# frequency analyzer or spectrum analyzer troubleshotting

Discussion in 'General Electronics Chat' started by relicmarks, Apr 26, 2008.

1. ### relicmarks Thread Starter Active Member

Oct 13, 2006
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0
When troubleshotting a circuit using a frequency analyzer or spectrum analyzer what can it tell the electronic tech how the circuit it wrong?

I use a function generator set at Squarewaveform that goes to the input of a circuit, i use the frequency analyzer on the output of each stage to troubleshoot each stage of the circuit

If the stages are faulty or components are shorted or open what would the frequency analyzer display?

When i have used a frequency analyzer is displays each frequency components amplitude + frequency

So if the amplitude is lower or higher on some of the frequencys what would that mean when troubleshooting a circuit? a resistor is open or shorted? thats why it raised or lowered the amplitude at those frequencys displaying on the frequency analyzer?

How would you guys use a frequency analyzer when troubleshooting ?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
What is the function of the circuit?

if you see some frequencies with more amplitude it means that the circuit has some filtering effects on the low amplitude frequencies.

3. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
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What is the function of the circuit? "in general"

if you see some frequencies with more amplitude it means that the circuit has some filtering effects on the low amplitude frequencies.

What you give me more examples please of circuits in general

Ampliifers preamps, power amps, etc.

4. ### mik3 Senior Member

Feb 4, 2008
4,846
70
A spectrum analyzer shows you what frequencies are contained in the signal you are measuring and what is the amplitude of each of these frequencies. For example, an amplifier designed to drive a woofer will amplify the low frequencies (you see them with high amplitude on the spectrum analyzer) and attenuates the medium and high frequencies (low amplitude on the analyzer).

5. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
A spectrum analyzer shows you what frequencies are contained in the signal you are measuring and what is the amplitude of each of these frequencies.

Yes, i know this is the basics

But when your troubleshooting stages or a circuit What are some basic or general spectrum analyzer problems? like would the frequencys be wrong or the amplitudes be wrong? and if they are what components or steps would you take after this?

Is it mostly a RC component problem ? Capacitors set the frequencys and resistors set the amplitudes of circuits and networks

So if the spectrum analyzer is displaying wrong amplitudes would mean its a faulty resistor in the stage? or network?

IF the spectrum analyzer is displaying wrong frequencys would it mean its a faulty capacitor in the stage? or network?

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Do you have a particular circuit in mind?

If not, why not experiment with a signal generator capable of a frequency sweep, along with a spectrum analyzer on a known good circuit, and a known bad circuit. You'll quickly recognize the difference.

Hint: The frequencies are always correct. It is the amplitudes at the frequencies which indicate a problem.

It's nearly always a component problem. Sometimes, it's an engineering problem; ie: some goober engineer hosed it up, and there isn't a snowball's chance in a warm place that it's going to work right. But the latter probability is fortunately usually on the low end of the probability scale.

If a component goes bad or is out of tolerance in a properly engineered circuit, it can be very difficult to track down the exact component or combination of components that are causing the problem(s).

One thing for certain: Open caps to ground will tend to shift the frequency response higher.

If you really want to find out about a circuit's response over frequency, then test it using a network analyzer that is equipped with an S-parameter test set.

7. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
One thing for certain: Open caps to ground will tend to shift the frequency response higher.

Do you mean bypass capacitors?

What other certain things?

Hint: The frequencies are always correct. It is the amplitudes at the frequencies which indicate a problem

If the amplitudes are wrong then what would be the problems in general?
What kind of components for amplitudes?

What other components or networks can shift the frequency

What other problems that are general for networks or circuits that are displayed on a frequency analyzer?

8. ### Wendy Moderator

Mar 24, 2008
21,823
3,039
A spectrum analyzer will show the signal level, much like a voltmeter only in DB, as well as the frequency it is at. This is extremely useful for RF amps. It will show oscillations, and if harmonics are generated then that is waveform distortion. It is actually a pretty useful piece of equipment. Frequencies never shift themselves, they can't, they are rock solid, but they can generate harmonics. They also give hard numbers for signal to noise ratios, also critical for RF circuits.

9. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
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How do "oscillations" look or display on a spectrum analyzer? What do they look like please?

How do "Harmonics" look or display on a spectrum analyzer? What do they look like please?

How do "waveform distortion" look or display on a spectrum analyzer? What do they look like please?

They also give hard numbers for signal to noise ratios, also critical for RF circuits.

How do "Noise" look or display on a spectrum analyzer? What do they look like please?

10. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
Spectrum Analyzer Applications:

Device Frequency Response Measurements: You can use spectrum analyzers for measuring the amplitude response (typically measured in dbm) against frequency of device.
broadband amplifier to a narrow band filter.

Microware Tower Monitoring: You can measure the transmitted power and received power of a Microware tower. Typically, you use a directional coupler to tap the power without interrupting the communications. In this way, you can verify that the frequency and signal strength of your transmitter are according to the specified values.

Interference Measurements: Any large RF installations normally require site survey. A spectrum analyzer can be used to verify identify and interferences. Any such interfering signals need to be minimized before going ahead with the site work. Interference can be created by a number of different sources, such as telecom microwave towers, TV stations, or airport guidance systems etc.

Other measurements that could be made using spectrum analyzer include the following:

Return-loss measurement
Satellite antenna alignment
Spurious signals measurement
Harmonic measurements
Inter-modulation measurements

11. ### Wendy Moderator

Mar 24, 2008
21,823
3,039
A spectrum analyzer output is a graph of frequency to amplitude, much like an oscope is a graph of voltage over time. It shows a line for the frequency on the graph, with the lines amplitude being read on the side scale.

Harmonics are multiples of the core frequency, always. Various waveforms have a distinct mathematical link on what frequencies are there (Wiki Fourier Analysis sometime), but they are always multiples of the base frequency and specific amplitudes. Harmonics are generated when the waveform is distorted from a pure sine wave.

If you have a frequency you didn't introduce, and it isn't a harmonic, then it is oscillation, probably with it's related harmonics.

Noise is the "grass" you get at the bottom of the display. Noise is the presence of all frequencies simultaneously.

To measure signal to noise ratios you introduce a calibrated noise level into the circuit being tested, with notch (filtered frequencies) being part of it. Then you measure the signal noise vs. the noise you get on the bottom of the notch. When I worked at Collins it used to drive us crazy that the customers had much better equipment than we did for CRG (customer returned goods). We tended to get the dregs of the equipment that was available, and customers were sending us printout of measurements we couldn't hope to get close to or duplicate. Makes it hard to fix stuff when it is a borderline failure.

12. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
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Spectrum analyzers are generally employed to measure signal characteristics such as carrier
level, sidebands, harmonics, and phase noise.

When combined with a tracking generator, spectrum analyzers can
be used for scalar component testing to show magnitude versus frequency information but not
phase information.

A spectrum analyzer with a dedicated tracking generator and an external coupler or directional
bridge can approximate the operation of a scalar network analyzer by showing amplitude as a
function of swept frequency.

However, the spectrum analyzer cannot make ratioed measurements or phase measurements.

13. ### Wendy Moderator

Mar 24, 2008
21,823
3,039
I get the feeling I'm talking to two people here. What's up?

Mar 24, 2008
21,823
3,039
15. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
What are some faulty or defective stages viewed or display on the spectrum analyzer? what would they look like?

Would the harmonics be "missing"?

Would there be added harmonics is its oscillations?

Would the harmonics have different amplitudes?

Would the harmonics had noise on them?

16. ### Wendy Moderator

Mar 24, 2008
21,823
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There are lots of exceptions in the field of RF, but in general we're talking linear amps. If a linear amp is working correctly then you should see an increase in gain with no harmonics generated. Harmonics in transmitters are a very bad thing, something to be eliminated. They would probably be illegal if allowed to be transmitted.

Where the exception exist on recievers is FM circuits. AM is pretty vunerable to distortion, but FM (especially before AGC) depended on amplifying a signal to the point of obvious distortion, almost a square wave, and then couldn't amplify it anymore. Read the ebook on clippers.

