18

The bits per sample will affect the dynamic range of your receiver. There's a lot of math that I'm sure you can find, but here's the intuitive explanation: A digital signal can represent only discrete quantities, where an analog signal can represent infinitely many quantities between any two discrete quantities the digital signal might represent. The ...


8

Switching to the inverted signal is equivalent to multiplying by -1. While in an ideal mixer, the RF input can be multiplied by a sinusoidal LO, the switching mixer multiplies by a square LO. It still works as any mixer would. That is, for each frequency component in the first input $f_1$, and each frequency component in the second input $f_2$, it produces ...


8

A lot of designs that include germanium diodes are older, or based on older designs, when germanium diodes were commonly available. On the page for Germanium, Wikipedia says, "From 1950 through the early 1970s, this area provided an increasing market for germanium, but then high-purity silicon began replacing germanium in transistors, diodes, and rectifiers"...


8

I encourage students to draw a simple schematic when it comes to word problems such as these. Here is an ideal series resonant circuit: simulate this circuit – Schematic created using CircuitLab Since the problem states that the circuit is in resonance, the inductive reactance equals the capacitive reactance making this ideal circuit a dead short (0 ...


8

The basic concept is to use RF bandpass filters for each frequency range. This is frequently done by hams for VHF and UHF applications. The common term for a grouping of these filters is diplexer, triplexer, or quadplexer as appropriate. Sometimes the term "duplexer" is used although this creates confusion with a different device that is typically much more ...


7

There is no hard and fast rule. Consider that in some LC circuit applications, a lower Q may be desirable in order to achieve a wider bandwidth. In other cases, a very high Q may be desirable for narrow selectivity, for example. Both the inductor and the capacitor in a resonant circuit may affect the Q of the circuit. The Q of the inductor is determined by ...


7

The GS-35A tube is the liquid cooled version of the same tube. Some hams have fabricated their own version of a liquid cooling jacket to replace the fins on the GS-35B: The designer of this jacket correctly highlights the challenges of selecting and maintaining the liquid medium since it is exposed to the full anode high voltage. Deionized water will ...


7

A wideband antenna is not what you're looking for – you really don't care about anything between 900 and 1500 MHz, or between 1600 and 2400 MHz. Wideband antennas are inherently hard to make, and even harder, even impossible, to make uniformly good across their whole range. What you much likely will rather want is a multi-band antenna. For example, I'd ...


7

Unless you haven't mentioned all the parts, you have built an unregulated power supply. The output voltage of a transformer varies depending on load, and the other components will just follow that voltage. For electronic equipment you should use a regulated power supply. This requires the rectifier to produce an output voltage somewhat higher than the ...


7

One case where this might happen would be in the world of RF over fiber. In this technique, a radio-frequency signal is used to modulate an optical (usually infrared) signal for transmission over optical fiber. Light is, after all, just really high frequency electromagnetic waves. To the best of my knowledge, the modulation used here is always AM, so if such ...


6

LM387 is just a really old, ordinary op-amp as far as I can tell. It has a funny pin configuration, so if you have a PCB already made you need to make modifications. I suggest cutting the traces, scraping off the solder mask, and soldering wires in place with the right layout. Look for datasheets on the internet for the pin configuration. For old stuff ...


6

If you are in mortal peril, use any means necessary to make your situation known and get help. It doesn't matter if you are licensed or not. Consider the alternative. But a shortwave transmitter is not especially effective. Among the many issues: A relatively high transmit power is required, meaning heavy batteries. Effective antennas are large. ...


6

The lower forward voltage drop (0.3 volts or even less), compared to silicon diodes (0.7 volts or more). Having said that, there are some schottky diodes with a low forward voltage drop, too.


6

The ideal number of turns depends on core material, geometry, and frequency. This is why you find such variance in how many turns should be used. More turns increases the choking impedance up to a point, but decreases the choke's self-resonant frequency (SRF). Once the SRF goes below the operating frequency, adding more turns increases the distributed ...


6

Sure, it's technically possible. But I can't think of a reason it would be useful. The resulting modulation would be more complicated to implement, and occupy more bandwidth, than ordinary AM. Demodulation would be accomplished simply by cascading two demodulators together.


6

A superhet has two distinct properties, which may or may not be advantages: The image frequency is far away, and The IF is not DC In a typical superhet design, the LO and signal frequencies will be quite far apart, making the image frequency in an entirely different band. Thus it's not difficult to exclude the image frequency with a simple analog filter. ...


