Making an SSB signal
Two stages, and the pool asks both.
What output is produced by a balanced modulator?
Double-sideband modulated RF.
A balanced modulator multiplies audio by a carrier and cancels the carrier, leaving two sidebands and no carrier — double sideband suppressed carrier.
What circuit is used to select one of the sidebands from a balanced modulator?
Filter
A sharp filter removes one sideband. Balanced modulator plus filter gives single sideband — the carrier cancelled by the balance, the unwanted sideband removed by the filter.
Recovering one
How is a product detector used?
Used in a single sideband receiver to extract the modulated signal
A product detector is a mixer supplied with a locally generated carrier at the frequency the transmitter suppressed. Multiply the incoming sideband by that reinserted carrier and the audio comes back out.
This is why an SSB receiver’s tuning must be accurate: the reinserted carrier’s frequency is the reference, and an error of 100 Hz shifts every audio frequency by 100 Hz. It is why the RIT control exists.
Frequency generation
Which of the following is characteristic of a direct digital synthesizer (DDS)?
Variable output frequency with the stability of a crystal oscillator.
A DDS builds a waveform numerically from a single crystal reference. You get arbitrary frequency in tiny steps with crystal stability — a combination that used to be impossible, and the reason a modern radio can be both accurate and continuously tunable.
Filter specifications
Four terms, each describing a different part of a filter’s response, and the pool asks all four.
| Term | What it describes |
|---|---|
| Insertion loss | attenuation inside the passband |
| Cutoff frequency | where output power falls to half the passband level |
| Ultimate rejection | maximum attenuation outside the passband |
| Bandwidth | between the upper and lower half-power frequencies |
What term specifies a filter’s attenuation inside its passband?
Insertion loss.
Even a good filter costs you something in the band you want. That cost is insertion loss, and it matters most in a receiver’s front end, where it adds directly to the noise figure.
What is the frequency above which a low-pass filter’s output power is less than half the input power?
Cutoff frequency.
Half power is −3 dB. The half-power point is the convention for every filter edge, which is why:
The bandwidth of a band-pass filter is measured between what two frequencies?
Upper and lower half-power.
Both edges at −3 dB. Any bandwidth figure quoted at a different point is measuring something else and should say so.
What term specifies a filter’s maximum ability to reject signals outside its passband?
Ultimate rejection.
How far down the stopband gets, eventually. A filter with steep skirts and poor ultimate rejection lets very strong signals through from far away.
Receiver sensitivity
Which parameter affects receiver sensitivity?
All these choices are correct — noise figure, bandwidth, and antenna gain.
All three enter the same calculation. Noise figure is how much the receiver adds; bandwidth is how much noise it admits; antenna gain is how much signal arrives. A low-noise preamplifier in front of a wide filter gains you less than narrowing the filter does.
Digital signal processing
Which of the following is an advantage of a digital signal processing (DSP) filter compared to an analog filter?
A wide range of filter bandwidths and shapes can be created.
An analogue filter’s response is fixed by its components. A DSP filter’s response is a set of numbers, so one piece of hardware provides every bandwidth from 50 Hz to 3 kHz, with shapes no analogue filter of reasonable cost could achieve.
Software-defined radio
What is the phase difference between the I and Q RF signals that software-defined radio (SDR) equipment uses for modulation and demodulation?
90 degrees
Two versions of the signal, ninety degrees apart — in-phase and quadrature. Together they capture both the amplitude and the phase of the signal, which is complete information about it.
What is an advantage of using I-Q modulation with software-defined radios (SDRs)?
All types of modulation can be created with appropriate processing
Because I and Q carry everything, any modulation is a matter of processing rather than circuitry. One hardware front end handles SSB, FM, digital voice, and modes not yet invented.
Which of these functions is performed by software in a software-defined radio (SDR)?
All these choices are correct — filtering, modulation, and frequency selection.
That is the definition. What was a chain of tuned circuits becomes a program, and the hardware reduces to converting between RF and numbers as accurately as it can.
The SDR lab in the practicum track is where this stops being abstract.
Check yourself
- What two stages generate an SSB signal from audio, and what does each remove?
- A filter is specified as 2.4 kHz wide. Between which two points?
- Why can one SDR front end handle modes that did not exist when it was built?
Answers
- A balanced modulator cancels the carrier, leaving double sideband; a filter removes the unwanted sideband.
- The upper and lower half-power (−3 dB) frequencies.
- It captures I and Q — 90 degrees apart — which together contain the signal’s full amplitude and phase information. Everything after that is software.