A spurious oscillation may not be a clean sine wave, so there will be harmonics.

A bad transistor or amplifier stage can generate noise, which could be the problem you are troubleshooting. Harmonics are signals, not noise, so their is not a direct corrilation.

You can use filters, bandpass or lowpass, to eliminate harmonics. Harmonics can also be a feature, this is where analog frequency multiplication is created.

Something that needs mentioned is spectrum analyzers tend to make lousy probes, since they tend to be terminated is a standard low resistance. They are useful, but you have to work within their limitations.

Do you have a new toy or something? Or is this a school project?

17. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
Spectrum Analyzer test:

1.) analyze the electrical signals that are passing through
2.) information is being transmitted correctly
3.) Measure the amount of power being transmitted
4.) Tests such as modulation degree, sideband amplitude, modulation quality, occupied bandwidth
5.) measuring distortion is critical for both the receiver and transmitter
6.) harmonic distortion at the output of a transmitter can interfere with other communication bands.
7.) receiver must be free of intermodulation distortion to prevent signal crosstalk.
8.) Common distortion measurements include intermodulation, harmonics, and spurious emissions.
9.) Measuring noise figure and signal-to-noise ratio (SNR)

18. ### Wendy Moderator

Mar 24, 2008
21,823
3,039
So have we helped?

19. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
Spectrum Analyzer test and notes:

1.) analyze the electrical signals that are passing through
2.) information is being transmitted correctly
3.) Measure the amount of power being transmitted
4.) Tests such as modulation degree, sideband amplitude, modulation quality, occupied bandwidth
5.) measuring distortion is critical for both the receiver and transmitter
6.) harmonic distortion at the output of a transmitter can interfere with other communication bands.
7.) receiver must be free of intermodulation distortion to prevent signal crosstalk.
8.) Common distortion measurements include intermodulation, harmonics, and spurious emissions.
9.) Measuring noise figure and signal-to-noise ratio (SNR)
10.) measurements of frequency, power, harmonic content, modulation, spurs, and noise
bandwidth, signal stability, output power, intermodulation distortion, power bandwidth, carrier-to-noise ratio
11.) A pure sine wave has no harmonic distortion
12.) fixed-tune mode (zero span) to provide time-domain measurement capability much like that of an oscilloscope.
13.) In the frequency domain, complex signals (that is, signals composed of more than one frequency) are separated into their frequency components, and the level at each frequency is displayed.
14.) The technique most widely used is superheterodyne.Heterodyne means to mix - that is, to translate frequency - and super refers to super-audio frequencies, or frequencies above the audio range.
15.) Know your analyzers have both hardkeys and softkeys
16.) measure your lower frequency baseband and IF signals
17.) Measure the frequency and power level of a carrier
18.) modulation frequencies, channel spacing, pulse repetition frequencies, and offset frequencies relative to the carrier.
19.) frequency-reference inaccuracy, span error, and RBW center-frequency error.
20.) Measure a log amplifier how true the logarithmic characteristic are
21.) Measuring how Linear the detector circuit
22.) Check for amplitude differences
23.) the IF filter bandwidth is also known as the resolution bandwidth (RBW). This is because it is the IF filter bandwidth and shape that determines the resolvability between signals
24.) Check for noise in the sidebands
25.) Selectivity is the important characteristic for determining the resolvability of unequal amplitude signals.
Selectivity is the ratio of the 60 dB to 3 dB filter bandwidth
26.) frequency stability of the spectrum analyzer's local oscillator. This
inherent short-term frequency instability of an oscillator is referred to as residual FM. If the spectrum analyzer's
RBW is less than the peak-to-peak FM, then this residual FM can be seen and looks as if the signal has been
"smeared". You cannot tell whether the signal or the LO is the source of the instability.

20. ### relicmarks Thread Starter Active Member

Oct 13, 2006
355
0
How do u use a spectrum analyzer when testing linear amps?

What are some bad displays to check?

What tests can i do with a spectrum analyzer when testing a linear amp?

MOstly checking the amplitude differences for each frequency component?

1.) Check each frequency component
2.) Check each amplitude differences for each frequency component