Posted by
Geoffrey Mackenzie-Kennedy-2 on
Mar 17, 2007; 10:38pm
URL: http://elecraft.85.s1.nabble.com/Filters-and-Spectrum-Analyzer-tp444864p444866.html
Jack,
Thank you, I should have clarified this point. As I do not own a tracking
generator with built-in programmable offset, when looking at a filter when
there is a receiver signal mixer between the generator and the spectrum
analyzer I use the outboard mixer plus generator approach, with the spectrum
analyzer's swept oscillator providing one of the two injection sources for
tracking. This swept oscillator works in the same frequency region
regardless of the input working frequency of the spectrum analyzer. A stable
generator provides the second fixed frequency injection source for the
outboard mixer, with the mixing scheme - high side or low side - allowing
choice of whether sweep inversion is introduced or not. Using a tracking
generator with programmable offset would be a better choice to avoid any
spurious products produced by the outboard mixer, but by using a H-mode
mixer in this application potential spurs have not intruded.
I use the same hardware for looking at stand alone filters and other
circuitry when an offset is not required, so in effect the outboard mixer,
generator and buffers is my tracking generator.
73,
Geoff
Jack Smith wrote:
> Geoff:
>
> Your approach assumes that the tracking generator has a programmable
> offset, doesn't it? RF Generator output, for example, sweeps from 7 - 7.4
> MHz with the K2 tuned to 7.2 MHz, but the detector needs to sweep 5115 -
> 4715 KHz, if I've got the maths right. (Sweep also must be inverted to see
> the display correctly due to K2's inversion on 7 MHz band.)
>
> My VNAs don't have that feature, but I believe frequency offset is
> available in some of Agilent's current production VNAs. With an auxiliary
> detector, such as an HP3400 RF voltmeter, however, I could use my HP8752B
> VNA's "vertical axis input is auxiliary DC voltage" feature, taking the
> 0...1V recorder output from the 3400 as the VNA's auxiliary input. That
> gives the advantage of a high Z detector, but the significant disadvantage
> of broadband input. The other option would be to lash up a program to
> command a signal generator and spectrum analyzer via their GPIB control
> buses with the flexibility in frequency stepping that provides.
>
>
> Jack
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