The question pool committee eliminated three questions from this section, including a question about PSK 31, and added a question about PACTOR. I find that kind of odd as many more hams are using PSK31 than are using PACTOR.
One of the most important circuits found in amateur radio equipment is the mixer. A mixer takes two input signals and outputs the sum and difference of the two input signals. Heterodyning is another term for the mixing of two RF signals. (G8B03)
The mixer is the receiver stage that combines a 14.250 MHz input signal with a 13.795 MHz oscillator signal to produce a 455 kHz intermediate frequency (IF) signal. (G8B01) 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, a 13.345 MHz signal will produce an image response in the receiver. (G8B02)
FM transmitters use multipliers to produce a VHF signal. The multiplier is the name of the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency. (G8B04) When modulating the oscillator, you must use a proportionally smaller deviation if you plan to multiply the oscillator’s output to the 2m band. For example, 416.7 Hz is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a 5-kHz deviation, 146.52-MHz FM-phone transmitter. (G8B07)
FM phone is often used on the VHF and UHF bands, but not below 29.5 MHz because it requires a fair amount of bandwidth. 16 kHz is the total bandwidth of an FM-phone transmission having a 5 kHz deviation and a 3 kHz modulating frequency. (G8B06)
“Digital modes,” such as PACTOR3 and WINMOR are getting more popular on the HF bands. The bandwidth of a PACTOR3 or WINMOR signal at maximum data rate is approximately 2300 Hz. (G8B05). In general, digital modes have much higher duty cycles than the traditional HF modes, such as CW or SSB. When operating digital modes, it is important to know the duty cycle of the data mode you are using when transmitting because some modes have high duty cycles which could exceed the transmitter’s average power rating. (G8B08)
It’s also important to know the symbol rate. Digital modes that transfer data at a high rate need more bandwidth than modes that send data at a slower rate. The relationship between transmitted symbol rate and bandwidth is that higher symbol rates require higher bandwidth. (G8B10)
Noise can be a problem when operating digital modes because noise can cause errors, and many of these digital modes do not have error correction. One way to minimize the effects of noise is to use filters to filter out the noise and to adjust your receiver’s bandwidth so that it only passes the signal that you’re interested in. It is good to match receiver bandwidth to the bandwidth of the operating mode because it results in the best signal to noise ratio. (G8B09)
Leave a Reply