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Modulation is using an intelligence signal to modify an RF signal. The intelligence can be voice, data, or image.
FSK is Frequency Shift Keying, where one frequency is the "mark", or 1 in a digital data stream; and another is the "space" or 0 in that stream. It involves directly altering the transmit frequency. This can be replicated using audio tones into an SSB transceiver. A variation is AFSK, used with FM transceivers, including for packet and APRS.
PACTOR is a data communications mode, for transferring messages and files via HF radio. As with various modes which work by connecting a modem to an SSB radio, the bandwidth of PACTOR version 3 is 2300 kHz, so that it fits within a voice frequency channel.
PACTOR is discussed on the following page.
Slow-scan TV, or SSTV, an image transfer mode, likewise has a bandwidth, of around 2500 Hz. If used, this must be in the upper end of each HF band, beside voice. There are also fax, or facsimile modes, similar to those used for transmitting weather maps to ships.
To generate a VHF FM signal, often phase modulation is used at a much lower frequency, such as a twelfth or sixteenth of the desired frequency, and this is then multiplied up to this frequency. To transmit on 146.050 using a crystal controlled radios, we need a crystal on 12.170833333 MHz or 9.128125 MHz, depending on the multiplication factor. For UHF a larger factor is used.
More modern radios use true frequency modulation.
A typical mixer takes two frequencies, and produces two outputs, these being the sum and the difference between the two.
Thus if we wanted to listen to 15 MHz (a time and frequency standard station), with a radio with a 10.7 MHz IF, we would need a Local Oscillator of 4.3 MHz. There is however a risk that a strong signal on 6.4 MHz would also be heard, this being the "image". It is also possible to go up deliberately, say a 7.1 MHz signal was desired, then the LO would be set to 3.6 MHz. (With potential unintended reception of 14.3 MHz.)
Off the exam: Note that from HF, "double conversion" may be use, converting to 10.7 MHZ, or similar, then to 455 kHz, as used in medium wave receivers, even of the pocket size. There are also big dollar sets, with triple conversion, the first IF being around 70 MHz.
Often transformers are part of the IF signal path.
The output of the IF mixer is filtered before final conversion to audio frequency, and amplification to a speaker to headphones.
The width of the IF filter must be compatible with the bandwidth of the signal of interest.
For SSB the width would be 2.2 or 2.3 kHz (2200 or 2300 Hz), or thereabouts, and generally a sharp roll-off is good.
For CW 300 Hz or 500 Hz are typical options.
AM uses around 6 kHz.
For 9 or 10.7 MHz IF frequencies several crystals are used, along with shunt capacitors.
Several stages of inductors and capacitors are another option.
For 455 kHz filters a ceramic filter, containing piezoelectric ceramic elements, is used.
The best filters were mechanical filters made by Collins, a famous receiver manufacturer to the military, etc. They have ceased their production, so supplies held by companies such as InRad (International Radio), who fit them to the small carrier boards, are dwindling. They also sell narrower, sharper filters for SSB. InRad Filters. Yaesu also market the same filters on their own carrier boards, prefixed "YSF-".
Using an SSB filter to receive CW means signals from several other stations may be in the receiver bandwidth, and even if we then have DSP, the ability of DSP to assist in the reception of a weak signal in the presence of significant noise, and/or a strong nearby signal is limited. A physical narrow filter, designed for CW reception is a valuable addition to a rig if you enjoy CW.
While the exam mentions matching the bandwidth to that of the communication, if you listen to broadcast AM on a ham receiver, then its narrowness may reduce the audio to communications style bandwidth. This may however be useful if you are listening to a "DX" station with a weak signal, as this also filters out noise in the upper audio frequencies.
Serious DXers, or DXpedition stations may even use a 1.8 kHz filter.
A friend had a good quality receiver (a Bearcat DX1000) which included wide 6 kHz and 12 kHz filters, for use when listening to good quality AM, in addition to a narrower 2.7 kHz one for SSB and CW.
Audio DSP filters the audio after the filtering above, and demodulation. Settings may be in menus, or via front-panel buttons and knobs. The FT-847 has a handy control using concentric knobs, the other for the high pass (removing low frequency noise and interference), the inner for the low pass one, removing high end interference and noise. Two quick adjustments may this help provide usable audio. The amount of noise in-band is also adjustable on many systems, but high setting may cause metallic, distorted audio.
