RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity. In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications. We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.

1. January 1, 2014 || Wayne Turner, Nisha Malik, Preet Rekhi,
Rahul Sharma, Sukhvinder Malik
Radio
Conformance
1. Introduction
RF testing has remained hype for most of us. But seriously it is not so. It
can be very interesting and one can develop a lot of interest in this if given
an opportunity.
In this paper, authors have started with the some basic concepts
of radio engineering which we studied in engineering and built upon
these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer
and Signal Generator which are the most common test tools used for any
radio testing.
2. Basic Concepts
.
Contents
1. Introduction
2. Basic concepts
3. RF components of a
Transceiver
4. RF test equipment
5. LTE Radio conformance Tests
6. Conclusion
A SIGNAL is a function that conveys information about the behavior
or attributes of some phenomenon. It can be an electric current or
electromagnetic field used to convey data from one place to another. In
terms of signal processing these can be analog or Digital.
A SPECTRUM is a collection of sine waves that, when combined
properly; produce the time-domain signal under examination.
7. References

2. MODULATION is the addition of information (or the signal) to
an electronic or optical signal carrier.
NOISE is a random process unwanted byproduct or unwanted data
without meaning. Noise can be intentional too called dither which can
reduce overall noise and allows retrieval of signals below the nominal
detection threshold of an instrument.
PHASE NOISE is the noise in the oscillator which increases at
frequencies close to the oscillation frequency or its harmonics. With the
noise being close to the oscillation frequency, it cannot be removed by
filtering without also removing the oscillation signal.
The noise close in to the center frequency is due to FLICKER
NOISE. It is usually referred to as 1/f noise or pink noise. The noise
signal adds to the modulated signal which adds together to end up in the
wrong place on the constellation.
DISTORTION is any sort of undesired change in waveform as it
passes from Input to Output of some device.
The total number of bit errors divided by the total number of transferred
bits is BIT ERROR RATE and is therefore unit less. BLOCK
ERROR RATE will similarly be total number of erroneous blocks
divided by the total number of received blocks.
An OSCILLATOR is used to convert a DC source to an AC
source. The output will swing from a maximum to minimum voltage
with a fixed period. An analogy can be a pendulum. In RF we mostly
use sine wave oscillators however we can have square, pulse, triangle or
sawtooth.
The Error
Vector Magnitude is a measure of the difference
between the ideal symbols and the measured symbols after the
equalization. This difference is called the EVM.

3. The FREQUENCY
ERROR is the difference in frequency, after
adjustment for the effect of the modulation and phase error, between the RF
transmission from the mobile station and the test set.
In digital communications, modulation is often expressed in terms of I/Q
FORMATS. I/Q diagrams are particularly useful because they mirror
the way I /Q modulator creates most digital communications signals.
Independent dc voltages (I and Q components) provided to the input of an
I/Q modulator correlate to a composite signal with a specific amplitude and
phase at the modulator output. Conversely, a modulated signal’s amplitude
and phase sent to an I/Q demodulator correspond to discrete dc values at the
demodulator’s output. The digital modulation maps the data to a number of
discrete points on the I/Q plane.
These are known as CONSTELLATION
POINTS. For e.g.
QPSK has 2 bits per symbol so 4 points on the constellation diagram, 16
QAM has 4 bits per symbol so 16 points and likewise 64 QAM has 6 bits
per symbol so 64 points.
THERMAL NOISE is a random fluctuation in voltage caused by
the random motion of charge carriers in any conducting medium at a
temperature above absolute zero. The formula to find the RMS thermal noise
voltage of a resistor is:
Vn = 4kTRB
Where: k = Boltzmann constant (1.38*10-23 Joules/Kelvin)
T = Temperature in degrees Kelvin (K= +273 Celsius)
R = Resistance in ohms
B = Bandwidth in Hz in which the noise is observed (RMS voltage measured
across the resistor is also function of the bandwidth in which the
measurement is made).

4. Thermal noise can be given as - 174 dBm/Hz. In context of LTE we
might need to calculate the thermal noise over a Resource Block or a
subcarrier or over the channel bandwidth. Thermal noise is the main
contributor for the Receiver sensitivity calculation.



