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Radio Frequency Engineering > Exam > Flashcards

Exam Flashcards

1
Q

L-Section Adv and Disadv (2)

A
  • Simplest Matching Technique

- Q is defined by the source and load impedence

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2
Q

Pi and T-section matching Adv & Disadv (4)

A
  • Pi and T networks allows Q to be chosen by the designer to be ANY value GREATER than the loaded Q defined by the source and load impedence
  • R_virt of Pi is always SMALLER than the source/load impedence
  • T-network is used to match a LOW source/load impedence for a HIGH Q
  • R_virt of T is always GREATER than source/load impedence
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3
Q

Wideband Matching Techniques and Limitation (2)

A
  • Used to design a system which has a loaded Q which is LOWER than that possible with a simple L-section
  • Transformers limit op. freq due to leakage inductance & winding cap
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4
Q

Configurations of Wideband matching techniques (2)

A
  • Multiple L-sections, therefore lower Q value

- Impedence transformer

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5
Q

Reasons to use impedence transformer (2)

A
  • When broader B/Ws are required than is possible with multiple L-sections
  • Used when very high frequency required which is not possible with standard configuration transformers
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6
Q

L-Section Design Method (8)

A
  • Find loaded Q (from source and load impedence)
  • Find reactances of source and load (X_s and X_p)
  • Capacitive reactance equivalent @ freq (for imag)
  • Absorb stay cap into series cap
  • Resonate out efffect of parrallel cap (find equiv reactance)
  • Find required ind.
  • Absorb inductor into mtching circuit
  • Draw circuit
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7
Q

Methods of Biasing Small Signal Circuits (3)

A
  • Constant Base Current Biasing
    > Uses resistor R_b to provide appropriate base current
    > Technique heavily dependent on beta (poorly defined for most transistors)
  • Shunt Feedback Biasing
    > A measure of collector current is fed-back to the base reducing the overall effects of beta & V_BE spreads & temperature variations of beta
  • Potentiometer Biasing
    > Uses potential divider with centre point connected to base
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8
Q

Method of biasing RF power amplifier (3)

A
  • Forward biased diode connected in parallel with base
  • Diode makes good thermal contact with the transistor to match temperature variations
  • If temp rises, R_diode & voltage falls which reduces the base voltage of the transistor and shuts down to prevent thermal runaway
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9
Q

Where are Active biasing Networks used?

A

Used in circuits where temperature stability is of prime concern

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10
Q

Purpose of C_B and RFC (3)

A
  • RFC provides an AC BLOCK for any RF signal that tries to flow into the supply
  • C_B prevents any RF signal passing into supply or biasing network
  • Both required to stop any undesired supply voltage variation & also any RF modulation of biasing network
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11
Q

Loaded Q (4)

A
  • A measure of circuit quality or selectivity
  • It provides a measure of circuit performance under loaded or real conditions
  • Q is dependent on the source and load impedence
  • Q is the circuit’s 3dB B/W
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12
Q

Effect of practical components on the performance of high freq circuits (3)

A
  • Cap and inductors normally have resistance associated with them due to the need for:
    > connecting leads
    > inductor windings
  • Resistance can be translated into an equivalent parallel effective resistance
  • Resistance appears in parallel with the source and load resistance which
    > lowers the resistance value
    > lowers loaded Q by equivalent amount
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13
Q

S and Y-parameter adv and disadv (3,2)

A
  • S-parameters
    > easy to measure (no ‘perfect’ RF short req.)
    > less likely to destroy device under test during measurement of parameters
    > can be used @ h.f relatively easily
  • Y-parameters
    > difficult to measure @ h.f. due to obtaining R.F short
    > short may destroy device
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14
Q 
Definitions of:
S_11
S_12
S_21
S_22
A

S_11 - Input reflection coeff
S_12 - reverse transmission coeff
S_21 - forward transmission coeff
S_22 - output reflection coeff

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15
Q

Actions taken for transistor instability at certain freq (5)

A
  • Choose new transistor
  • Determine values of source and load impedence
  • Determine regions of stability on a smith chart
  • Change the operating point (bias level) of the transistor to alter S-parameters and create new stable design
  • Use a stabilisation network
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16
Q

Stub vs Lumped Element Matching Techniques Adv and Disadv (3)

A
  • Stub matching attractive as freq increases because stub becomes more realistic in length
  • As freq increases, lumped elements start to become less practical due to increasing parasitic effects
  • Stub matching also has cost saving due to no L or C
17
Q

Factors for picking substrate (5)

A
  • Cost
  • Dielectric constant
  • Dissipation factor (tan (delta) - determines the insertion loss of the microstrip line)
  • Ease of working / mechanical strength
  • Temp stability
18
Q

Methods of coupling resonant circuits - 1st (8)

A
  • Capacitive Coupling
    > Simple and lost-cost

> Cap C_12 determines the amount of coupling

> Frequency skew due to form of coupling

> 18dB/oct increase, 6dB/oct decrease

> Gradient difference due to effective presence of 3 reactive components (L-C-L) below resonance, only C above

> Cap too large, too much coupling and response broadens drastically = two resonant peaks

> Cap too small, not enough signal energy is passed from one resonant circuit to the other & insertion loss increases to unacceptable level

> C_12 = C/Q

19
Q

Methods of coupling resonant circuits - 2nd (3)

A
  • Inductive Coupling
    > Directly analogous to capacitive coupling except freq response is in opposite direction.
> Design of transformer circuits is difficut & not very scientific. Coupling between coils depends on:
~ Coil Geometry
~ Coil Spacing
~ Cre material used
~ Degree of shielding

> L_12 = QL

20
Q

Methods of coupling resonant circuits - 3rd (3)

A
  • Active Coupling

> Use of transistors (FETs) to provide interstage coupling w/ high resolution between each stage

> Results in absence of freq response skews

> Signal only flows in 1 direction

> Increased cost & complexity

21
Q

Courses of action taken for amplifier w/ specific gain required but not the MAG (2)

A
  • Choose transistor w/required gain but is very time consuming
  • Best method is to choose transistor with higher gain than req then selectively mismatch o/p by plotting a CG circle
22
Q

Practical passive component construction and effects @ h.f (4)

A
  • Devices w/ wire leads not commonly used @ r.f
  • Modern surface mount devices chosen more
  • These devices has extremely short leads, therefore less parasitic effects
  • Need to be checked for resonance effects prior to use, to identify resonances that occur within bounds of particular design freq.
23
Q

Design for Specified Gain (6)

A
  • D_s
  • D_2
  • C_2
  • Calculate G
  • Centre of circle, r_0
  • Radius of circle, p_0
24
Q

Reflection co-efficient (2)

A
  • Find distance to intersect of CG circle

- Find angle of intersection

Radio Frequency Engineering (1 decks)

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