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13 votes
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What limits a PIN diode or varicap from being used in a "Software Defined [Radio] Filter"?

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 ...
Neil_UK's user avatar
  • 246
10 votes
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Where are the baluns for VHF and higher? How can one be made?

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 ...
Phil Frost - W8II's user avatar
9 votes

What is a "roofing filter"?

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 "...
Phil Frost - W8II's user avatar
8 votes

Looking for a schematic of a DIY circulator

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 ...
Marcus Müller's user avatar
8 votes
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Looking for a convenient audio frequency spectrum analyzer

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 ...
Marcus Müller's user avatar
7 votes
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How to measure quartz crystal motional parameters using a VNA?

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 ...
Phil Frost - W8II's user avatar
7 votes
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How do you choose an inductor wire gauge for high-power RF?

First: yes, it's mostly about $I^{2}R$ loss, but if your inductor is part of any kind of resonant circuit, you can't assume that $I^2 = \frac{P}{50}$. The current circulating between reactive ...
hobbs - KC2G's user avatar
  • 12.4k
6 votes
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How to effectively test and interpret the "working range" of a ferrite core with a signal analyzer?

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 ...
glen_geek's user avatar
  • 941
6 votes

What limits a PIN diode or varicap from being used in a "Software Defined [Radio] Filter"?

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 ...
Brian K1LI's user avatar
  • 7,858
5 votes

SSB filtering in the frequency domain?

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 ...
Kevin Reid AG6YO's user avatar
  • 24.6k
5 votes

How does selecting the opposite sideband when receiving CW on HF reduce interference?

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 ...
hotpaw2's user avatar
  • 13.4k
5 votes

Diplexer termination?

Terminated! An open end would have a reflection coefficient of |R| = 1 (because, where would the energy go).
Marcus Müller's user avatar
5 votes

Under what circumstances does intermodulation become an issue in a filter after LNA setup?

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 ...
hobbs - KC2G's user avatar
  • 12.4k
5 votes

Looking for a schematic of a DIY circulator

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 "...
Mike Waters's user avatar
  • 7,986
5 votes
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What is the current state of the art in CW filters?

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 ...
Marcus Müller's user avatar
5 votes
Accepted

How to select capacitor for diplexer use

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{...
Phil Frost - W8II's user avatar
4 votes
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Filtering for PSK31 demodulation

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 ...
Marcus Müller's user avatar
4 votes
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Are directional couplers ever used in duplex repeater systems?

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 ...
Phil Frost - W8II's user avatar
4 votes
Accepted

What function do high-pass filters in a direct conversion receiver perform?

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 ...
Marcus Müller's user avatar
4 votes

What function do high-pass filters in a direct conversion receiver perform?

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 ...
Hamsterdave's user avatar
  • 1,844
4 votes
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What dimensions to choose when winding your own inductor

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-...
Dave Tweed N3AOA's user avatar
4 votes

SSB filtering in the frequency domain?

If you want to do SSB in the frequency domain: 1) Compute the complex FFT. 2) Pick the frequency band of interest. 3) Perform a backwards FFT on the bins that contain the signal of interest. ...
sm5bsz's user avatar
  • 1,009
4 votes

Small band-pass filter for HT?

[This was from comments by @MarcusMüller and myself. I've moved them to this answer.] Don't underestimate the filtering effects of an antenna. It's most important how the spurs "close" to your ...
Mike Waters's user avatar
  • 7,986
4 votes
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Small band-pass filter for HT?

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 ...
Phil Frost - W8II's user avatar
4 votes

Baud rate/frequency/bandwith/characters per second

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: ...
Glenn W9IQ's user avatar
  • 18.6k
4 votes
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In which cases do SAW filters improve performance?

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 ...
tomnexus's user avatar
  • 11.3k
4 votes

Problem of two SAW fitler?

So, probably a combination of different things: You physically can't isolate things arbitrarily much. My experience with well-designed RF PCBs is that they have -60 dB to -49 dB crosstalk – simply ...
Marcus Müller's user avatar
4 votes

What is the current state of the art in CW filters?

Ringing in narrow filters may already have an answer here: https://dsp.stackexchange.com/questions/2170/why-do-i-see-ringing-in-the-output-of-a-digital-filter-with-a-narrow-transition "looking into ...
hotpaw2's user avatar
  • 13.4k
4 votes

Tradeoffs: varicap vs. air variable capacitor for Rx bandpass?

First and foremost, varicaps aren't mechanical. If you want a rapidly tunable thing, that's a necessity; even if you just want something tunable by electronics, it's very desirable (otherwise you'll ...
Marcus Müller's user avatar

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