Frequency Analyzers measurements

[danny davis]

Frequency Analyzers measurements


On a frequency analyzer, can you set the time per division squares?

Because all i can find is you can only set the SWEEP TIME not the time per division time value like on a oscilloscope

Frequency Analyzer
What have you used a frequency analyzer and for what kind of measurements do you do?

Why is a frequency analyzer better to use to measure NOISE instead of an oscilloscope? because you can view noise waveforms on an oscilloscope but why you a frequency analyzer?

 

[davenn]

Hi danny

a frequency analyser is better known as a spectrum analyser
they do work a bit different to an oscilloscope. X axis ( along the bottom is frequency from lowest ( on left) to highest ( on right). There is no set amount per division as as it is user adjustable. depending on the abilities of the spec an. it may be adjustable from ~ 10kHz / division out to maybe 100MHz or so / division

so no, there is no time /div as with an O.scope

the Y axis is signal strength and is usually displayed/calibrated in dBm / division and is also adjustable often from ~ 1 dB to 10 dB / division

here's a pic of my own spec an. .....

it covers from 9kHz to 1.9GHz, one day I would like one that went up over 10GHz from the higher freqs I work on

cheers
Dave

 

[danny davis]

so no, there is no time /div as with an O.scope

'

Can you set the freq per division?

Because I can't adjust what is per division right? or what is per division on a frequency analyzer?

 

[davenn]

What have you used a frequency analyzer and for what kind of measurements do you do?

now my spectrum analyser as shown in the last post is for RF, not AF work
but the principles are much the same
its for accurately measuring frequency and strength of a signal.
You can see any harmonics ( multiples of the fundamental signal) and their relative strength.
When a RF carrier signal is modulated, "sidebands" are created. the analyser will tell you their frequency and their relative strength.
It can show you other transmissions and their strength close to your signal that may be causing interference
When used in conjunction with a sweep generator it can be used to determine the frequency response and bandwidth of filters.

Why is a frequency analyzer better to use to measure NOISE instead of an oscilloscope? because you can view noise waveforms on an oscilloscope but why you a frequency analyzer?

I wouldnt say they are necissarily better, they just measure noise in a different way
Spec an's are good for measuring something called phase noise. All oscillators either audio or RF generate phase noise
This is particularly important in my RF work as I want to know how clean the output of an oscillator is. Some things that are looked at in an oscillator performance are...

harmonics -- multiples of the fundamental signal, they can be wither even or odd multiples.

phase noise -- is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a waveform, caused by time domain instabilities
( thats a real mouthful its quite a complex subject) Its presence can cause real problems when working with narrow band weak signals

spurii -- random spikes of signal not harmonically related to the fundamental signal
( often caused by the mixing of 2 signals)

With an RF spectrum analyser you can see ALL other RF signals present and how strong they are. lots of very weak ones close to the frequency you are using will be present as noise in your receiver

cheers
Dave

 

[davenn]

'

Can you set the freq per division?

Because I can't adjust what is per division right? or what is per division on a frequency analyzer?

yes reread the first couple of sentences of my first post


D

 

[danny davis]

it may be adjustable from ~ 10kHz / division out to maybe 100MHz or so / division

How do you set the frequency per division? which menu or button is that?

Is it the center frequency?

 

[davenn]

How do you set the frequency per division? which menu or button is that?

Is it the center frequency?

ok if you look at my analyser pic above ( click for full size image for better clarity)
to the right of the large knob there are 3 oblong buttons
from top to bottom

CHAN/FREQ - push that button and then use numeric keypad to select centre freq of interest

SPAN/DIV - then use keypad to select freq span per division

REF LEVEL - this sets the Y axis reference level for reading signal strength

these buttons are common to most decent spectrum analysers, they may not be placed in the same place on the control panel but are likely to be labelled the same or very similar

Dave

 

[danny davis]

SPAN/DIV - then use keypad to select freq span per division

So the SPAN sets the frequency per division square?

 

[davenn]

So the SPAN sets the frequency per division square?

thats what it says

D

 

[danny davis]

But if you set the SPAN to 10Khz

You can count the squares and multiple them them by 10khz and get what?


Like on a oscilloscope it says time per division , So you can count the squares and multiple them by the times per division setting

On a frequency analyzer it doesn't say frequency per division it says SPAN


I thought SPAN ment something different

The Sweep TIME sets the display to visually be viewable nice and not flashing or strobbing

 

[davenn]

if its set to 10kHz / division, most units have 10 divisions horizontal so you can look over a 100kHz section of spectrum (bandwidth)

D

 

[davenn]

conversly, if you set it to 10MHz / division you can look at a 100MHz block of spectrum. you could then use the other button CHAN/FREQ and select the frequency you want to observe ... it may be the output of say an oscillator.
it will then be in the middle of that 100MHz span

eg you freq of interest was 450 MHz and you span is set to 10MHz / division
then you will be looking at a range (bandwidth) of 400 to 500 MHz ( with 450MHz in the centre of the display)

D

 

[danny davis]

eg you freq of interest was 450 MHz and you span is set to 10MHz / division
then you will be looking at a range (bandwidth) of 400 to 500 MHz ( with 450MHz in the centre of the display)

I get confused because you can set the Bandwidth range with the ( BW BUTTON) and the Center Frequency with the Center frequency Button

10Mhz X 5 square is only 50Mhz
10Mhz X 10 square is only 100Mhz

Where do you get 400mhz to 500 Mhz from a SPAN at 10Mhz?