Thermal Noise over 1 Resource Block (180 KHz) = - 121 dBm
Thermal Noise over 1Sub-carrier (15 KHz) = - 132 dBm
Thermal Nosie over 20 MHz Channel BW (18MHz) = - 101dBm
Thermal Noise for a Bandwidth of Interest can be calculated as
= -174 + 10*log10(bandwidth in Hz)
.
The NOISE
FIGURE of a circuit element represents the amount
the S/N ratio degraded from the input to the output of that circuit element,
when and only when the noise input power = KTB. The higher the noise
figure, the less sensitive the system. FRIIS formula for noise factor states
that
NF Total= NF1 + (NF2-1)/G1 + (NF3-1) / (G1*G2) + . . .
Where NF1 = noise figure of amp1 in linear ratio
NF2 = noise figure of amp2 in linear ratio
NF 3 = noise figure of amp3 in linear ratio
G1 = gain of amp1 in linear ratio
G2= gain of amp2 in linear ratio etc.
It is always advised to put a lot of gain in the first stage in a multistage
amplifier system, which should have a low noise figure.
The Base station have a NF in order of 2-3 dB while mobile handsets have
NF in order of 7-8 dB. Reason for this high NF in mobile handsets is the
limitation of the size and weight which leads us to use an inefficient
power amplifier while in Base station we do not have limitation of size and
weight so we can use a better linear power amplifier stage which reduce
the NF with high gain capability.

5. 3. RF Component of a RF Transceiver
The basic components of a radio transceiver are local oscillators (LO), RF
mixer, phase local loop (PLL) and synthesizer. Under this section we will
discuss about all these components.
HOMODYNE RECEIVER or Zero IF or Direct Conversion
Reception is the most natural solution to detect information transmitted by a
carrier in just one conversion stage. It has at least one mixer stage less than in
a heterodyne system. But from the other hand the flexibility to change
receiving frequency is reduced significantly, because the narrow band
filtering is not possible before down conversion and the only narrow band
filter is the output low pass. Though, in case of the BPM signal processing
there is no need to change the input frequency. That means a narrow band
filter can be used already at the front-end of the electronics.
The principle is that the signal is first amplified at a low noise stage and
then directly converted to the baseband or even to a direct current signal.
When the frequencies of the RF and the LO signals are equal, this scheme
works as a phase detector.
Suppose that the IF in a heterodyne is reduced to zero. The LO will then
translate the center of the desired channel to 0 Hz, and the portion of the
channel translated to the negative frequency half-axis becomes the “image”
to the other half of the same channel at the positive frequency half-axis.
To achieve maximum information, we should take both parts of signal. It’s
done by a method, which is called quadrature down conversion. The
principle of this method is that the signal is at first divided into two
channels and then down converted by an LO signal, which has a phase shift
of 90 in one channel with respect to another. The vector of the resulting
signal is described as:
Signal  I 2  Q 2
arg( Signal )    arctg
Q
I
The main problem in homodyne technique is an offset caused by
the LO signal leakage to the RF port of the mixer. The propagated signal
reflects from the components in the front-end of the receiver and goes back
to the mixer, where it is mixed down to DC. The offset can be considerable
with respect to the signals to be measured. This leads to a narrower
dynamic range of the electronics, because the active components get
saturated easier than it would be in case of a zero offset.
Another problem of the homodyne receiver, or, more concretely, of the I/Q
(in-phase/quadrature) mixer, is mismatches in its branches.

6. In HETERODYNE
RECEIVER the Radio frequency (RF)
signal is first amplified in a frequency selective (but usually broadband)
low noise stage, then translated to a lower intermediate frequency (IF) : fIF
= fRF – fLO. After a significant amplification and an additional filtering on
the intermediate frequency it is finally down converted to the baseband (i.e.
to the original or desired signal frequency).
This technique is flexible in changing the receiving frequency, because the
only change to be done is the frequency of the first local oscillator (LO) in
the way that at the frequency of the signal at the output of the mixer would
not change. The rest of electronics is free from additional readjustments,
what is very important in the applications of the heterodyne receiver in the
radio, TV, satellite and other communications.
The main problem in heterodyne technique is the half IF and image
frequencies, received signals on these frequencies act as a blocker e.g.
reducing the sensitivity of the receiver. If the LO frequency is a 1100 MHz
and the received signal is at 900 MHz, this is a high side injected system,
the Image frequency will be at:
High side injection Image LO + IF
IF = 200 MHz
Image = 1100MHz + 200MHz = 1300MHz
PHASE NOISE is short term random fluctuation of the signal
which is expressed in dBm/Hz i.e. the power measured in a 1 Hz resolution
bandwidth, the noise profile follows the 1/frequency profile. To measure
phase noise of an oscillator the measuring equipment has to have a phase
noise performance exceeding the phase noise performance of the oscillator
you are testing. The phase noise is proportional to the Q (quality factor) of
the resonant circuit used in the oscillator circuit.
A PLL or PHASE
LOCKED LOOP is a control system that
generated an output signal whose phase is related to the phase of an input
signal. When using a phase locked loop as a frequency synthesizer the
phase noise may be amplified by 20 logs (N-divider number) and is directly
to the ratio of the output frequency of the VCO and the comparison
frequency of the phase detector used in the PLL.

