[hpsdr] phase noise

[John Miles]

Frank,

I don't have one of the receivers in question, but generally the apparent
noise level is tied to the sampling bandwidth.  If your sampling clock is 10
MHz, your noise bandwidth is 5 MHz, and you can normalize it to 1 Hz by
subtracting 67 dB*.

But you also have to realize that the noise energy is distributed evenly
between the FFT bins.  Digitally speaking, what does this rig do when you
"change bandwidths", as you say?  Does that mean it just uses a different
number of bins to draw the spectrum graph?  This is important: if the ADC's
sampling clock and front-end bandwidth didn't change, and the FFT kernel
size (# of bins) didn't change, then the noise level in each bin won't
change, either.

I'm not personally crazy about the idea of using an 8640B cavity as an LO
for a narrowband receiver LO.  They drift, and they are nothing special when
it comes to close-in phase noise (< 10 kHz).  Instead, consider adding a PLL
to clean up any DDS spurs at inter-channel offsets, or upgrading to a DDS
with better SFDR.

-- john, KE5FX

*: In reality, FFTs have "equivalent noise bandwidths" just like
conventional filters do, so the noise-normalization function is not strictly
10*log(BW).  This is a property of the window.  There's a decent tech note
on that phenomenon here:
http://www.bores.com/courses/advanced/windows/files/windows.pdf

  -----Original Message-----
  From: hpsdr-bounces at lists.hpsdr.org
[mailto:hpsdr-bounces at lists.hpsdr.org]On Behalf Of FRANCIS CARCIA
  Sent: Thursday, October 25, 2007 5:20 PM
  To: hpsdr at lists.hpsdr.org
  Subject: [hpsdr] phase noise


  I find it interesting to monitor phase noise with the flex display. The
level does not change with bandwidth. Many articles I have read take a
correction factor for the bandwidth of the monitor device, log BW hz
improving the phase noise. This is broadband noise and my hours sitting
behind the old NF102 doing MIL-STD 461A testing has me wondering if this
correction can be taken for a broadband  signal. When we changed bandwidths
on the old NF102 a BB signal would not change amplitude while a narrow band
signal would. When I run the phase noise test per ARRL method the result
follows the uncorrected value not the lower assumed value. This also tracks
the degraded dynamic range on close in signals.
  Even the Russian guy who had an article in QEX used a 1 KHz filter to
measure his phase noise so he took 30 dB. I'm considering going back to my
homebrew RX and ripping out the synthesizer and installing an HP8640B cavity
because that is the closest thing I see to a crystal oscillator. Yes a DDS
is clean close in at the expense of bb spurs.
  Any smart guy want to set me straight here. Frank WA1GFZ
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