# Tag Info

19

A cavity filter is inductors and capacitors. Something like this: (from amateur-radio-wiki.net) There are several reasons these are not constructed from more common discrete inductors and capacitors. Firstly, these filters must handle a bit of power. Not that you can't get high-power discrete capacitors and inductors, but they aren't cheap, or small. But ...

18

CW signals are not “transmitted on the upper sideband”, nor the lower one. A CW signal is approximately at a single frequency (with only the additional bandwidth required to allow the key-up and key-down transitions). However, the standard method of receiving a CW signal is identical in structure to a single-sideband receiver. The local oscillator (LO) of ...

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 ...

8

"Pole" comes from the Laplace transform, or it's discrete (for digital filters) equivalent, the Z-transform. Without delving into the mathematics of it, these transforms enable a filter designer to express the response of a filter in terms of some number of poles and zeros on the complex plane. Using these transforms simplifies many tasks of the filter ...

7

As Phil Genera says, the KX3 uses a different type of receiver that requires a different kind of filter. A traditional superheterodyne receiver, which is what you'd find in most older radios, uses a mixer to convert the desired signal to a fixed IF of say 10.7MHz and then uses a very narrow bandpass filter at that frequency to select only that signal. ...

7

Puh! That's a big one you're planning: Most circulators are passive, magnetic material circulators: These circulators are basically a three-port resonant cavity in a cylindric shape, where the cavity is filled with ferrimagnetic material, typically special ferrite disks. The electromagnetic properties of magnetized ferrite are used to break up the ...

7

I don't know if this counts as "easy", but: Find the series resonant frequency. Multiply this frequency by $2\pi$ to convert it to an angular frequency, and call that $\omega_s$. Note the resistance at this frequency. That's $R$. Find the parallel resonant frequency, multiply by $2\pi$, and call it $\omega_p$. Halfway between these resonant ...

7

So, requirements: runs on OS X and windows easy to work with quicker than "tone and excel" method If you don't like writing software yourself, this should still be pretty doable with just your soundcard and a quick signal flow graph design (in GNU Radio Companion) of your own. So, first off: Exciting with a single tone and stepping through all ...

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 ...

6

It appears that all three 'scope probes are set to their "X1" position. As such, each probe includes the oscilloscope's internal 1M input resistance in parallel with about 20pF input capacitance. But more importantly, the capacitance of the probe's cable is in parallel too. The total 'scope parallel capacitance is roughly 100pf. Since there are two ...

5

Let's clarify some terminology: a balun is any device used to convert between a balanced system (ladder line, dipoles) and an unbalanced one (coax, vertical monopoles). There are a lot of ways to make a balun, and a common-mode choke is one of them. A choke can also be used in balanced to balanced, or unbalanced to unbalanced connections where suppression of ...

5

Terminated! An open end would have a reflection coefficient of |R| = 1 (because, where would the energy go).

5

If you are using (or building) a vintage radio that does not have a narrow CW audio filter, the audio bandwidth above and below a listenable CW audio side-tone frequency can be quite unsymmetric. By flipping the side-band, one might be able to move an interfering signal down close to 0 Hz audio frequency (inaudible), even if the audio is not bandpass ...

5

A circulator can probably be constructed from coaxial cable. However, you will also need ferrite and magnets. From https://wa8dbw.ifip.com/Circulator.html : A circulator is best thought of as a "Magic Box" containing three transmission lines spaced 120 degrees apart. These transmission lines are placed between two disks of ferrite material. On the other ...

5

I like your PLL approach, because it doesn't try to "recover the original signal from noise", but actually goes ahead and detects what you're actually interested in, the presence of a specific frequency, and uses that to generate a "perfect" tone. Much cleverer than spending hundreds on the best thinkable crystal filter on the market! (I'm always baffled ...

5

Many of these specs are different ways of expressing the same thing. Q and ESR (equivalent series resistance) both express the loss in the capacitor. $$Q = {\left| 1 / (2 \pi f C)\right| \over \text{ESR}}$$ Strictly speaking ESR can change with frequency due to things like skin effect, but this equation can get you close enough to make a comparison as long ...

