analog filters vs software filter/editing

[antcollinet]

So change RFI to greater than 100mhz. I think everyone else understood what I meant. How do digital filters work without an analog filter first?

They don't. See my post no 12 immediately prior to your no 13

But it is specifically an anti aliasing filter, not necessarily an RFI filter.

 

[Eujene]

Even in spectrum analyzers. Software spectrum analyzers can do everything an analog spectrum analyzers do? Absolutely no exception?

 

[MaxwellsEq]

Even in spectrum analyzers. Software spectrum analyzers can do everything an analog spectrum analyzers do? Absolutely no exception?

Are you confusing the word "analog" with "physical" or "hardware". So you are contrasting a dedicated hardware device that analyses spectra, vs the same thing running as an application on a PC?

If not, your question makes no sense.

 

[Eujene]

Yes. A dedicated hardware audio spectrum analyzer costs over $10,000. I was asking if the free REW can equal it in performance after you got the analog signal passed via say the $100 E1DA ADC.

 

[MaxwellsEq]

Yes. A dedicated hardware audio spectrum analyzer costs over $10,000. I was asking if the free REW can equal it in performance after you got the analog signal passed via say the $100 E1DA ADC.

OK, that means most of this thread is irrelevant.

To be clear, you can not normally use the word "analog" as a substitute for the word "hardware".

A very high grade, state of the art hardware analyzer can do things that REW + E1DA can not.

 

[Eujene]

OK, that means most of this thread is irrelevant.

To be clear, you can not normally use the word "analog" as a substitute for the word "hardware".

A very high grade, state of the art hardware analyzer can do things that REW + E1DA can not.

I was referring to pure high grade analog 1980s state of the art spectrum analyzer that doesn't use any ADC. But process every signal pure analog.

This is contrast to modern spectrum analyzer that uses ADC hence not pure analog.

 

[MaxwellsEq]

I was referring to pure high grade analog 1980s state of the art spectrum analyzer that doesn't use any ADC. But process every signal pure analog.

This is contrast to modern spectrum analyzer that uses ADC hence not pure analog.

That is not what you asked.

 

[Eujene]

I wrote:

"Even in spectrum analyzers. Software spectrum analyzers can do everything an analog spectrum analyzers do? Absolutely no exception?"

Software spectrum analyzers refer to REW/E1DA ADCs or those stand alone units that still use ADC inside.

Contrast this to pure analog spectrum analyzers or pure 1969 technology they used Astronauts on the moon. I wonder if their analog instruments can detect more stuff that our present software analyzers can't detect. They used analog sensors and analog filters. Contrast this to software which uses program to shift through the signals.

 

[Eujene]

To add above. I was asking about it because of vintage spectrum analyzers like the HP 3588A /001/003 10Hz-150MHz Spectrum Analyzer

HP 3588A /001/003 10Hz-150MHz Spectrum Analyzer | eBay

FFT Sweep Span 1Hz-40KHz. Spectrum:10Hz-150MHz res1.1Hz Zoon 11mHz Duynamic 140dB(type). Hitech&Facility Co,Ltd since 1997. If you have any carrier account, We can use it.

www.ebay.com

These are being sold very expensive at ebay. I presume they are pure analog without any ADC inside? They seem to have some appeal to them. I wonder if they can detect signals modern ADC + REW can't. If none. Then maybe they really belong to museum.

 

[MaxwellsEq]

You have the subject title "analog filters vs software filter/editing"

You asked "Can you give examples where analog filters can do things that can't be done by any software filter? By software filters are meant after the analog signals got into the ADC (Analog to Digital Converters"

Do NOT change your subject half way a thread. Start a new thread if you want to discuss something else. Spectrum Analyzers are a totally different topic to analog filters!

 

[Eujene]

But I thought analog spectrum analyzers have hundreds of analog filters inside! How can analog spectrum analyzers filter each frequency except using analog filters. Isn't it?

 

[Eujene]

To add above again. By analog filters I meant filters that measures say 1kHz. 3 kHz, 10 kHz pure analog. Since analog spectrum analyzers measure all frequencies. Then shouldn't there be hundreds if not thousands of analog filters inside??

 

[voodooless]

You can't use a digital filter for anti aliasing on the input to your ADC.

Yes, you can, and it's extremely common. That's why these things oversample, so you don't have to do this in the analog domain.

.. granted though, you'll always need an analog filter at some point to filter out anything beyond the high sample rates bandwidth, but the complexity is vastly lower (probably a first-order filter is fine, and you want that anyway for other reasons).

But I thought analog spectrum analyzers have hundreds of analog filters inside! How can analog spectrum analyzers filter each frequency except using analog filters. Isn't it?