7. The idea is to keep the N-Divider ratio to a minimum value as possible; this
can be achieved by using a fractional-N SYNTHESISER.
A frequency synthesizer is an electronic system for generating any of a
range of frequencies from a single fixed time base or oscillator.
There are discrete spur like signals which appear on the output of the local
oscillator spectrum (integer N), these are the reference spurs i.e. they
appear on both the upper and lower side of the spectrum at frequency offset
from the center equal to that of the caparison frequency. These occur as the
charge pump is constantly pushing current in or pulling from the loop filter
( due to capacitor current leakage and constants loop correction), a tight
loop filter can reduce these spurs but would increase the lock time of the
phase locked loop.
If the local oscillator used in a transceiver has a bad phase noise
performance then the following performance can be degraded.
1.
2.
Transmitter and Receiver EVM
ACS (Adjacent Channel Selectivity)
A RF MIXER is a 3 port device which is used to produce a sum a
difference signal of 2 signals. For example is a mixer is fed with 900MHz
and 1100 the sum will be at 2 GHZ and difference will be at 200MHz, in a
down converter the difference will be used and in a up convertor the sum is
used.
When a transmission line (cable) is terminated by impedance that does not
match the characteristic impedance (Z0) of the transmission line, not all of
the power is absorbed by the termination. Part of the power is reflected
back down the transmission line. The forward (or incident) signal mixes
with the reverse (or reflected) signal to cause a voltage standing wave
pattern on the transmission line. The ratio of the maximum to minimum
voltage is known as VSWR, or VOLTAGE
STANDING
WAVE RATIO. A VSWR of 1:1 is ideal and means that there is no
power being reflected back to the source. A VSWR of 1.2 could be
excellent.

8. 4. RF Tests Equipment
There are a lot of radio test equipment like power meter, VSWR meter, Signal
Analyzer and Signal Generators. The Signal Analyzer and Signal generator are
the most power test equipment for radio engineers which are mostly used for
lab testing.
In this section we will discuss about the principle of operation and basic
building blocks of signal analyzer and signal generator which help us to
understand the functioning of these test equipment.
A SPECTRUM ANALYSER measures the magnitude of an
input signal versus frequency. Its primary use is to measure the power of the
spectrum of known and unknown signals. The swept-tuned analyzer
“sweeps” across the frequency range of interest, displaying all the frequency
components present.
This enables measurements to be made over a large dynamic range and wide
frequency range. Inside the analyzer, the mixer converts the input signal
from one frequency to another. The input signal is applied to one port, and
the local oscillator’s (LO) output signal is applied to the other. The mixer is a
nonlinear device, so frequencies will be present at the output that weren’t
present at the inputs. These frequencies are the original input signals, plus the
sum and difference frequencies of the two signals. The difference frequency
is called the IF signal.
Block Diagram of a Spectrum Analyzer

9. The analyzer’s IF filter is a band pass filter used as a “window” for detecting
signals. Its bandwidth, the analyzer’s resolution bandwidth (RBW), can be
changed by means of the instrument’s front panel. A broad range of variable
RBW settings allows the analyzer to be optimized for different sweep and
signal conditions and enables the user to trade off frequency selectivity,
signal-to-noise ratio (SNR), and measurement speed. Narrowing RBW, for
example, improves selectivity and SNR. However, sweep speed and trace
update rate degrade. The optimum RBW setting depends heavily on the
characteristics of the signals of interest.
Agilent’s and R&S Spectrum Analyzer
The detector allows the analyzers IF signal to be converted to a baseband or
video signal so it can be digitized and viewed on the LCD. This is
accomplished with an envelope detector whose video output is digitized with
an analog-to-digital converter (ADC) and represented as the signal’s
amplitude on the Y-axis of the analyzer display. For e.g. Agilent’s N9020A
MXA Signal Analyzer which supports a frequency range from 10 Hz to 26.5
GHz.
The CW
SIGNAL GENERATOR, the RF CW source splits
into three sections: reference, synthesizer, and output. The reference section
supplies a sine wave with a known frequency to the phase-locked loop (PLL)
in the synthesizer section. Its reference oscillator determines the accuracy of
the source’s output frequency. The synthesizer section produces a sine wave
at the desired frequency and supplies a stable frequency to the output section.
Block Diagram of a CW Signal Generator

