Re: Tektronix 7L12 Spectrum Analyzer Question

In article <428bad41_3@xxxxxxxxxxxxxxxxxx>, chuck@xxxxxxxxxx says...
> I'm in the market for a used spectrum analyzer like the 7L12
> or the HP 141-T. One important use will be to make two-tone
> IMD measurements on HF SSB transmitters. I am concerned that
> with tone separations on the order of one kHz, the 7L12 may
> not have sufficient bandwidth in the 300 Hz mode to resolve
> adjacent IMD products expected to differ in amplitude by 40
> dB or more.
>
> Has anyone on the group used a 7L12 for this purpose and is
> the 300 Hz RBW sufficiently narrow?
>
> There seems little doubt that the 141, with 10 Hz or 100 Hz
> RBW, will handle this.
>
> Many thanks in advance.
>
> Chuck
> NT3G
>

It wouldn't be ideal for 1 kHz spacing. If the -6 dB point is 300 Hz,
and the shape factor of the 7L12's filters is about 8:1 like other Tek
analyzers I've used, then the -60 dB point is around 2.5 kHz. Figuring
a linear falloff (in log space), 1 kHz tones couldn't be resolved below
about 30 dB, give or take 10 dB at the most.

I've never used a 7L12; maybe the filters are steeper than that. But
you can't beat an HP 141T / 8553B / 8552B (not -A!) combination for HF
work at less than 3x the price. If you've got the space, that's the way
you should go.

-- jm

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