What is phase noise? I must take into consideration the use of phase-locked loop (PLL) frequency synthesizers according to the Extra Class examination manual (Wolfgang, Reed, & Carman, 2007). This type of oscillator incorporates a feedback loop that samples output frequency then compares the output frequency with the desired frequency. Automatic corrections adjust the oscillator frequency.
I can understand and what about phase noise? The book tells me too think about averages as well as instance that is, on average the oscillator output frequency is near the desired value. Is that meant as desired frequency? However, if I take a sample at any moment, the actual frequency is most likely a little high or a little lower than actual, desired frequency.
Now the light bulb is on inside the mind of KA3DRR and the book follows up. I'm thinking cycles at the moment, "The phase of any cycle is likely to be slightly different from the phase of adjacent cycles" (Wolfgang, et al. 2007, p 4-25).
I'm digging this because when tuning toward a strong signal the receiver noise floor tends to increase according to the manual (Wolfgang, et al. 2007). For example, think outside the receiver passband when tuning a weak signal, and a kilowatt station begins transmitting nearby. There is an increase in receiver noise. Bingo. Its called phase noise.
I thought the nuisances of phase noise ended in the last paragraph? The principle of phase noise applies to the transmitter as well. This type of interference is aptly dubbed splatter. It's like painting RF graffiti inside the receiver.
Understanding receiver specifications is important for me and a limitation to consider is image to image-rejection ratio. I'm breaking this one because I'm a visual learner.
- 2 signals = new signal @ the sum (+) of frequencies & a new signal @ the difference (-) in frequencies.
I'm creating an example based on the manual (Wolfgang, et al. 2007, p 4-26)
- My rcvr uses a 20-MHz local oscillator (LO) for tuning at 30-MHz. The difference (-) is 10-MHz as the intermediate frequency (IF) for my rcvr.
- Suppose a signal at 5-MHz gets into the mixer as well. The sum (+) of the new signal is 10-MHz as well. Houston this is a problem. Why?
- As understood by the KA3DRR mind, the IF for both signals, is 10-MHz and both are amplified in the IF stage. Not good news for me because the 5-MHz signal is known as an image signal. [note: I answered my own question in a previous posting]
Hot Note: Radio frequency (RF) amplifier stage determines the receiver image rejection ratio and must include appropriate filtering for passing of desired signal according to the manual.
73 from the shack.
Reference: Wolfgang L, Reed D, and Carman J (2007). [8th ed.] Extra Class License Manual. ARRL-The national association for amateur radio. Newington, Ct. 06111. (pp 4-25, 26)