ELEC2140 High Frequency Electronics
High Frequency Electronics
ELEC2140 High Frequency Electronics
Design Project 1
STUDENT NUMBER
e.g. if the last 2 digits of your student number are 28, and you design a ~9 GHz satellite link, use 9.28
GHz in your calculations. This helps reassure us that you did the work entirely on your own.
Overall Objective
For a simple line-of-sight link using parabolic dish antennas, as shown below, from an application of
your choice, you are expected to:-
1. Carry out a basic link budget calculation to determine received signal power
2. Design and simulate a simple lumped element low-noise amplifier (LNA)
3. Explore the effects of the parasitics of the practical components used in the typical
implementation of the LNA.
Lumped circuits are covered in Unit 2. Your design might use surface mount devices (SMD transistor,
inductors, resistors and capacitors) for a PCB design, or you might be using on-chip IC components if
your design frequency is suitably high.
Use these last 2 digits as part
of your signal frequency
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MARKING SCHEME
Component
Weighting
A description of your chosen case study (~500 words) 10
Link budget calculations 20
Link budget explanation and justification of all values 10
LNA design: Suitable circuit schematic and simulation results 20
LNA design: Brief narrative explaining your design and results 10
Identification of suitable passive components and summary of their
parameters
10
Simulation results with parasitics 10
Summary and conclusions (~250 words) 10
Total 100%
Project 1 counts for 30% of the total module mark.
Marks will NOT be awarded for any results that are not fully supported by your workings-out.
For example, results from online link budget calculators may not be submitted, although you may use
such tools to check your own calculations.
OTHER OPTIONS
These guidelines have been written to make your project go as smoothly as possible with no
unforeseen problems. “Sensible” frequencies would be in the range 1-100 GHz (below that, a dish
antenna will be very large, above that may be too high for the transistors for which data is supplied.
However, if you are confident in what you are doing, you are welcome to try more advanced
options such as higher frequencies, different antenna types or different transistors.
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PART 1: The Link Budget
Important pre-reading: deciBels dB
Marks will be deducted for any figures without the proper units clearly stated.
dB is a unit based on the logarithmic ratio of powers. dB units are able to conveniently represent a
huge range of signal levels and high gain/attenuation ratio: e.g. from pW to 100's of Watts is often
encountered in radio.
Gain in dB = 10 log OUTPUT POWER , for an amplifier for example
INPUT POWER
Note that this is a ratio and has no absolute units.
Since power = V2/R, it is important to remember to use 20 log, not 10 log for voltages:-
Gain in dB = 20 log OUTPUT VOLTAGE
INPUT VOLTAGE
Providing you are consistent, you can use peak-to-peak voltage, r.m.s, or amplitude; it’s a ratio unit.
Note that because S-parameters are voltage-based, to get dB, you must use 20 there too:-
20 log (magnitude of Snm).
For example, the gain in dB of an amplifier is 20 log (mag S21) (S-parameters are introduced in Unit 2)
dB units are mainly used because they are convenient – especially for link budgets and system-level
calculations. For example, the gain of the following chain of amplifiers is just 10 + 15 + 15 = 40 dB
(because dB are a logarithmic unit, the multiplication of gains becomes addition of dB).
G = 10dB 15dB 15dB
This total gain of 40 dB means the power is increased by 1040/10 = 10,000 times
If a circuit has loss (e.g. a filter) the “gain in dB” will be negative, so this chain has 23dB gain:-
10dB gain 2 dB loss 15dB gain
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By itself, the dB is a relative unit but the input or output power of an amplifier, for example,
must have some absolute units.
This is achieved by adding an extra letter (suffix) to denote the power reference:-
Power in dBW = 10 log(P in Watts)
Power in dBm = 10 log(P in mW)
Now consider the link budget, using the nice diagram from Infinet Wireless:-
https://academy.infinetwireless.com/en/online-education/wireless-networking-fundamentals/8
The received signal power is calculated by taking the transmitter power (in dbW or dBm) and
adding/subtracting the gains and losses in dB from the output of the transmitter amplifier to the input
to the receiver.
The two antenna gains are key to making the link work successfully, and the gain in dBi of a
parabolic dish is given by:-
)log(10
2
22
)(
D
G dBi =
Where D is the diameter of the dish, η is its efficiency and λ is the signal wavelength.
dBi means dB relative to an ideal isotropic radiating element (an ideal point source that radiates
equally in all directions). This shows that a key purpose of the antenna is to increase the signal
intensity through its highly directional radiation pattern.
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“EIRP”, which you can see is labelled on the Infinet diagram, stands for Effective Isotropic Radiated
Power. This way of stating transmitter power is useful because it is this that determines the radiated
signal intensity – i.e. a narrow-beam radiation pattern is as important as how much power the
transmitter generates.