10. Agilent’s Signal Generator
Creating a VECTOR SIGNAL GENERATOR simply
involves adding an IQ modulator to the basic CW generator. To generate
baseband IQ signals, a baseband generator takes binary data containing the
desired “information” to be transmitted, maps it to digital symbols and then to
digital I and Q signals, converts the digital IQ signals to analog IQ signals,
and sends them to the IQ modulator to be coded onto the carrier signal. The
same thing we can see in the block diagram of a vector signal generator. After
the data undergoes symbol mapping, the digital signals are digitally filtered
using two sets of filters in the baseband generator.
The filters are designed to limit the bandwidth of the I and Q symbols and
slow down the transitions between symbols. Many types of baseband filters
exist, with each having different attributes that must be set in the signal
generator. Common filter types are Root Raised Cosine, Gaussian, and
Rectangular.
Block Diagram of a Vector Signal Analyzer
The above picture shows some of the Agilent’s signal analyzer which is
capable of generation a wide range of signals for different technology like
LTE, WiMAX and Microware Radar signals.

11. 5. LTE Radio Conformances Tests:
3 GPP standard 36.141 and 36.104 talks about the Radio conference test case.
In this standard these testcases are broadally classified in three categories.
 Transmitter conformance Test
 Receiver conformance Test
 Performance Test
For these test case standard defines some test model called EUTRA-Test
Models (ETM) and Fixed Reference channel (FRC). The ETM models are
basically used for the transmitter conformance testing and FRC are used for
receiver and performance testing. The standard also defines the minimum
criteria for passing for a particular test
In this section we will discuss about some transmitter and receiver test case.
SPECTRUM FLATNESS is how much amplitude variation you
have in the wanted bandwidth i.e. all subcarrier have the same average power.
CCDF (Cumulative Distributive Function) is a way to
express statistically how often a static peak to mean ration happens. For e.g. a
peak to mean ratio of 6 dBs occurs 0.001% of the time.

12. The amount of energy in the adjacent channel compared to wanted will be
the ACLR (ADJACENT CHANNEL LEAKAGE RATIO).
Reasons could be improper filtering, frequency control or improper
tuning. For e.g. the ratio of power in my channel and the adjacent channel
must be greater than 44dB in LTE. However the ACLR is more of an
integrated measurement.
The SEM (SPECTRUM EMISSION MASK) is
kind of absolute measurement so a spike of RF energy could pass the
ACLR but would surely fail the SEM measurement.

13. TRANSMIT OFF POWER measurement is used to
verify whether the RRC filtered mean power versus time meets the
specified mask. For e.g. in LTE Transmit OFF power should be less
than -85 dBm/MHz One can use a Limiter which brings down the On
Power only and there is no effect on the Transmit off Power.
RECEIVER SENSITIVITY is the lowest power level at which
the receiver can detect an RF signal and demodulate data. As the signal
propagates away from the transmitter, the power density of the signal
decreases, making it more difficult for a receiver to detect the signal as the
distance increases. Improving the sensitivity on the receiver (making it more
negative) will allow the radio to detect weaker signals, and can dramatically
increase the transmission range. Sensitivity is vitally important in the
decision making process since even slight differences in sensitivity can
account for large variations in the range. For e.g. for Base station planning if
the sensitivity is more negative, we need less Base Stations and vice versa.
The basic formula to calculate the Receiver sensitivity
RX Sensitivity = -174 + 10*log10 (Bandwidth) + SINR + Noise figure
The range over which the Receiver is sensitive enough to operate will be its
DYNAMIC RANGE. The low end of the range is governed by its
sensitivity whilst at the high end it is governed by its overload or strong
signal handling performance. The overall dynamic range of the receiver is
very important because it is just as important for a set to be able to handle
strong signals well as it is to be able to pick up weak ones. This becomes
very important when trying to pick up weak signals in the presence of nearby
strong ones. Under these circumstances a set with a poor dynamic range may
not be able to hear the weak stations picked up by a less sensitive set with a
better dynamic range.