No, they don't have hundreds of filters, see Wikipedia:

• A swept-tuned analyzer uses a superheterodyne receiver to down-convert a portion of the input signal spectrum to the center frequency of a narrow band-pass filter, whose instantaneous output power is recorded or displayed as a function of time. By sweeping the receiver's center-frequency (using a voltage-controlled oscillator) through a range of frequencies, the output is also a function of frequency. But while the sweep centers on any particular frequency, it may be missing short-duration events at other frequencies.

To add above. I was asking about it because of vintage spectrum analyzers like the HP 3588A /001/003 10Hz-150MHz Spectrum Analyzer

HP 3588A /001/003 10Hz-150MHz Spectrum Analyzer | eBay

FFT Sweep Span 1Hz-40KHz. Spectrum:10Hz-150MHz res1.1Hz Zoon 11mHz Duynamic 140dB(type). Hitech&Facility Co,Ltd since 1997. If you have any carrier account, We can use it.

www.ebay.com

These are being sold very expensive at ebay. I presume they are pure analog without any ADC inside? They seem to have some appeal to them. I wonder if they can detect signals modern ADC + REW can't. If none. Then maybe they really belong to museum.

From what I can see, this is actually a hybrid device, it can do both swept-tuned and FFT and probably uses it in parallel.

 

[Eujene]

I think I used the term analog filter incorrectly. So by analog filter is mean a wider passband. I know now. But what term do you refer to specific frequency filter.

In my original message I was describing "sharp audio band filter". So maybe the term to describe single frequency filtering is indeed "sharp audio band filter"? I wrote "back in the 1970s. They used sharp audio band filters or LCR band filters and then used spark/arc plotters to plot the envelope amplitude output from each of those filters."

What I meant by "envelope amplitude output" is from one frequency only and analog only. That is I want to compare output for every 1kHz, 2 kHz to 20 kHz. I want a device that can only show spectrum around 1 Khz, 2 kHz, 3 kHz or very narrow single frequency, not a passband, and using only pure analog, without any ADC.

What analog technology or term that can do that?

 

[solderdude]

Sharp notch filters that are voltage controlled over a certain band.
You can also look for phase changing when sweeping using PLL.
One would only need to switch capacitors to create more bands.
One can use the output voltage v.s. control voltage to find the spectrum.
The question is how 'accurate' it must be.
Why would that have to be done analog ?

 

[voodooless]

Why would that have to be done analog ?

.. and where would the output go?

like this: https://hackaday.com/2014/01/03/build-an-audio-spectrum-analyzer-the-analog-way/ ?

All in one chip: https://www.sparkfun.com/datasheets/Components/General/MSGEQ7.pdf

 

[Eujene]

The output of the analog single frequency filter goes to some kind of (as I mentioned in my first message) spark/arc plotters to plot the envelope amplitude output from each of those filters.

Is there within the realm of possibility that this can more accurately detect single frequency signal than frequency passing all of them via ADC and then using software spectrum analyzer?

Again remember people still use vacuum tubes of the 1950s in audio because it can capture certain aspectof sounds not possible in present op amps. Likewise the above questions.

 

[fpitas]

The output of the analog single frequency filter goes to some kind of (as I mentioned in my first message) spark/arc plotters to plot the envelope amplitude output from each of those filters.

Is there within the realm of possibility that this can more accurately detect single frequency signal than frequency passing all of them via ADC and then using software spectrum analyzer?

Again remember people still use vacuum tubes of the 1950s in audio because it can capture certain aspectof sounds not possible in present op amps. Likewise the above questions.

Uhhh. Somehow I don't remember that. Must be my failing memory with extreme age.

 

[voodooless]

Is there within the realm of possibility that this can more accurately detect single frequency signal than frequency passing all of them via ADC and then using software spectrum analyzer?

No, because the analog filter will never be narrower and steeper than what can be done with an equivalent digital filter, never mind using an FFT, which can be vastly more accurate (Shannon and Fourier are your friends here).

in the end, it's mostly just not worth it. You'll need increasingly insane electronics to do things the analog way. Getting some chips for a few $£€ is much simpler and gives better results.

What makes you think analog electronics would be more accurate?

 

[voodooless]

The output of the analog single frequency filter goes to some kind of (as I mentioned in my first message) spark/arc plotters to plot the envelope amplitude output from each of those filters.

How would a plotter ever be more accurate than a 20+-bit ADC? There is no way to have a million steps on a few cm of paper, never mind that you would be able to actually see them with your eyes. And never mind the time resolution of something like that..