OpenHam Wiki:Discord explanation archive

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Grounds

2021-05-26 toinaeraser

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The important thing to remember is there really isn't such thing as "ground". No realistic path is properly equipotential like how we treat "ground" in the ideal case.

With that in mind, we need to think about the purpose of the ground we're talking about. Here are some examples:

RF Ground

This is a few concepts lumped together. Normally it refers to the shield of the coax cable. The purpose of RF ground is to conduct the return current from your radio signal. RF ground is not equipotential, meaning we can't consider it the same thing everywhere. This makes it a bit hard to talk about because it's super contextual.

Safety Ground

This is the 3rd pin in most outlets. The purpose is only to conduct dangerous currents away from the user. Most devices that are connected to mains and have an exposed metal chassis require a connection to safety ground. If something fails internally and touches the metal case, a GFCI or breaker will detect the current flow and trip, protecting the user from an energized device. Safety ground is not useful for RF. Connecting your radio here has almost no value for RF and may even degrade the performance. Most radios powered by 12V don't require a connection to safety ground.

Lighting Ground

This is used specifically for protection from lightning strikes. Safety ground is not suitable for lightning protection because the parasitic inductance is far too high. The best strategy for lightning ground is to use several ground rods driven at the point the coax enters your house. They need to be bonded together and to the main house ground with minimum 6 ga wire.

Lightning protection is pretty hit or miss, a direct strike will almost always destroy something. This is also way more than most hams are willing to do, so I think it's best to disconnect the coax outside your house during a storm or when you're not using it.

What you are looking for is RF ground for your antenna, but I'm at char limit so next postt.

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Filters

2022-08-07 Adrian VA3QAK

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so, the process of generating SSB is to first generate an AM signal and filter out one sideband and the carrier. this is way easier said than done

so we can generate an AM signal by just feeding our audio input and an RF oscillator into a mixer. the resulting double sideband signal plus carrier can be sent to the antenna and we have AM centered at the RF oscillator frequency, albeit at pretty low power if we're just directly feeding the mixer output to the antenna

problem is, to cut off the carrier and unwanted sideband, we need a razor sharp filter. all real filters have some "rolloff", they don't instantly transition from passband to stopband

we're trying to minimize this transiton consider that the usual legal requirement for SSB is to suppress the unwanted sideband to 40dB below the wanted sideband. way our filter passband starts at an audio frequency of 300Hz. this means that in the space of 300Hz, not only do we need to get rid of the carrier but we also need to reduce the other sideband by a factor of 10 000 (40dB less) -40dB in 300Hz is possible, but it takes some care first thing; we can get rid of the carrier if we are careful about our mixer design. we can use a balanced mixer to cancel out the carrier, making our filtering job much easier. balanced mixers can be built easily but need some fine adjustment to null out the carrier. this still leaves us with DSB-suppressed carrier still on board?

so I want to finish it

to cut out the opposite sideband, we can make a filter. conventional LC networks aren't really going to cut it here; hypothetically you could but it's really hard to get a high enough quality factor (Q) out of them

another part we can use that constitutes a high Q filter is a quartz crystal we can make a very sharp (high Q) filter out of quartz crystals, cascaded one after the other highqfilter.png we call these "crystal ladder filters" adrian-bandpassfilters.png more crystals -> sharper passband-stopband transition (ie higher Q) -> better unwanted sideband suppression

so, we can take our balanced mixer and input an audio frequency and a radio frequency, getting DSB-suppressed carrier. then, we can feed the output of that to a crystal ladder filter that cuts off the unwanted sideband. we've got SSB, so we're done right?

actually we're not done; the crystal ladder filter only works at one exact frequency

if the frequency of our crystal filter falls within a ham band, we can build something like VK3YE's Knobless Wonder: a fixed frequency direct conversion SSB transceiver

https://www.youtube.com/watch?v=UxDD2Qe1VX4

but if we want to be frequency agile, we need to do more

to cut to the chase, what we do is to pick a crystal filter frequency outside a ham band (typically 9ish MHz, this frequency is our IF or Intermediate Frequency because it's inbetween the audio and final RF frequencies) and then mix it with a variable frequency oscillator, resulting in an SSB signal that can be changed in frequency as easily as turning the VFO

a mixer generates two sidebands, but in this case the sidebands are several MHz apart (the spacing is determined by the VFO frequency fed to the mixer) and thus the unwanted one can be easily filtered out by a sloppy, easily built filter

as an example, we have a SSB signal generated at a 9.0MHz IF. we want to transmit on the 20m band so we use our VFO to generate 5.2MHz. the VFO mixed with the IF produces two images, the sum frequency at VFO + IF = 5.2 + 9.0 = 14.2MHz and the difference frequency at VFO - IF = 9.0 - 5.2 = 3.8MHz (in the 80m band, you'll notice; with a well chosen IF frequency we can cover multiple bands with the same VFO)

the difference in frequencies between the images is in this case VFO * 2 = 10.4MHz and this is way, way easier to filter out the unwanted image from

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2022-09-25 Adrian VA3QAK

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Amplifiers

2022-09-25 Adrian VA3QAK

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Receivers

2023-10-11 Adrian VA3QAK & VA3NB

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One of the main things that defines how good a receiver is, is the "spurious-free dynamic range". Let's break that phrase down; the dynamic range is the difference in input power from the antenna between the lowest power that a receiver can successfully receive (the sensitivity limit) and the highest power it can receive. The high power limit is different depending on whether we mean the highest power the receiver can tolerate before being damaged (the damage threshold) or the much lower power that is the maximum it can receive before spurious signals caused by intermodulation and distortion become unacceptable. The latter limit sets the upper bound on the spurious-free dynamic range.

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