[minima] Re: A homebrew spectrum analyzer
- From: allison <ajp166@xxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Sat, 29 Aug 2015 16:16:23 -0400
On 08/29/2015 02:56 PM, Ashhar Farhan wrote:
allison,
i tried two different tracking generators based on the hetrodyne
principle. they both leaked into the IF badly, raising the noise floor
by 15 db.
I used power splitters so there is more isolation between the source and
the two destinations.
Given a chance the RF doing to mixer one can easily find a path to the
tracking generator
mixer if not stopped in its tracks.
Also even though the 570 and MAV11 are driving the mixer a low pass
filter at the mixer at IF
plus 80mhz or so can help as higher order harmonics from the source are
an issue. Also
try to not over drive the MMIC they can do weird things, even oscillate
at selected frequencies.
Seems like a painful loss but a input attenuator of 6db is advised at
the mixer. It protects the mixer
from an oops (transmit into it) and also it guarantees the input filter
has a good termination.
I've measured a few Spec analyzers and typical front end noise figures
are high in the 22db
range. If you need lower noise figure (improved input sensitivity) the
solution is a clean
(more than quiet) preamp for time when you looking down below -90 to
-100 dbm or so.
Hint if it slides around the bench easily likely not enough shielding.
Each major stage lives
in it s own box. For spec analyzers its a must. Soldered up boxes with
lids made of copper
clad work very well for this.
Now, instead, I am going to try an Si5351 directly at the signal
frequency.
I'd think that would work. But adjacent channel coupling is likely.
I did try the CMOS version of Si570. but i noticed more spurs than
with the LVDS version. I am not sure if I must attribute it to the
LVDS version alone. I had substantially increased the shielding
between changing the Si570 type.
Played with CMOS and LVDS and noted no real difference save for
interface. Shielding is
everything. Generally speaking the NCOs (Si570) are close to spur free
if harmonics are
ignored. They unfortunately due to the near perfect square wave are
strong on harmonics.
Spec analyzers are a good exercise as they allow one to look deep (small
signals) over wide
bandwidth and a consequence is that you start seeing things you might
have not noticed in
a narrow band system or thoroughly filtered systems. All the good ones
I use even the
portable Rhode&Schwarz FSH6 are remarkably heavy and its all shielding.
FYI take care with microprocessor clock and data lines especially the
SCLOCK. They
can make a racket.
Allison
- f
On Sat, Aug 29, 2015 at 11:59 PM, allison <ajp166@xxxxxxxxxxx
<mailto:ajp166@xxxxxxxxxxx>> wrote:
Farhan,
One suggestion:
Add tracking generator (difference of first osc and second, filter and
pump up with amp)
as then it adds sweeperino capability another useful function in one
instrument. It adds
little to the complexity but adds huge functionality.
One comment. Shielded modules, dead bug or PC are a must to get the
best result.
Allison
On 08/28/2015 01:01 AM, Ashhar Farhan wrote:
> While working on the Minima project, I needed a spectrum analyzer to
> measure the performance. The new specan is as complicated as a
simple
> SSB rig, probably even simpler.
>
> This is an Si570/Arduino controlled spectrum analyzer that can be
> built for less than hundred dollars. It is simple to build and
it can
> be used to measure IMDR, spurs, harmonics, etc. It doesn't need an
> oscilloscope.
>
> I like you all to read about
> it http://hfsignals.blogspot.in/p/specan-reboot-of-w7zoi.html
>
> - f
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