[Premium-Rx] bandscope frequency conversion question

[Jack Smith]

Brooke:

As I understand it, the 4395A uses a dual FFT/Stepped system. It tunes 
in discrete frequency steps and for each frequency step, it performs an 
FFT.

This gives you the advantage of an FFT (fast, but limited bandwidth) and 
a conventional swept spectrum analyzer (slower, but wide swept bandwidth).

I've been working about a month now on a DSP-based updated panadapter, 
probably ready sometime next year. I'm in the early stages of 
development, but one of the things on my list to look at is a stepped 
approach with a Softrock type design. Assuming you can do an FFT in 25 
ms (not unreasonable) for a 25 KHz frequency span, then one could do a 
100 KHz stepped approach (4 steps of 25 KHz each) in 100 ms. Stitching 
the different FFTs together gets a bit messy, but, as they say "it's 
only software ..." Plus, I imagine you would get some odd artifacts for 
pulse waveforms, but they are a problem with any method of spectrum 
analysis.

Jack K8ZOA

Brooke Clarke wrote:
> Hi Jack:
>
> The connection between RBW and sweep speed comes about because of the 
> physical nature of the band width filters.   Those restrictions are 
> not present when using a digital system.  For example the HP/Agilent 
> 4395A combo box when used as a spectrum analyzer can sweep a 100 kHz 
> span with a 100 Hz RBW in 0.3 seconds whereas an analog SA will need 
> 30.0 seconds to make the same sweep.
> http://www.prc68.com/I/4395A.shtml#SA p
>
> This kind of thing is great for doing a spectrum survey.  The narrow 
> RBW puts the noise floor way down.  Peak capture remembers any 
> transmissions.  Come back after a day or so to see where the action is.
>
> The thing I really like is that the RBW (as well as but different than 
> VBW) can be set to 1 Hz, much too narrow for an analog filter.  But 
> now I wonder what it would be like to have sub 1 Hz RBW capability.
>
> I think this is one of the reasons why the SDR-IQ and other SDRs are 
> popular.
>
> Have Fun,
>
> Brooke Clarke
> http://www.PRC68.com
> http://www.precisionclock.com
>
>
>
> Jack Smith wrote:
>> You might find my article on  the Z90 digital panadapter of some 
>> interest, as it discusses span versus resolution and sweep time. A 
>> copy is available at the ARRL's QEX site.
>> http://www.arrl.org/qex/2007/03/smith.pdf.
>>
>> You might also find some useful things if you poke around my web site 
>> a bit. www.cliftonlaboratories.com. It shows some typical spectrum 
>> displays of different modulation types among many other things.
>>
>>
>> Jack K8ZOA
>>
>> H. L. wrote:
>>
>>> Just discovered the premium-RX group and am enjoying the archive 
>>> very, very much.  Quick question. Has a circuit been published for 
>>> an outboard mixer/oscillator that will convert a premium receiver's 
>>> 455 kHz IF output to 21.4 MHz for connecting to a WJ (or other) 
>>> panoramic bandscope spectrum analyzer. I understand that the display 
>>> frequency range will be highly limited (or compressed) and that a 
>>> lower quality Heathkit SB620 bandscope will give a better, higher 
>>> resolution display than an expensive WJ scope because it is designed 
>>> for a 455 kHz input.  I don't understand the mathematics of this 
>>> issue and would appreciate an article reference if available.  
>>> Regards, Hal, KK6HY (main premium receiver: Stoddart NM-26T -- with 
>>> a 455 kHz IF output jack). My Stoddart radio history website: 
>>> http://online.sfsu.edu/~hl/src.html
>>> _______________________________________________
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>>> Visit the Website: http://www.premium-rx.org
>>> Email Help: paul at premium-rx.org
>>>
>>>
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>>
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>