14. Problems like blocking, inter-modulation distortion and the like within the
receiver may mask out the weak signals, despite the set having a very good
level of sensitivity. These parameters are obviously important when
determining what equipment should be used in a radio communications
system.
The BLOCKING characteristic is a measure of the receiver’s ability to
receive a wanted signal at its assigned channel frequency in the presence of
an unwanted interferer on frequencies other than those of the spurious
response or the adjacent channels, without this unwanted input signal causing
a degradation of the performance of the receiver beyond a specified limit.
Digital receiver measurement are usually performed by varying a signal
(unwanted, or wanted) and observing the bit error rate.
To measure the effect of a potential inferring on a receiver you follow the
steps bellow:





Find baseline sensitivity with a standard receiver data pattern i.e.
0.1 % BER
Increase the received signal level to a specified level maybe 3 dB,
specification radio access technology test specification will state
this level.
Introduce the blocker at the frequency you wish to test and start
below the expected limit
Measure the BER
Keep increasing the blockers level and measuring the BER until
the BER level reaches 0.1% and this is the level the system fails at.
There are three types of blocking.
 In band: - In Band the wanted and the Interferer are both inside the
band and the interferer and the wanted are usually mirror images
of each other.
 Outer band: - The Interferer is placed outside the band at a
particular required offset. For e.g., it can be a 5 MHz E-UTRA can
act as an interferer.
 Narrow band: - A single RB of the 5 MHz will cause the
interference in this case.
SPURIOUS RESPONSE is a measure of the receiver’s ability
to receive a wanted signal on its assigned channel frequency without
exceeding a given degradation due to the presence of an unwanted CW
interfering signal at any other frequency at which a response is obtained i.e.
for which the blocking limit is not met.

15. TAYLOR SERIES is a representation of a function as an infinite sum of
terms that are calculated from the values of the function's derivatives at a
single point. A one-dimensional Taylor series is an expansion of a real
function
about a point
is given by
When Harmonics of in band signals mix together THIRD
PRODUCTS are formed.
ORDER
If the input frequencies are f1 and f2, then the new frequencies produced will
be at 2f1 - f2, 3f1 - 2f2, 4f1 - 3f2 and so forth. On the other side of the two
main or original signals products are produced at 2f2 - f1, 3f2 - 2f2, 4f2 3f1 and so forth. These are known as odd order inter-modulation products.
Two times one signal plus one times another makes a third order product,
three times one plus two times another is a fifth order product and so forth.
The main signals are first the third order product, then fifth, seventh and so
forth.
One of the intermodulation products will fall in to the wanted channel, an
example is shown below:
F wanted = 2140 MHz
Fu interferer_cw = 2150 MHz;
Fu interferer _mod = 2160 MHz
Higher Side intermodulation test frequencies close in third order products
will be at
2*Fu interferer cw – Fu interferer _mod and 2*Fu interferer_mod - Fu
interferer_cw
2 * 2150MHz – 2160MHz = 2140 MHz !!!! Will fall on your wanted
channel, produce and interfere
2 * 2160MHz -2150MHz = 2170MHz
Lower Side intermodulation test frequencies
Close in third order products will be at
2*Fl interferer _cw – Fl interferer _mod and 2*Fl interferer _mod - Fl
interferer _cw

16. Authors
2 * 2130MHz – 2120MHz = 2140 MHz
wanted channel, produce and interfere
2 * 2120MHz -2130MHz = 2110MHz
Nisha Malik
Student M.Tech
!!!! Will fall on your
To overcome these 3rd order distortion which produce an on channel
interferer a more linear RX chain is required i.e. output intermodulation
intercept point is high (OIP3).
6. Conclusion
The authors have explained the basic fundamentals of RF and Radio
conformance. They explained about test tools, test thresholds and some
basic calculations .We believe this paper will give the basic
understanding and confidence for anybody who has interest in radio
testing.
Wayne Turner
System Design Engineer
7. References



Preet Rekhi
System Test Engineer
Rahul Sharma
System Development Engineer



3 GPP 36 141 LTE: Evolved Universal Terrestrial Radio
Access (E- UTRA); Base Station (BS) conformance testing
3GPP 36104 LTE: Evolved Universal Terrestrial Radio Access
(E-UTRA); Base Station (BS) radio transmission and reception
R& S LTE Base Station Tests according to TS 36.141
application notes
Agilent X-Series Signal Analyzer application notes
Agilent X-Series Signal Generator application notes
LTE, The UMTS long Terms Evolution: From Theory to
Practise
Disclaimer:
Authors state that this whitepaper has been compiled meticulously and to the best of their
knowledge as of the date of publication. The information contained herein the white paper
is for information purposes only and is intended only to transfer knowledge about the
respective topic and not to earn any kind of profit.
Every effort has been made to ensure the information in this paper is accurate. Authors
does not accept any responsibility or liability whatsoever for any error of fact, omission,
interpretation or opinion that may be present, however it may have occurred
Sukhvinder Malik
System Test Engineer