 

[davenn]

I get confused because you can set the Bandwidth range with the ( BW BUTTON) and the Center Frequency with the Center frequency Button

10Mhz X 5 square is only 50Mhz
10Mhz X 10 square is only 100Mhz

Where do you get 400mhz to 500 Mhz from a SPAN at 10Mhz?

10MHz / division x 10 divisions = 100MHz of bandwidth you are looking at

if you set you freq you want to look at say 450 MHz then it will be the centre freq and you will see 50 MHz either side of it ie. the piece of spectrum from 400 to 500 MHz --- 100 MHz total span (bandwidth)

Dave

 

[davenn]

now in the meantime dont worry about the RES BW button that is lower down on the control panel thats resolution bandwidth and in there you can select either AUTO or VIDEO ( we wont go into that at the moment) a whole different thing

D

 

[davenn]

ok, I have to go home from work ... its the end of the day for me

we will continue this later
post any other Q's you have and I will reply when next at the puter over the weekend

D

 

[danny davis]

10MHz / division x 10 divisions = 100MHz of bandwidth you are looking at

It is bandwidth or spectrum or Span?

Because I thought the BW ( bandwidth) Button set the bandwidth

I'm confused on this now

if you set you freq you want to look at say 450 MHz then it will be the centre freq and you will see 50 MHz either side of it ie. 400 to 500 MHz 100 MHz total span (bandwidth)

You mean if I set the "Center frequency" to 450MHZ?

 

[davenn]

It is bandwidth or spectrum or Span?

Because I thought the BW ( bandwidth) Button set the bandwidth

I'm confused on this now
You mean if I set the "Center frequency" to 450MHZ?

you shouldnt be


step back a few steps and reread what I wrote earlier.....
here's a repost....

ok if you look at my analyser pic above ( click for full size image for better clarity)
to the right of the large knob there are 3 oblong buttons
from top to bottom

CHAN/FREQ - push that button and then use numeric keypad to select centre freq of interest

SPAN/DIV - then use keypad to select freq span per division

REF LEVEL - this sets the Y axis reference level for reading signal strength

these buttons are common to most decent spectrum analysers, they may not be placed in the same place on the control panel but are likely to be labelled the same or very similar

ok see the second one SPAN/DIV that means freq span per division OK
that can be set at almost any frequency amount you want it basically determines how much you want to "zoom" in on your frequency of interest.

As I also commented earlier my analyser covers 9kHz to 1.9GHz (1900MHz)
So the TOTAL span (bandwidth) capability of the unit is 1.9GHz minus a few kHz

Now we dont have to always look at that whole range we want to be able to zoom in and look a bit closer around our freq of interest, I gave the example of 450MHz lets say thats the frequency of our transmitter and we want to make sure it has a clean looking signal.
So pushing the SPAN/DIV button we can then use the keypad to enter say 5MHz, so that means each division is 5MHz in width, we have 10 horizontal divisions so thats a total SPAN of 50 MHz of viewable bandwidth. we chose 450MHz as our freq of interest so that will be our centre freq on the display ie. half way across the display and there will be 25 MHz of viewable spectrum either side of our centre freq of 450MHz so we will view the spectrum from 425MHz to 475MHz OK

alternatively we want to zoom out a to see more spectrum so ....
pushing the SPAN/DIV button we can then use the keypad to enter say 50MHz, so that means each division is 50MHz in width. We have 10 horizontal divisions so thats a total SPAN of 500 MHz of viewable bandwidth. We chose 450MHz as our freq of interest so that will be our centre freq on the display ie. half way across the display and there will be 250 MHz of viewable spectrum either side of our centre freq of 450MHz so we will view the spectrum from 200MHz to 700MHz OK

The freq you choose to look at will ALWAYS be the centre freq and central in the display in the total span of spectrum you are viewing

cheers
Dave

 

[danny davis]

The Frequency of interest is 450Mhz

1.) You push the Center Frequency button First and set it to 450Mhz
2.) Now set the SPAN frequency per division
3.) Set the Reference Level

What I am confused about is if the SPAN frequency sets up the Bandwidth , But What does the BW bandwidth button do or set on a frequency analyzer?

 

[danny davis]

I'm still confused on how to measure Noise on a frequency analyzer

What do you do?

Is an oscilloscope better

A Frequency analyzer doesn't have volts per division , so when you look at waveforms on the frequency analyzer you can't measure the voltages of the frequency waveforms?

It only has DB for signal strength not Voltage level right?

The frequency Analyzer will tell you the Frequency of the Noise waveform right? But an oscilloscope will tell you the Voltage level and what Frequency the noise waveform is

So isn't an oscilloscope better to use?