6

I would like to applaud your interest not only for homemade transmitters but also a theory! I started to build my own receivers and transmitters not a long time ago and completely understand how frustrating it can be at the beginning. However this is a broad and complicated topic. If we consider only oscillators for transmitters, there are LC oscillators - ...


5

The original RCA specs for 12BY7 and 6146 did not disclose estimated lifetime of the tubes. An average tube has a lifetime of 2,000-4,000 hours, sometimes much-much more depending on usage. Given a low transmit duty cycle on CW/SSB (compared to prolonged TX with digital modes), and the radio is properly tuned and maintained, the tubes may last a lifetime. ...


5

Q isn't related to the voltage. A simple definition of Q could be the ratio of the total energy in a system to the energy lost per cycle. So the biggest factor in Q is the loss in the dielectric. In general, vacuum variable capacitors (VVC) have a higher Q than air variable capacitors (AVC) but there is some overlap. A common range for VVCs is a Q of ...


5

You can't use a fixed resistor to control the input the voltage for an arbitrary electronic device, only one which is a passive load, such as a resistor, or an LED. (Note that even a simple LED invalidates your Ohm's-law-based reasoning, because it does not have a fixed value of $R$, though it can still be handled with a resistor.) A battery charger is not ...


5

For an ideal circulator, all the power entering one port exits the next port in the rotation and no other. So all the power in port 1 exits port 2, all into port 2 exits port 3, and all into port 3 exits port 1. This is expressed by the scattering matrix: $$ S={\begin{pmatrix}0&0&1\\1&0&0\\0&1&0\end{pmatrix}} $$ Or a 4 port ...


5

Feed-through capacitors are still commercially available but their use in commercial applications has fallen off so prices have risen over the last few years. The concept with the feed-through capacitor was to provide an RF capacitive bypass to ground while providing a stout connection through the wall of the enclosure. When I see them at a swapfest for a ...


5

...variable caps are ... no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are difficult to adjust precisely). Thankfully, Oren Elliott is a surprisingly affordable source of brand new air-variable capacitors. I have used them successfully in ...


5

Fair-Rite makes ferrite rods which would be suitable for HF applications. It should be possible to create or repurpose a screw-operated mechanism to move the rod into and out of a cylindrical coil. Preferably, the material will have steady permeability and low loss over the frequency range of interest. Loss is proportional to the ratio of the real and ...


5

You could rewind it, but the result may not necessarily be better. This is because the core must be optimized for different needs in each balun design. In what you're calling a "current balun", the idea is to maximize the impedance in the common mode. Ideally, that impedance is mostly resistive, meaning high loss. Saturation current isn't so important, ...


5

The first part of why a quarter wavelength is special is actually understanding that it's not a quarter wavelength, but a half wavelength. Consider a quarter-wavelength monopole. If a wavefront originates at the feedpoint, a quarter-cycle later it will have reached the end of the monopole. Here "something interesting" happens, because the antenna ends. That ...


4

This is likely a ferroresonant transformer. http://www.electroncoil.com/ferroresonant_transformers.php http://www.hammondmfg.com/CVR.htm http://www.oltronix.nl/en/ferroresonant-principle If you wanted a surge protector, a motor-generator would work pretty well. Is there a way to get rid of the moving parts of a MG? Yes. Most transformers are fairly ...


4

The number of bits in the converter will set the maximum dynamic range of the resulting data stream. This is approximately $\textrm{SNR} = 6\,\textrm{dB} \times \textrm{Bits} + 4.8\,\textrm{dB}$ (for a full scale input sine wave). However, as with everything in life, this is only the beginning of the story. The data sheets for the converters will typically ...


4

The SA602AN looks to be still available from many of the usual distributors. (Check octopart.com or findchips.com to see which distributors have them in stock.) The LM387 is apparently long obsolete. A quick web search didn't reveal any pin-for-pin compatible parts still being manufactured, so if you're using the printed circuit board from the kit, then ...


4

A balanced mixer is one that suppresses the local oscillator (LO) input in the output. A double-balanced mixer (like the one you show here) suppresses both the RF input and the LO in the output. This is often accomplished with balanced transformers, hence the name. Because T1's secondary has a grounded center tap, the voltages on its two "hot" output ...


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