[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[ed breya]

Erik, that looks pretty nice, with good stout bypassing. I'd recommend 
adding some ceramic caps bypassing for RF, just in case. Also, I believe 
the standard three-terminal regulators have no limit on capacitive 
loading, so you probably don't need R1. The standard 7805 can put out a 
lot of current (>1A), so I'd suggest using the 78L05 low power version 
(100 mA or so). That will make it a little safer, fault-wise. Especially 
when building and experimenting, it's good to have some limiting 
appropriate to the circuit loads. If you should say, inadvertently short 
the circuit common to ground, a way bigger than needed regulator will 
supply lots of current, and possibly crash the +12V if it can't keep up. 
Finally, whenever these regulators have large output capacitors, it's 
good to have a reverse protection diode from output to input, in case 
the input voltage is suddenly dumped - another thing that can easily happen.

So, with good DC and LF regulation, and plenty of audio and RF 
bypassing, the common line should be very solid. I don't think the very 
low frequency regulator noise or DC drift will cause any grief, since 
the opamp's CMRR should take care of it. I looked up the OP-27, and it 
shows over 120 dB CMRR up to a few kHz, so the CM to input signal 
conversion is less than 1 uV/V. You should check all the part specs to 
be sure if it will be OK. The PSRR is much weaker, so the cleanliness of 
the +12V supply should be considered too.

I see that you've changed the LNA circuit (to inverting) and feed to the 
audio (ground-referenced - nice). Also, it appears that 10X voltage gain 
is sufficient, instead of the 100X previous. This should greatly reduce 
the grief. One thing that's different about the inverting mode versus 
the previous non-inverting, is that now you have a definite DC/LF 
termination/load resistance (the 100 ohms into the summing node) on the 
mixer IF port, while the non-inverting mode can provide very high input 
R, so you can have more flexibility on termination R choice, from "open" 
on down to whatever you want. I don't know what the ideal is, but as in 
previous discussions, it doesn't need to be 50 ohms as in RF work, but 
can be much higher for the audio, to get a bigger voltage signal. For 
inverting mode, the input R must be all or part of the termination R, 
and "open" is not an option.

I'm wondering what the PC USB audio box is. Could you please let us know 
make, model, etc? The "balanced microphone" description sounds like it's 
600 ohm input R.

Ed