Re: Antuino - the antenna analyzer

Ashhar Farhan
 

jerry, 
we need 80 db dynamic range for useful work with specans. at 6db per bit, this pushes for a 14 bit dac or better. these, including thr rtl-sdr family are hardly useful.
for a radio amateur, a double conversion specan can easily be buily for less than $100 providing as good a performance as thr professional ones with limited range and bandwidth selection.
with 110 mhz as the first IF and 12 mhz as the second, onr can span 0-70 mhz easily 
- f

On Sat, 29 Dec 2018, 11:16 Jerry Gaffke via Groups.Io <jgaffke=yahoo.com@groups.io wrote:
I was assuming the Antuino would have a set of front end filters for each region of interest.
Though the 25mhz IF is kind of low for this to be convenient.

Arv's DC receiver as a spectrum analyzer is also worth considering. 

I've had parts here for a year or so to build a 0-55mhz spectrum analyzer 
around two of the PX1002 86.85mhz SAW filters.  
Seeing the Antuino, I'm inspired enough to have parts splayed out on my workbench,
an enclosure selected, am about to cut the bare PC board to fit. 
Will build ugly style, use 3/4" copper pipe cut into 1/4" high cylinders
to provide a shield for each section, bolt a plate over the top once it works.
Signals going off to the next shielded section will have a wire through a hole
in the board to to where it's held down to the back groundplane using copper tape. 
So almost coax, the one inch runs short enough that impedance won't be an issue.
But by selecting appropriate wire diameter and insulation thickness, might hit 50 ohms.

And if it works, maybe add another stage up front using a 915mhz SAW filter.
That first VFO would have to do something like 915 to 1500mhz.
Either a clock doubler on the 1000mhz si5340, or perhaps the si549.
But of course, I'll have a hard time looking at that stuff when it doesn't work.

Anybody have a feel for how well the cheap $200 laptop based spectrum analyzers
are doing?  Is there much reason to homebrew other than as a hobby?
How do those $200 units compare to a Rigol SA?

Jerry, KE7ER


On Fri, Dec 28, 2018 at 08:05 PM, Ashhar Farhan wrote:
the challenge would be to eliminate the if image. this could be done when you sweep to both sides and eliminate the image reading in software. here is an example :
 
1. lets imagine that there is a signal at 10 mhz. for this, we set the local oscillator at 35 mhz (as the IF is at 25 mhz). how can we be certain that the signal we are resolving is not at 60mhz (60mhz - 35 mhz = 25 mhz)?
2. we can take a second reading by setting the local oscillator at 85 mhz. in this csse, thr 60 mhz response (if any) should show up. else we can confirm that the signal we resolved in step 1 is indeed a 10 mhz signal.
 
one has to be aware that such a system will be slow. lets imagine that you have to sweep 30 mhz. the crystal filter is 10 khz wide. hence, we will need 30,000/5 = 6000 readings. if we can get, 100 readings a second, this will take a minute to complete the scan. an alternative would be to have two filters, a crystal filter and a 4 or even 5 section LC filter at around 12 mhz. we could switch to the lc filter for large sweeps. my specan circuit has a suitable design.
 
you are aking what happens if there was signal at 110 mhz in step two? aaaah, you tell me!
 
- f

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