[time-nuts] OT: Spectrum Analyzer

David I. Emery die at dieconsulting.com
Sat Oct 31 04:28:32 UTC 2009


On Fri, Oct 30, 2009 at 11:39:37PM -0400, Glenn Little WB4UIV wrote:
> Keep in mind that anything much newer than the 141T will have custom 
> parts and processors.
> This makes the instrument smaller and more versatile, but, less repairable.
> The 141T is a workhorse.
> 
> I would not trade mine for any number of the processor controlled analyzers.
> Having a nice clean analog display is nice and you do not have to 
> worry about did I get an accurate display of what I am looking at or 
> did what I am looking for occur between sample on the digital display.
> 
> I hope you find what you are looking for. It is hard to beat the 141T 
> for a spectrum analyzer.

	I disagree.   Modern analyzers have a LOT of features the 141T
family lacks - starting I guess with full frequency synthesis for near
time nuts level frequency accuracy (most all take external 10 MHz from a
standard and are fully coherent with this).    This makes a REAL
difference if you are interested in observing and analyzing unknown
signals... and allows the analyzer to also serve as a (microwave)
frequency counter.   If you are observing a crowded part of the
spectrum, having say 5 MHz or so frequency accuracy (typical of
unsynthesized instruments at VHF and above)  doesn't cut it AT ALL...
gets to be completely impossible to reliably identify a particular weak
signal or spur with 50 others nearby.

	And frequency synthesis also allows precise keypad entry of a
frequency of interest (or via GPIB or other remote control)... try that
on an analog only crank the dial instrument.  At best you will do a LOT
of dial cranking...

	Another feature less pre-Cambrian analyzers have is markers...
allowing accurate measurement of level and frequency at a particular
point on the screen, and often differential measurement too (eg so many
db down). Most instruments with markers will also do auto-peak search so
you can find the next left or right peak and read its frequency and
level with one key press.   And usually the marker readout of frequency
and level is significantly more accurate and higher resolution than the
display can be read easily... and often more than one marker is allowed
on screen at once.

	And yet another feature modern analyzers have is digital display
memory - which makes it usefully possible to make slow narrow band
measurements with say 100 Hz or  30 Hz bandwidths.   Really antique
analyzers had display storage tubes or similar arrangements as options,
but all even slightly modern ones have fully integrated digital storage
and displays.

	And yet another feature that present in all modern instruments
is peak hold, and usually also valley hold... allowing one to look over
time for elusive signals that appear only momentarily, or see how strong
or weak the noise or some signal got in a particular time interval.

	Most modern instruments also include extensive self check and
auto-calibration firmware which can compensate for drift and aging of
the various analog circuits and handle temperature and other
environmental changes much better than older instruments without it.

	And modern instruments have as at least as an option (usually
installed) some form of remote control allowing software to access the
measured data and control the analyzer - making possible all kinds of SW
applications that process measurements into more useful forms and
perform various corrections. GPIB is very common here.

	Another feature absent in 141T era analyzers is plotting or
printing the screen via a printer of some kind (or these days a printer
emulation program such as the excellent ones John Miles provides).  
This allows capture of a  fully annotated screen image (these days as a
.png file)...

	Really modern instruments (last 10 years or so) also often have
FFT processing for fast narrow band analysis and modulation domain
analysis including digital demodulation and constellation and error
vector displays.

	And many more recent instruments have built in firmware
measurements of things like occupied bandwidth and noise power per root
Hz... this all can be done with an attached laptop, but it is nice to
have it integrated and available with a button press.

	It is true, of course, that the more recent the instrument the
more likely it is to have at least some proprietary parts that are
probably unobtanium by the time the instrument hits the affordable
surplus market.   And unfortunately many really modern instruments do
not have easily obtained schematics and chip level documentation
available - this alas being true of most T+M gear made recently, not
just SAs.   But lots of instruments with all the features I have
described above that date from the late 80s through 2000 or so are
readily available in your under $3K price range now and ARE documented
by available service manuals and schematics.


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."




More information about the time-nuts mailing list