The alternative is to digitise signals at the IF stage, and do all filtering and conversion to audio digitally.
If buying IF filters, characteristics to note are the width (-3 dB), the frequency of operation (be it 455 kHz or 9 point something MHz, for example), and the matching impedance. A -60 dB bandwidth may also be specified.
Any circuit which contains semiconductor junctions can be an accidental mixer. While it may be a poor quality or faulty radio gear on a communications site, this includes a rusty bolt on a tower; or a wire resting on a galvanised water trough, near a transmitter site. Twice one frequency, minus the other is a classic combination.
Thus a 150 MHz and a 160 MHz transmitter can interact to cause interference at 140 MHz thus:
2f1 - f2 = 2 × 150 - 160 = 300 - 160 = 140 MHz.
Or if some nasty combination of music and data appears on the VK calling frequency:
2 × 152.5 - 158.5 = 146.5 MHz.
This can be frustrating to find, and if one or both the signals are intermittent, these bursts will only happen periodically, and perhaps only during business hours.
In some cases there are more than two signals involved. It is possible that both transmitters are in perfectly clean, and that the mixing occurs externally, or that one contains some fault which results in the generation of the interfering signal, in the presence of a second signal, or is of faulty design.
Yep, here are actual questions from the General exam pool.
How is an FSK signal generated?
A. By keying an FM transmitter with a sub-audible tone
B. By changing an oscillator’s frequency directly with a digital control signal
C. By using a transceiver’s computer data interface protocol to change frequencies
D. By reconfiguring the CW keying input to act as a tone generator
True FSK alters the frequency of the transmitter's oscillator directly, in response to a digital signal, answer B.
The speed at which this must be done is too fast for a "CAT" interface, but but this can be done using direct digital synthesis.
What is the name of the process that changes the phase angle of an RF wave to convey information?
A. Phase convolution
B. Phase modulation
C. Phase transformation
D. Phase inversion
This is phase modulation, answer B.
Many transceivers, especially "PMR" or commercial / government FM transceivers actually use PM, followed by multiple stages of frequency multiplication to generate FM. Some Ham rigs are marketed with a claim of using "True FM".
What is the name of the process that changes the instantaneous frequency of an RF wave to convey information?
A. Frequency convolution
B. Frequency transformation
C. Frequency conversion
D. Frequency modulation
A nicely written question, avoiding the concept of changing the frequency of the "carrier" which once it is off its centre frequency is no longer the carrier. This is frequency modulation, answer D.
What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage?
A. Multiplex modulation
B. Phase modulation
C. Amplitude modulation
D. Pulse modulation
This is phase modulation, answer B.
What type of modulation varies the instantaneous power level of the RF signal?
A. Frequency shift keying
B. Phase modulation
C. Frequency modulation
D. Amplitude modulation
AM involves generating sidebands on either side of the carrier, and the power level of these varies with that of the modulating signal (voice), answer D.
Which of the following is characteristic of QPSK31?
A. It is sideband sensitive
B. Its encoding provides error correction
C. Its bandwidth is approximately the same as BPSK31
D. All these choices are correct
These are all correct, answer D.
Which of the following phone emissions uses the narrowest bandwidth?
A. Single sideband
B. Double sideband
C. Phase modulation
D. Frequency modulation
SSB - single sideband is the narrowest analogue phone mode, answer A.
Which of the following is an effect of overmodulation?
A. Insufficient audio
B. Insufficient bandwidth
C. Frequency drift
D. Excessive bandwidth
Overmodulation is all sorts of bad, and results in excessive modulation, answer D.
What type of modulation is used by the FT8 digital mode? A. 8-tone frequency shift keying B. Vestigial sideband C. Amplitude compressed AM D. Direct sequence spread spectrum
The 8 in the name indicates 8-tone frequency shift keying, answer A.
What is meant by the term "flat-topping," when referring to a single sideband phone transmission?
A. Signal distortion caused by insufficient collector current
B. The transmitter's automatic level control (ALC) is properly adjusted
C. Signal distortion caused by excessive drive
D. The transmitter's carrier is properly suppressed
In both SSB transmissions, and audio amplifiers, if excessive level or "drive" is applied, then the waveform reaches the supply rail voltage, and this results in a flat-top on the signal, visible on an oscilloscope, answer C.
This results in a distorted signal. Electric guitar players may be aware that some effects pedals involve deliberately over-driving amplifiers to generate grungy sounds.
