This document provides an introduction to spectrum analyzers by explaining their key components and functions. It notes that spectrum analyzers are needed to analyze signals in the frequency domain because time domain analysis using an oscilloscope is not suitable for high frequency applications. The main components described are the pre-selector, mixer, local oscillator, intermediate frequency filter, detectors, video filter and ramp signal. It explains how these components work together using the superheterodyne receiver principle to convert a range of input frequencies to a single intermediate frequency so they can be analyzed one frequency at a time as the local oscillator is swept.

1. Introduction to Spectrum
Analyzer
Vinoth Kumar K
iNARTE Certified Associate EMC Engineer

2. Why we need Spectrum…?
• A special thanks to Fourier
• Who only found frequencies behind the time
• Any signal can be broken down into the sinusoidal
waveforms
• Sum of sinusoidal waves with various amplitude also
can give any defined waveforms
• Why we need Spectrum analyzer…?
• The answer is behind why we need frequency…!

3. Cont…
Oscilloscope display Spectrum analyzer display
When it comes for the high frequency applications
we can’t do anything with Oscilloscope
We need frequencies…..!
Because no books explained RF device
design using the time domain…!
Not even at least filters…!
Filter is the
window to
the
frequencies!

4. Why it’s so costlier…?
• Dynamic range:
• Approximately -120dBm to 10dBm
• It’s like visualizing a Pico meter sized object as well as one meter sized object on
the same screen with the good resolution
• Frequency range:
• Approximately 9kHz to 18GHz
• It’s nothing but counting 18000000000 variations in a second
• Sensitivity:
• dBuV
• Please think yourself…

5. Block diagram…
Mixer1
2
3
4 5
6

6. How it works in short…
• It’s a simple receiver like FM radio.
• In our home we manually tune from 88 MHz to 108MHz
• But the spectrum analyzer is a swept tune receiver i.e. It self tunes from 9kHz to
18GHz
• For any receiver we need Local oscillator and Mixer for allowing the
frequency we look in the screen
• Selecting any frequency we interest with the help of Local oscillator with a mixer
called super heterodyne receiver.
• Filters, detectors for the precise measurements
It needs no human to tune its range
That’s the heart of any receiver
Will see block
by block…!

7. 1. Pre-selector and Attenuator…
• The important difference from Spectrum analyzer to the EMI receiver
is a pre-selector
• It’s a Bandpass filter. It attenuates the unwanted frequency range by
40dB.
• Hence spectrum analyzer measures from the output of Pre-selector
• Improves the ambient noise level by eliminating the unwanted spectrum
• Attenuator stops the overloading of receiver
• When the measurements are made beyond +10dBm we may lose the
sensitivity
• But too much attenuation will add up more ambient noise…!
Try the measurements with Pre-selector ON/OFF for the difference in ambient noise…!
It’s always better to keep Auto attenuation ON since, it works in a adaptive manner

8. 2. Super heterodyne receiver…
• Mixer:
• The multiplier which multiplies the two sinusoidal inputs
f1
f2
f1+f2 and f1-f2
Hope you don’t forget the formula,
cosA*cosB= cos(A+B) + cos(A-B)
i.e. A=f1, B=f2
Try out in oscilloscope multiplying
two different frequency signals.
Looks complicated…!
(Freq_INPUT)
(VARIABLE TUNING)
That’s right. We will get any two frequency components from
Freq_INPUT by multiplying freq_INPUT with Local oscillator.
But we are going to remove one of them by LPF/HPF. Finally
we will get one freq information from freq_INPUT
But when we tune f2 continuously we can drag the whole
freq_INPUT…!

9. Cont…
• Local Oscillator:
• How t o tune continuously…?
• There we use VCO (Voltage Controlled Oscillator)- By the name it generates
frequencies with respect to the voltage applied.
In FM, we tune the freq by tuning the variable capacitor…!
Right, We need a ramp signal as an input to VCO i.e. Linear
increment of voltage results in Linear frequency sweep
VCORAMP
This is given to Mixer input

10. Cont…
• A little math:
0 to 110MHz
(Freq_sweep)
200MHz
(Freq_INPUT)
200 to 310 MHZ
(Intermediate Freq)
Right, If you want to see the 0Hz we will
tune the VCO to 200MHz alone.
To see 110MHz we will tune to 310MHz
To see 50MHz we should tune to 250MHz
f_IF = f_SWEEP – f_INPUT
f_INPUT = f_SWEEP – f_IF
Finally we understood, Intermediate frequency wont change. Now we are so
surprised…!
We have designed a broadband spectrum analyzer to receive all the f_INPUT
by designing for 200MHZ.
Then our work is to build the remaining circuit for 200MHz alone
This is the big
success of
Super
heterodyne
receiver…!
We will remove 310MHz components by LPF

11. 3.RBW/IF filter…
• Intermediate freq is fed to the Resolution Bandwidth(RBW) filter for
further clarity.
RBWIntermediate freq
Peak - 6dB
RBW
Intermediate freq
CISPR needs 6dB RBW filter in
EMI Receiver for the Emission
tests
When we choose 9KHz filter we are allowing only 9kHz
band signal centered by Intermediate freq and It
attenuates the remaining freq.
i.e. IF-4.5kHz to IF+4.5kHz
Filter response
When we choose a narrow band instead of
a broad band, Noise floor get decreased
It takes too much time for the test.
It is important to know what RBW we should choose?

