- [Lines] OK. I will make the tracks bigger for the majority of the board (signal path, power management, etc). I will leave it the same for the RF path (I think 10mil on this board thickness is the correct impedance characteristic), as well as for the comparator-to-transistor-switch for the boost control. The latter is because I need to run all 4 of them underneath the IC, and a thicker track just won't work. I expect that the current here will be fairly low...
- [Chip directions] I understand why this generally a good practice. However, I did not place them randomly in different directions, and I rather like the signal-oriented layout of the board. Unless you think this is a disastrous idea, I would kind of like to keep it. FYI, the boards I was talking about are things like our PD and the universal PDH box, not some generic mass-produced board.
- [Connectors] The connectors are mounted to the edge of the board, and they poke through holes in the front panel of the box. The connectors have threads and will be fixed to the front panel as well as to the board. This is for all connectors (and the potentiometers) EXCEPT for the internal RF connections that we will not generally need access to. The 2 RF connections we will need access to---the RF IN from the RFPD and the LO IN from the oscillator---will be connected from the panel to the board via a small coax cable that goes from SMA->SMP, as will the internal connection from mixer IF -> servo IN. Does this make sense?
- [Extra pads] I see. So, you think I should put in another one or two filter stages (that we will set to unity gain?), as well as a second summing amp stage near the output? I still don't really see what functionality this will add, given that we already have an excitation input and extra pads on the input stage to do things like whitening. This reminds me, though, that I have to do something about being able to disconnect the EXC input when we don't want it so as not to inject noise (as you mentioned before).
- [Component values] I understand this philosophy, and I agree with you that it is a shame to not utilize the AD829's low noise fully. However, Rana mentioned that the servo should have < 10 deg of phase lag @ 300 kHz. A gain of 10 at the input stage adds 3 degrees all by itself; a gain of 100 misses the requirement without even considering the later stages. We might get away with slightly higher gain if we use faster op amps for the switching stages, but we simply can't use some very high gain for the input stage if we want our bandwidth to be this high. I am curious to hear your and Rana's suggestions as to a compromise on this.
- [TP7-10] You are right; they are misplaced. I meant to put them before the resistors between the stages, not after. Will fix.
- [+/-15V supply] I will make certain that we can do this, but it may be a better idea to use 15V anyway, since we have to use the 24V supply from the NIM crate and we can dissipate less waste power.
- [Chattering] Is this really a common problem? I admit that I know nothing about it, so I will do some research, but I find it hard to believe that the DAC is so noisy that the comparator would flip so many times. My intuition is that even if this were true, the mechanical transfer function of the relays would be such that the chattering is filtered out (like you say). That is just a guess---I will actually figure it out.
- [Vref] OK.
- [More boards] I will!
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- [About the thin lines] Thin lines are not so immune to heat. They may peel off from the board if someone put too much heat on the soldering. Probably 10mil is ok as you are going to ask a company to make resist-coated PC boards. Also, we always modify the circuit in some unknown reasons. If the lines are thicker, we can patch the stages in a flexible ways. Of course, the
- [Chip directions] You will know if you build 20 of different circuits with the chips arranged randomly. The board made by industries are fine as their components are put on the board by a machine and use paste solder and an oven. Our board is made by ourselves. Also we test the board by ourselves. It is always good to have the chips in a single direction. This helps the debugging a lot.
- [Connectors on the panel] I could not get whether the connectors are mechanically fixed on the front panel or not. We don't want to put strain by the cables to the board because it may crack the soldering of the connectors SOME YEARS LATER. If you have threads on the connectors and fix them on the panels that is nice, but we will have some trouble to fix the board in the box in this case.
- [Extra pads / Extra pattern] There was some confusion because of my short explanation. What I wanted to say was that if you have one or two opamps extra stages which is not connected anywhere, we can use them as a universal board for some unknown purpose in future. i.e. Someone wants to put some signal goes into the loop ==> insert a summing amplifier somewhere in between the stages. Someone needs whitening stage for DAQ. Etc.
- [Components values] This is the philosophie of the low noise circuit: The input stage should decide the input referred noise level of the circuit. It is VERY sad too see the performance of AD829 is spolied by the misuse of the high resistance with low gain at the first stage. The shot-noise level may be higher than 10nV/rtHz, it is good to know the basic rule that the first stage should decide the input referred noise.
- [TP7-TP10] They must be misplaced. You will not be able to measure any sensible transfer functions. Review the schematic and replace the TPs.
- [+/-15V power supply] Review the data sheet. If you can exploit full +/-10V range of the ADC by +/-12V supply, that is still good.
- [Pull up / Pull down] If you pull down the port, we can't turn on/off by plugging a short plug there. Pull up is the way.
- [Chattering] Between the steps the control voltage acrosses the thresholds. If the comparator is enough fast (I suspect so), the comparator switches multiple times. That may cause the multiple switchings of the relays. The relay may be enough slow to remove the chattering. I have no experience of relays.
- [Vref] It is always nice to have Vref on the board.
- [Final remark] Don't forget to make the extra oards for the purpose of the others as the board will be very useful to everyone!
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