What is the modulation envelope of an AM signal?
A. The waveform created by connecting the peak values of the modulated signal
B. The carrier frequency that contains the signal
C. Spurious signals that envelop nearby frequencies
D. The bandwidth of the modulated signal
The envelope of an AM signal is is a curve linking the peaks of the modulated signal. If we observe this on an oscilloscope, we see the audio waveform on the top of the screen, and this mirrored below, with a filling consisting of the RF in between, answer A.
Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
FT8 is the only item which is both narrow-band, and works with weak signals, answer B.
The other modes require greater bandwidth and/or strong signals.
Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)?
A. Image frequency
B. Local oscillator
C. RF input
D. Beat frequency oscillator
Adjusting the local oscillator (LO) puts different frequencies into the IF passband, answer B.
If a receiver mixes a 13.800 MHz VFO with a 14.255 MHz received signal to produce a 455 kHz intermediate frequency (IF) signal, what type of interference will a 13.345 MHz signal produce in the receiver?
A. Quadrature noise
B. Image response
C. Mixer interference
D. Intermediate interference
This is termed image response, answer B.
Sometimes it is handy, as the US requires scanners to block frequencies once used for analogue (AMPS) mobile phones, despite this being discontinued forever ago, but these frequencies can be received using the image.
What is another term for the mixing of two RF signals?
D. Phase inverting
The mixing of signals is also called hetrodyning, answer A.
What is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency?
B. Reactance modulator
C. Pre-emphasis network
The is a multiplier, answer D. Often there are several doubler stages and/or tripler stages.
What is the approximate bandwidth of a PACTOR3 signal at maximum data rate
A. 31.5 Hz
B. 500 Hz
C. 1800 Hz
D. 2300 Hz
High data rates require a wider bandwidth, and for HF data wide is 2300 Hz, answer D.
This also occupies the bandwidth available using a standard SSB transceiver.
What is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency?
A. 3 kHz
B. 5 kHz
C. 8 kHz
D. 16 kHz
This adds to 8 kHz, but modulation is both down and up, so totals to 16 kHz, answer D.
The code for it is 16K0F3E for 16.0 kHz bandwidth, F for FM, 3 for a single analogue channel, and E for telephony (voice).
What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter?
A. 101.75 Hz
B. 416.7 Hz
C. 5 kHz
D. 60 kHz
The multiplication factor is 12, so the initial deviation must be 5000 / 12 = 416.6667 Hz, answer B.
Why is it important to know the duty cycle of the mode you are using when transmitting?
A. To aid in tuning your transmitter
B. Some modes have high duty cycles which could exceed the transmitter's average power rating.
C. To allow time for the other station to break in during a transmission
D. The attenuator will have to be adjusted accordingly
Modes which use AFSK (Audio Frequency Shift Keying) or PSK (Phase Shift Keying), such as RTTY often have a high duty cycle, and these can exceed the transmitter's average power rating, answer B.
Modes like FM on 10 metres also has a 100% duty cycle. This might be mitigated by reducing transmitter power for these modes. In some cases, after-market fans are available for rigs with passive heatsinking. (Duty cycle also affects electro-magnetic radiation exposure calculations.)
Why is it good to match receiver bandwidth to the bandwidth of the operating mode?
A. It is required by FCC rules
B. It minimizes power consumption in the receiver
C. It improves impedance matching of the antenna
D. It results in the best signal to noise ratio
Matching the receiver's bandwidth to the mode gives the best signal to noise ratio, answer D.
What is the relationship between transmitted symbol rate and bandwidth?
A. Symbol rate and bandwidth are not related
B. Higher symbol rates require wider bandwidth
C. Lower symbol rates require wider bandwidth
D. Bandwidth is always half the symbol rate
The more data elements, or symbols sent, the wider the bandwidth needed, answer B.
Note that under good conditions especially, some modes supports more than one bit for each symbol, the bit rate thus exceeds the baud rate.
What combination of a mixer’s Local Oscillator (LO) and RF input frequencies is found in the output?
A. The ratio
B. The average
C. The sum and difference
D. The arithmetic product
This is the sum and the difference, answer D.
What process combines two signals in a non-linear circuit or connection to produce unwanted spurious outputs?
Intermodulation can cause interference, answer A.
On to: Modes 2 - More on Digital
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, March 2022.
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