It's up front that counts in one's receiver and distinguishing between a "desired" and "undesired" signal on a different frequency is selectivity. And sensitivity is the minimum input-signal level that produces audio output in a receiver. I'm thinking minimum discernible signal (MDS), noise floor, sensitivity, and selectivity.
Selectivity.
- Is the ability of a receiver to separate signals.
- Paste terms like "broadband filter" or "narrowband filter" with receiver selectivity.
- Everything is in degrees and the same is true for receiver selectivity that is, filter-network bandwidth determines the degree of receiver selectivity.
Filters.
- Think high-Q filters, tuned circuits and/or resonator in association with IF selectivity.
- Frame the 'Q' as the quality factor of inductors and capacitors in a circuit.
- When a tuned circuit acts as a filter, then the greater its loaded 'Q' or quality factor, the greater selectivity or the ability of a receiver to reject undesired signals near the desired frequency.
- A filter with a 'wide' bandwidth located in the IF stage results in undesired signals reaching the audio stage.
Hot Tip.
- Achieve the narrowest bandwidth as possible with caution because one does not want too narrow of a filter.
- Generally, according to the Extra Class License manual, the IF filter bandwidth is "...Slightly greater greater than the bandwidth of the signal one is receiving." (p 4-19)
- A filter that is too narrow will ring and if there is too much ringing it is difficult to copy Morse code.
When Examining Receiver Specifications?
- Find a selective front end for rejection of "out-of-band signals", "prevent overloading", and "spurious responses." (p 4-20)
- According to the manual, associate selective IF circuit with two or more IF filters and different bandwidths for example, 2.4 kHz [SSB], 300 Hz [CW, RTTY], and 15 kHz [FM]. (p 4.20)
- RC active or passive audio filters facilitate wide-band noise and provide audio selectivity. Please refer to manual for selectivity frequency ranges.
- Research selective circuits and/or filters in local oscillator chain for rejection purposes beyond the desired frequency.
Front End.
- What is a pre-selector? It is resonant networks before & after the radio frequency (RF) amplifier stage.
- Hot--greater network Q or quality factor, the sharper frequency response, and the better adjacent-frequency rejection.
- A typical pre-selector circuit is a parallel-tuned LC circuit.
My next task is reading over the Yaesu FT100 receiver specifications and put my notes to practical use. I'm beginning to understand this stuff! Next stop is blocking and intermodulation (IMD) dynamic range, very cool.
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 4-19, 20)
My first field day in almost a decade and this weekend is special. Tents. Barbecues. Conversations. Antennas. Generators. Transceivers. The air around a field day site is electric. Let's not forget the picnic tables and plates of good food as well. My favorite time during field day is shortly after sunset when little light illuminates the scene. Each station is a mini-drama of operator pitted against propagation. One will share single-sideband chatter or the music of Morse code. Whatever your flavor this weekend most likely field day will have a serving. I'm looking forward to meeting members of our local ham radio community. Let's say community shares the spotlight with emergency communications this weekend. Let's Field Day.
