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<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2>Frank,</FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>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*. </FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>--
john, KE5FX</FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff size=2>*: 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: <A
href="http://www.bores.com/courses/advanced/windows/files/windows.pdf">http://www.bores.com/courses/advanced/windows/files/windows.pdf</A> </FONT></SPAN></DIV></FONT></SPAN></DIV>
<DIV><SPAN class=561270301-26102007><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px solid; MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT face=Tahoma
size=2>-----Original Message-----<BR><B>From:</B>
hpsdr-bounces@lists.hpsdr.org [mailto:hpsdr-bounces@lists.hpsdr.org]<B>On
Behalf Of </B>FRANCIS CARCIA<BR><B>Sent:</B> Thursday, October 25, 2007 5:20
PM<BR><B>To:</B> hpsdr@lists.hpsdr.org<BR><B>Subject:</B> [hpsdr] phase
noise<BR><BR></FONT></DIV>
<DIV>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. </DIV>
<DIV>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.</DIV>
<DIV>Any smart guy want to set me straight here. Frank
WA1GFZ</DIV></BLOCKQUOTE></BODY></HTML>