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17

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 ...


14

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 ...


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 ...


6

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. ...


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

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


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

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

In tuning a trap there are two concerns which are somewhat contradictory: You want the trap to have a high impedance on the target band. You want losses to be low. The objective of a trap is to reduce the currents on the inactive portion of the antenna to a negligible amount. This doesn't require an impedance that's as high as possible: it requires only an ...


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 ...


3

Mini-circuits is the place for filters and other RF gadgetry. See http://www.minicircuits.com/products/filters_coax_high.shtml for their high pasd catalogue. For example, the BHP-250+ BNC connectorised high pass filter. It has -40 dB at 100 MHz, -20 dB at 150 MHz and -3 dB at 205 MHz. These little filters can only carry about 1 Watt, so would only be ...


3

While that would work somewhat, it won't work great. What you are describing, with a single resistor and capacitor, is a first-order filter. Such a filter has a roll-off of approximately 6dB per octave. This means for each halving of the frequency, attenuation is increased by 6dB. Say you design your filter to have a a cut-off frequency of 200 MHz. The FM ...


3

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 ...


3

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 ...


3

Well, different books / schools use different definitions. Some authors just copy definitions from others and make mistakes when doing that. In this case, this is pretty likely: Possibly, someone remembered "hey, shape factor is between the point where there's half the power and -60dB of the power", then when writing down accidentally converted "half the ...


3

While writing the question I found a picture that clarifies the “in line” part on this web page I found in a search, Radio Frequency Interference: A small vessel guide: To cure alternator noise, which is pulses radiate from the output lead, it is necessary to filter the output lead as close as possible to the alternator. The most effective filter is a 0.5 ...


3

How do cell phone filters filter out all the extraneous EM waves and noise (especially noise) when you get a call? Short answer: they don't. Of course, they use a bandpass filter to reject EM radiation outside the designated channel. But every radio does that. Modern cell phones also dynamically adjust their antennas so they are more sensitive in the ...


3

For DSP with complex samples, whether you are working in the time domain or the frequency domain, you don't need an AM demodulator, or complex conjugation, or a Hilbert filter. You say you have available a shifted low-pass filter — in other words a band-pass filter suitable for SSB. From the output of that filter, discard the imaginary part. That's it. You'...


3

Cascading more filters may help. Also simply adding an attenuator may help, as long as the RF noise floor is above the receiver's internal noise floor. However, spurious emissions from the station can also be an issue. In the US, spurious emissions are regulated by 73.44, which states: Emissions removed by more than 75 kHz must be attenuated at least 43 +...


3

Two daisy-chained bandpass filters might clean up your SSB signal enough to be legal, or they might not. Much depends on the design of the bandpass filters, of course. What you should do is to get help from someone with the equipment (a suitable dummy load and a spectrum analyzer, primarily) and expertise who can measure the important parameters of your ...


3

As pointed out, the short answer is no. The Q value for the series resonance of a crystal can be calculated from $Q = \frac{2\pi f_s L_m}{r_m}$. Q value is of course related to the bandwidth: $Q = \frac{f_s}{BW}$. If you take the equivalent circuit for a crystal, add another similar next to it, and then simplify the circuit as far as you can, you can see ...


3

Your measurement looks about right. I think your circuit looks like a loop antenna, of about 20 cm in circumference, so it will start to radiate well when the wavelength gets below ~ 80 cm, about 400 MHz. The loss you see is radiation - the circuit/antenna is radiating the signal into space, not absorbing it. Stray inductance calculations don't work when ...


2

Very good answer @phil (Phil Frost - W8II). @Adam (Adam Davis) - Another reason is that repeaters are usually located on high-sites where there are a lot of repeaters installed. On these high-sites it would also have line of site to the other high-sites in the area. There are Inter-modulation involved one these sites where phantom transmissions could ...


2

As ~430 MHz SAW filters generally have a 3dB bandwidth of 0.2–2 % $f_\text{center}$, tuning them by some 0.1% does seem like a relatively big change. You could try to build a small "oven" for your filter to operate in, e.g. by placing a resistor atop of your filter, using that to heat up the filter, and encasing everything in spray foam to make the thing ...


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