Spectrum Analyzer or FFT of DSO?

[Sajjadkhan]

Ok Guys i am a bit confused. i want to learn about filters and their designing so i need some thing to measure the frequencies. i have few questions:

1. i already have a 25MHZ DSO with FFT as its Maths function, so do i need a spectrum analyzer, if yes than why?

2. why does basic standalone spectrum analyzers starts from 9 KHZ why not from DC. Those who does are way too expensive.

3. Finally given the links below what spectrum analyzer would you prefer? the usb one starts from 1HZ.

https://www.rigolna.com/products/spectrum-analyzers/dsa800/dsa815-tg/

**broken link removed**

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Admin please move this post to its respective category. thanks.

 

[HMS1021]

To measure filters a network analyzer (NA) and a RF sweep generator is the common approach for tuning and test. Add a directional coupler to measure return loss. This setup gives a fast real time plot of insertion and/or return loss vs freq.

A spectrum analyzer (SA) can be used, it can measure the out of band rejection to much lower levels than most network analyzes. A SA is basically a receiver that you can set the bandwidth of that displays the received signal as it tunes across a range of frequencies. You set the SA to a freq, set the sweep generator's output to the same freq note the signal level on the SA and then insert your filter between them. The signal level change displayed on the analyzer is the filter's insertion loss at that Freq. it can be a slow process useless it is done automatically under computer control. (You can get a amplitude vs freq plot of a filter by sweeping the RF Generator at a slow rate while the SA is sweeping across its freq range especial if the SA has display storage.) I can attach examples if needed.



1. A DSO/FFT will show you the frequency components of a signal that is sampled in the time domain. Ie put in a square wave ( or other wave form) and it will provide a plot of amplitude versus freq so you can see the fundamental freq and the harmonics. This is really not helpful in testing filters where you want a plot of amplitude versus frequency.

2. A SA is a multi conversion superheterodyne receiver. There must be separation between the local oscillator and the RF freq sufficient to permit image rejection.

3. The Rigol. It goes to 1.5GHz and will have many more uses. The 19kHz minimum is not a problem. Below 10MHz you can just use your o'scope for testing filters. There are also very inexpensive software that uses your computers sound card for a 30Hz to 30kHz spectrum analyzer. With either unit you still need a RF sweep generator that covers your freq range.