The free-space path loss, a distance d from the transmit antenna, is given by:-
This is the simplest possible case with no intervening lossy medium (e.g. water vapour) and no
multipath effects. There are two components to this formula:-
1. The radiated signal is naturally spreading out across a larger area as it moves away, giving an
inverse square law with distance, d.
2. The receiving antenna has what is called a capture area which increases with the square of
wavelength.
Including (2) as part of “free-space path loss” could, admittedly, be considered confusing. The effect
is greatest for low frequencies – think about the “cars can see round corners” article in IEEE
Spectrum.
Returning to the task in-hand “Carry out a basic link budget calculation to determine the
received signal power”
Using appropriate dB units, the graphical form of the link budget on page 4 is easily implemented in a
simple table or spreadsheet. From your chosen case-study, you need to decide on sensible values
for:-
• Signal Frequency
• Required transmission distance
• Transmitter Power
• Cable Loss (TX)
• TX antenna size
• RX antenna size
• Cable loss (RX)
With these decided, you can calculate the received signal power in dBm.
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Calculating the received signal-to-noise ratio (SNR) is optional since it is quite complicated. To
calculate the minimum SN) required for successfully demodulating the signal requires a knowledge
of:-
• The data rate since, the data rate sets the minimum bandwidth of your receiver chain (and
therefore the noise bandwidth)
• The signal format, since some modulation schemes are more sensitive to noise than others. For
example, in 256-QAM the constellation points are very close together, so high SNR is needed to
avoid bit errors.
If you would like to determine the minimum SNR and receiver noise figure from your chosen case
study, you can make simplifications as necessary to make the calculations straightforward.
Once you have determined the minimum SNR for your link budget, you can calculate the necessary
receiver noise figure. The noise input level in practice depends primarily on the “antenna
temperature”. A dish pointing at the sky does not pick up the same thermal noise as one pointing at
the earth. If you are not confident about the correct figure for your case study, assume the TA is
300K. Then, for a 50Ω system at room temperature you know that the thermal noise floor is at - 174
dBm/Hz. Once you have the receiver noise figure specification you can use Friis’ cascade formula to
determine a target LNA gain and noise figure.
Cascade calculators are one of many RF tools available via the web. Some link budget tools will have
much more detail than this simple example and you may find the many parameters confusing and
realistic values difficult to determine. Pasternack, a leading US supplier of RF modular products and
passive components, has some very nice tools available:-
https://rfinstitute.pasternack.com/
https://www.pasternack.com/t-rf-microwave-calculators-and-conversions.aspx
REMEMBER, you can use these tools for checking your calculations but you must show your own
workings-out in detail in your coursework submission.
Note: Cascaded noise figure calculations are covered again in Project 2
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PART 2: The Low-Noise Amplifier
To introduce the basics of RF amplifier design, it is useful to revisit a typical first year bipolar
transistor amplifier circuit, as shown below, and identify some of its limitations:-
https://www.electronics-tutorials.ws/amplifier/amp_2.html
The reasons we can’t use this directly for our RF LNA include:-
1. Bipolar transistors generally have quite poor noise performance*, because of Rbb and Rbe.
2. The bias resistors add further noise
3. The transistor input and output capacitances limit the bandwidth
*for completeness, it should be emphasised that we are considering thermal noise, but in many applications the effects of
“flicker noise” (or 1/f noise) and shot noise must be considered. In RF systems this is especially true for oscillators, where a
device’s 1/f flicker noise is upconverted via nonlinear processes and appears on the left and right hand side of the RF sine
wave’s spectrum. This is called phase noise and can impair signal demodulation. Bipolar transistors can be much better
than field effect transistors when 1/f noise is a concern.
The simplified RF low-noise amplifier shown below addresses these 3 issues as follows:-
1. It uses a field-effect transistor
2. There are bias “chokes”: inductors instead of resistors to set the DC operating point
3. It has impedance matching elements at the input and output to “absorb” the FET capacitances
***more detail in Unit 2***
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For your LNA design, information on a number of FETs has been provided on Minerva. For your
simulations, you can use the S-parameter files provided. However, to understand the very basics of
matching, it is useful to consider the basic equivalent circuit of a FET, shown below:-
Matching is a complex subject, covered in depth in L3/LM, but for a simple design we can “absorb”
the FET capacitances and achieve useful gain at our design frequency with:-
1. A SERIES inductor on the input to resonate with Cgs
2. A parallel inductor on the output to resonate with Cds
This is referred to as reactive matching and allows the design to achieve good noise performance,
but it is clear that absorbing the capacitances through resonance leads to a narrowband design. This
is why many of the phone/radar chipsets featured in the module have multiple LNAs to cover different
frequency bands.