12. Cont…
F_INPUT
RBW filter
RBW filter moves step by step based on setup
For example, CISPR suggests ½*RBW stepsize
i.e. 9kHz RBW moves for every 4.5kHz steps
0Hz, 4.5kHz, 9kHz, 13.5kHz,….etc.
In display it actually shows the area
of freq spectrum covered by filter
for each steps.
RBW_BW
Filter moves for every 0.5*RBW It points the area covered by the filter then
it connects every points.
When we zoom the display of Spectrum
analyzer we can see the discretized
connections.
With the narrow RBW it’s possible to see the
freq separations hence named the Resolution
Bandwidth
When we found any peaks it’s good to
sweep again with Narrowband filter
Broad filter
Narrow
filter

13. 4. Detectors…
• Once the filter attenuates/allows the unwanted/wanted frequency
contents the detectors are going to measure the spectrum
• The thing, we need to give enough time to measure i.e. measuring
time
• Peak detector:
Output
When we give measuring time 30msec
Detector will display the maximum value measured in 30 msec
Hope you remember,
Diode will be ON when Anode voltage > Cathode value.
Detector waits for 30msec and it measures the fluctuations happening
But the diode will be ON when Anode voltage > Capacitor value.
Then capacitor will update the maximum value over 30 msec.
For every freq step/every sweep the capacitor value should be made to 0V by grounding
the capacitor. So after every sweep the capacitor voltage is grounded (after every 30msec)
We should give enough
time to measure correctly
Time to charge matter….!

14. Cont…
• Quasi- Peak:
• When we talk in phone the continuous disturbance sound disturbs the
communication i.e. burrrruh….! Sound
• Still we can adjust the random big sound (Peak) i.e. tik/tuk/dum sound
• So it’s important to measure the continuous disturbances
RC Output
When Output of the Peak detector is connected with the shunt resistor, the
Capacitor discharges to the resistor and the capacitor voltage gets down.
At the same time, When Anode voltage > Capacitor Voltage, Diode turns ON and it
again charges up the capacitor and it goes on
Ex:
- 1000 repetitions of peak in measuring time, charges the capacitor 1000 times.
After every peaks capacitor discharges to the resistor but 1000 times peak keeps the
capacitor voltage being filled and a very less time only capacitor discarges! –
Measured high QP value…!
- 10 repetitions of peak charges the capacitor 10 times but it gets in-between more
time to discharge hence measured low QP value…!

15. 5. Video Filter…
• Video filter filters the noise from the detector output to see the smooth
picture in display
• It doesn’t impact on Noise floor like RBW but it stops the very small
variations in signal.
Right, It’s a LPF and it just removes the noise (High frequency variations)
LPF
You can try…
The thing, When we keep a narrow
Band filter it takes more time to
display the signal instead a broad
band with little added noise
LPF

16. 6. Ramp signal…
• Freq span:
• It’s known, The ramp signal is fed to VCO for the linear freq sweep
• The same ramp is directly applied to the Vertical deflection plates of the display
(X-axis) hence we can run the frequency along with amplitude.
Freq span variation X-axis
Y-axis from the detector
Right, at the low voltage VCO generates a small freq as well for high amplitude the high freq
By changing the span actually we change the
range of ramp amplitude
The actual ramp feed looks like a discretized
since we need to move the freq by step by step
i.e. that’s nothing but freq step setup
When the freq
step is higher the
separation will be
higher
It’s important to understand, The ramp only controls
1. the freq_span
2. The freq step
3. The measurement time

17. Food for thoughts…
1. How to control the freq harmonics ?
• The freq harmonics are born from Mixer.
• Hope you don’t forget, mixer is a non linear device
• Which creates the freq over f1±fLo, f1±2fLo, f1±3fLo, f1±4fLo and so on…
2. What will happen when the QP measuring time is 1hr?
• CISPR suggests 1sec generally.
• When the DUT generates a spikes for every 1 min we should keep 1 min
measuring time.
• We will get the same result when we keep 1 hr measuring time also.
The problem is solved by RBW filter. That allows only that band of freq.
It attenuates remaining all the freq….!

18. Cont…
3. How to extent the freq range of spectrum analyzer?
• By extending VCO as well Mixer range is enough
4. What made the Spectrum analyzer being a small piece ?
• Logarithmic design of freq and Amplitude
I once again remind, Remaining all the hardware components are the same.
By symbol mixer looks a
multiplier but it’s the
costliest thing in a
spectrum analyzer
As I discussed first in slide 4, When we have the dynamic range in a linear manner we would have a
mountain sized Analyzed height as well length by your next bus stop…!
A good
engineer
must be good
at math

19. Cont…
• What would happen without super heterodyne receiver?
• If you have really understood the concept of Spectrum analyzer
• You can think of that size as well the re-design
I once again remind, From the mixer output
we design the remaining hardware for a
single freq

20. Thank you….
vlbvinoth@outlook.com
Reach me on for
any clarifications