#3: Principles of Radio & Wireless Flashcards
What’s an ‘image’ frequency?
When you have your typical heterodyne receiver with a local oscillator, you’ll receive your tuned frequency, plus a higher ‘mirrored’ frequency at your tuned frequency + (I.F. * 2)
The frequency of the local oscillator should be (higher / lower) than the desired radio frequency.
Higher
Why should your local oscillator frequency be higher than the tuned radio frequency?
Because if it’s lower, you’ll need a much bigger range of intermediate frequencies to generate.
Draw a basic AM demodulating circuit. Explain.
Diode leaves only +’ve part of cycle, and clamps lower to +0V. C, R form a basic low-pass filter.
Explain differentiation in the context of demodulating FM signals. What is one problem?
Differentiating = rate of change
Therefore differentiating signal leaves carrier + modulating signals.
Problem: any signal fluctuating quickly (i.e. noise!) is exaggerated.
Draw a time vs. gain graph illustrating the frequency response for pre-emphasis and de-emphasis filters. What are their equations?
Pre-emphasis: G = sqrt(1+(f/fc))^2
De-emphasis: G = 1/( sqrt(1+(f/fc))^2 )
Explain the concept of ‘multiplexing’.
Sending multiple signals over one common channel.
What is the concept of ‘mutliple access’?
Multiple users want to access a single medium. Originally done by hand via a switchboard (e.g. operator would switch someone on/off to a particular frequency).
What is Frequency Division Multiplexing? Draw a diagram.
Sending multiple modulated signals (channels) over one common channel, and then de-modulating at the other end via a mixer to their original signals.
Explain the concept of OFDM, including its abbreviation.
OFDM = Orthogonal Frequency Divison Multiplexing.
Orthogonal = “perpendicular” (i.e. for signals, the net area under the curve of their product = 0, or the peak of one signal occurs at the null of its nearby signal).
Means you can encoding two bits at once.
Graphically illustrate the orthogonal nature of two signals transmitted via OFDM.
How do several people share (i.e. multiple access) the same system (e.g. mobile network)? Explain some of their options.
FDMA (Frequency Division Multiple Access) - each user has seperate transmission (uplink) and receiving (downlink) frequencies. Uses lots of bandwidth.
TDMA (Time Divison Multiple Access) - each user has up/downlink time slots. Less bandwidth, but a problem for real-time applications (e.g. voice calls).
CDMA (Code Division Multiple Access) - aka Spread Spectrum. Instead of using specific frequencies, it transmits across a wide bandwidth.
Explain the concept of Spread Spectrum. What are some pros and cons?
Instead of tx’s across certain frequencies, it transmits across a wide bandwidth. For example: start transmission on CH4; then CH7, CH 13, CH 1, and back to CH4. This would make the hopset = (4, 7, 13, 1).
Pros: minimises interference to/from others; others can use same frequencies, just in a different order.
Cons: complex
Explain what ‘SS-FH’ and ‘SS-DS’ are. What are some pros and cons?
- SS-FH:* Spread-Spectrum Frequency-Hopping (good use of RF spectrum, but slow).
- SS-DS:* Spread-Spectrum Direct-Sequence (several hops/bit, much fast, but complex).
Draw a block diagram of the SS-FH (Spread-Spectrum Frequency-Hopping) concept.
Draw a block diagram of the SS-DS (Spread-Spectrum Direct Sequence) concept.
Draw a possible spectrum graph of: a) conventional FSK (Frequency Shift Keying), and b) SS-FSK. Why the difference?
Differences: a) spread spectrum approach (wide bandwidth); b) the hopset (outputs at a wide range of frequencies)
Draw an example waveform modulated by QPSK-FHSS. How would this be different to a waveform modulated by QPSK-DSSS?
The random binary number generator multipled by the input signal from the QPSK-DSSS approach creates kind of sharp, quick lines between bit transitions, as opposed to the ‘wider’, smoother curves + transitions of QPSK-FHSS.
