# Tag Info

13

What limits the use of "solid state variable capacitors" like these for dynamically filtering (low power/received) RF signals? The main issue is bandwidth. Although there are some varicaps that claim a 10:1 variation (or more) in capacitance, not all of this is useable. One end of the range may be low Q, or have a very steep curve. Once you have a ...

7

The roofing filter is the filter through which the first IF must pass. The roofing filter is an IF filter, specifically the first one. AB4OJ provides a block diagram: As for why it's called a "roofing" filter, Elecraft suggests it's about protecting the subsequent stages from spurious signals: The term "roofing filter" has most often ...

5

Technically, the utility of a variable capacitor in a receive filter depends on the amount of available capacitance variation and its Q - the quality factor, which is the ratio of the capacitor's reactance at the operating frequency to the series parasitic resistance. (In some applications, parasitic series inductance may also be a factor.) Commercially, ...

4

Another difference between using a varicap for oscillators versus for input filters is feedback. You can feed the output of a variable oscillator into a phase comparator or frequency counter to determine if the component values are correct, and run a feedback loop correction algorithm or circuit to maintain lock. This can be done while the oscillator is ...

4

Lumped elements are theoretical constructs from the lumped element model. So you can't really build a filter from lumped elements at any frequency. But, you can ask at what frequency discrete components stop being approximated by the lumped element model. As a rule of thumb, anything that is physically bigger than 1/10th of a wavelength is definitely not ...

3

Two datapoints that might help: A previous company we had a 20-6000 MHz diplexer with a changeover at about 900 MHz, using 0402 parts. Some trial and error to get it to match the simulated design. A current bandpass (highpass+lowpass) filter for 570-1050 MHz is made entirely from discrete parts, including hand-wound wire inductors instead of chip inductors....

3

The capacitor C2 would be of a value small enough to prevent the antenna from damping the Q of the tuned circuit. In other words, the purpose of C2 would be to improve selectivity.

3

I found the fault. It was due to the enamel not being stripped back far enough on one of the inductors, preventing a good soldered connection.

3

The 300 650 combination is often used for CW (Morse Code) reception. It tunes the SDR receiver’s (Tayloe mixer or FPGA or software synthesized) oscillator(s) 650 Hz below (or above for LSB) the dial frequency so that you will hear a 650 Hz beat frequency tone for RF signals transmitted at the dial frequency, and inserts a 300 Hz bandpass filter that cuts ...

3

SSB has a bandwith of roughly 2.7KHz. However, if you are listening to a station, but there is another station (or loud noise) overlapping with it slightly either below it or above it, you can narrow the receive bandwidth to try to clip off the interfering signal. The result will make the fidelity of the station you are listening to slightly worse, but ...

2

Simulating the circuit with 50$\Omega$ load and source impedance yields no resonant peaking behaviour. Also a filter made to pass HF might start looking funny by 100 MHz and will certainly be awful at 500 MHz. -- @tomnexus Yeah, that will definitely be a problem with this topology. If you are trying to operate >100MHz things don't look too good (...

2

Alan Wolke, W2AEW, explained this topic very well in one of his videos: Measuring Crystals with NanoVNA and other tools. Alan shows several methods. Personally, I measure: C0: directly with a suitable RLC-meter. If you don't have one or are in a hurry, assume 2.5 pF, that will be good enough. VNA can be used for the task if it's capable to measure the ...

1

Consider increasing the dynamic range of the ADC to obtain many of the same performance benefits. This can be done by either upgrading the ADC, or low-pass filtering the ADC input below the Nyquist frequency and oversampling. Often this is the cheaper and more flexible approach, especially given the limitations of the varicap's Q-factor and capacitance range,...

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