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, ...

5

Nobody mentions distortion (IM2 and IM3) of PIN diodes and varicap diodes. Tuning (center frequency) limitation range, required band switching and further requirements. Best solution is to start with a good system that can handle the dynamics without filtering. As W8II Phil Frost proposes. ** Mentioned Q-factors (Brian K1LI) of 1000 are not feasible in a ...

4

The KX3 receiver is a software defined radio SDR. The IF filters are made in the software. Since software IF filters are steeper than crystal filters the crystal filters are not necessary anymore. Another advantage is that SDR IF filter width is continuous adjustable. But band filters and audio filters are still LC filters like in the past. That has not ...

4

Filtering square wave aims to produce as pure as possible sine wave as the local oscillator frequency. If you would feed all the harmonics to the non-linear modulator, the output would contain all the strange sum frequencies that can be very close to the wanted signal. For example in SSB transmitter the unwanted sideband could mix with a harmonic of the ...

4

It boils down to practical issues — you want to make it as small as possible, but no smaller. The power-handling requirements will dictate the wire diameter (to handle the current) and the end-to-end spacing (to handle the voltage). For an air-core coil, you also want the wire to be stiff enough to hold its shape well. You will want to wind the coil ...

4

Are you thinking something like this? simulate this circuit – Schematic created using CircuitLab Let's say you have a really good directional coupler with a directivity of 55dB. And the antenna is a good match with a VSWR of 1.1, or equivalently a return loss of 26 dB. And the transmitter power is 10 W, or 30 dBm. And we'll say we select a ...

4

So, I don't know the Softrock personally, but from your description, it's a switching mixer architecture. Receiver side What that means is that you mix not by multiplying the input signal with a single harmonic oscillation (a tone), but by switching it on and of. Pre-Mixer HPF Mathematically, this description is omitting one interesting detail: The ...

4

They prevent (or at least reduce) front end overload due to strong and/or messy nearby signal sources at lower frequencies. Think of the 80 meter station at Field Day blanking the 40 and 20m receivers every time it's keyed up. Chances are good that you'll be ok without them if your environment is pretty RF quiet, but it may not take all that much to cause ...

4

What they mean by "coaxial capacitor" might be described by more people as a feed-through capacitor. For an example, see Tusonix 4300-002LF at Mouser. The datasheet contains these images, which are pretty informative: Thus, the two leads are effectively a wire, ideally having zero impedance between them. However, this conductor also has some capacitance to ...

4

So, to maximize SNR you'd use a matched filter (which is the conjugate complex of the time-reversed TX pulse shaping filter), and assuming the pulse shaper has been reasonably chosen, that'd also a ISI-minimizing filter. Now, psk31.txt from the original DOS program says The solution is to filter the output, or to shape the envelope amplitude of each ...

4

The issue of filtering RS-232 (or any slow digital signal) is not so much the baud rate as it is the required rise and fall time of each bit. There are two ways to quantify this rise/fall time issue: what does the RS-232 standard say or what does your RS-232 chipset and UART require? I have formulated my answer from the perspective of the former. RS-232 ...

4

If you want cheap, building your own is not difficult. Here's a simple low pass pi filter: simulate this circuit – Schematic created using CircuitLab The values of the components should be such that they have a reactance of 50 ohms at the cutoff frequency, which should be just a little above the top of the 2-meter band, if that's your objective. For ...

4

Simply put, the LNA amplifies everything that comes into it. If you think about it, it's actually kind of amazing. It's dragging its output value up and down in response to nanosecond-by-nanosecond changes in its input, reproducing signals at every frequency up to its bandwidth limit. A good amplifier is as linear as possible (meaning its output is ...

4

You've got it right already: Does it only help when there is a lot of noise on nearby frequencies or does a filter like that improve always performance? It helps by attenuating strong out-of-band signals. In your case, cellphone signals from 880 MHz upwards are the problem, and in an office or factory these could be much stronger than the signal of ...

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