vasile said:
Depends. A lazy one, with a fake noise level measuremet (only for some
frequency ranges and modulations) is here:
http://www.aaronia.de
Roughly like this:
Breaking the 1Ghz range in multiple 100MHz ranges, with an overlap
between adjacent ranges (see how works Tektronix analyzers)
Designing a narrower filter is difficult, and could be useless.
I wouldn't shy away from whipping up a filter myself. Done it many
times, including really steep crystal filters. One issue with the
simpler analyzer models is that they don't have a nice low noise PLL but
a YIG oscillator. I found those to be a bit noisy. But OTOH it'll be
enough for us.
USB does not mean it will be for sure in "real time"... think at ISM
band (2.5Ghz) and how fast are
WIFI signals
If it needs a few seconds to transmit it's data that'll be fine. I grew
up with Polaroid cameras hooked to side hinges on the screen. Worked.
Except when you got the goo from the back foil onto your clothing.
This will require a stable reference, preferable digitally
synthesable.
I've done it with really old ones. The trick is to turn it on, make
coffee, do some other design work for an hour or so and then it's warmed
up. You just have to make sure that nobody touches the thermostat on the
wall or opens a window. Would be no problem here. Believe it or not but
this office is usually heated by a wood stove. You can keep the room
temp at 69.0F and it stays there. To the point where I sometimes thought
the digital thermometer had frozen up on me.
The maximum available dynamic range is finally a matter of bits
numbers of the A2D converter:
SNR = 6.02 x n bit + 1.76dB
It means you need a fast ADC with more than 12 bits.
No ADCs in the old ones
The impedance analyzer here in the lab uses a slope converter that (in
theory) has almost infinite resolution. I get north of 90dB out of that.
Of course the analyzer arhitecture it may be zero IF or with one (or
more) IF stage...
True, but my experience with I/Q demod scheme is, well, mixed.
Without a complete calibration (amplitude and frequency) a spectrum
analyser is useless.
If the frequency calibration could be easy done based on the odd/even
harmonics of a stable 10Mhz crystal
(you'll see up to 100MHz all harmonics), the amplitude calibration is
not easy. An 1Ghz Tektronix scope, DPO4000, has about 2% amplitude
nonlinearity, and it's factory calibrated.
This is how I've done it: Take (or borrow) a generator with known output
amplitude over the whole range. Or make one from a few PECL gates. Then
plot the amplitudes on the analyzer. Now turn on your 10MHz harmonics
thingie and compare. From then on you know the amplitude of the
harmonics. In the 80's I did that stuff on a notepad sheet because all
they had was a daisy-wheel printer and the accounting folks had that
occupied all the time.
On really old analyzers I sometimes let the harmonics generator run into
the input line via a 20dB splitter, to see where I am on the frequency
scale. I mean, in the olden days they navigated big airliners in a
similar fashion by following headings to regular AM band stations. It
always got them home.
Anyhow, looks like we might just buy a real oldie like the HP141. I've
used that a lot when I was young and it has never failed me. It is just
very big and heavy. And no automatic stuff whatsoever. That was why the
others back then hated this thing so I could have it as long as I wanted
to ;-)
