shifter in the way described above. At the output of the mixer we will have an inline SMA LPF and then an SMA->SMP patch to take the LF signal to the servo input (This SMP connector is not yet on the board).
      • The BNC connectors on the box will be:
        • Input monitor
        • Pre-injection monitor
        • Post-injection monitor
        • Local sweep
        • DAC EXC
        • Boost control
      • Switches control the inversion function of the output stage and the manual local sweep.
  • I added some pads to the input stage for future modification at Rana's suggestion. I am not sure where else we're likely to want them. Without considering the input's flexibility, we have four independently-placeable poles and zeros. Any specific suggestions?
  • By this you mean for a future modification of the board (i.e. redesign) correct? If this is the case, then I think the board's layout lends itself very nicely to future modifications. Stages U2-U5 are essentially identical in footprint, so this could be carried on indefinitely with little thought, so long as you connect the final stage to K5, as I have done. Of course, more than 4 stages will require a more complicated switching scheme, but that will be for future work.

Schematic diagram:

  • All component values were chosen after the analysis in my previous elog post. The total input-referred voltage noise at the PDH2 output is below the expected shot noise level between 100 mHz and a few kHz. I believe that within a decade in either direction it is still below the requirement curve. The basic reason is that the optical gain will be much higher now, so we need less servo gain in order to keep the loop stable. Although it is counter-intuitive, it is better to place the low-frequency gain on the filter stages because they roll off at high frequency; putting any appreciable gain on the first two AD829 stages results in too much phase lag at high frequency since they have a flat response. Of course, the pads are all there, so we can change this as we aim for a higher UGF (once we have designed the notch filters).
  • TP7-10 are there so that we can check the transfer functions of the individual filter stages.
  • The wiring around U3 was a mistake that I must have made while I was trying to print the schematic. It is fixed.
  • No big reason for this except that it is the choice of the universal box. This is probably because the power input for that box is +/-15 V, so they want to regulate to 12. We will use the +/-24 V supply of the NIM crate, so it may be better to use +/-15 V. I can change this.
  • See first bullet.
  • Ok--will do.
  • This is a good idea. I was actually thinking recently that it's not great to have to supply a voltage to keep the thing in the 0 state. Shouldn't we actually pull it down, though?
  • OK
  • The incremental steps of the stages are 2 V, and I have staggered them so that they are always 1 V away from the switching point. I can't imagine that we will have enough noise in this channel to cause the relays to switch.
  • I am not sure. The universal board just uses the +12 supply, but I guess this could add phase noise. I will consider adding a reference.


I will try to make the changes above quickly. Please let me know if my rebuttals are sound.



  • The lines looks too thin
  • The chips should always be the same direction
  • The connectors / the potentiometer should be on the panel. Not on the board.
  • There is no extra pads for future modification of the TF
  • Have a universal pattern for future expansion

Schematic diagram

  • Increase the gain of the 1st stage for the low input noise
  • What's the purpose of TP7-10? They only shows 0V when their are correctly working...
  • Strange wiring at around U3
  • Are +/-12V power intentional choice? Why not +/-15V?
  • Why U7 is an attenuator in stead of an amplifier?
  • Use 7805 in stead of using the voltage divider for the power supply.
    The current consumed by the comparator changes the divided voltage. This changes the reference level of the comparator.
    In the worst case this causes the oscillation.
  • Is it a good idea to pull up the boost input with the +5V source through a reasonably high resistance?
    This will allow us to use a terminator plug (or a potentiometer) to control the boosts.
  • Write the threshold voltages of the comparators on the schematic diagram for the easy understanding of the boost control.
  • The comparators have no hysteresis (positive feedback). What do you do about chattering of the comparators?
  • Is it OK to get the phase shift ctrl form the voltage division of the +12V supply? Don't we need a reference voltage?


ELOG V3.1.3-