Date: Thu, 22 Oct 2015 15:15:04 -0500 From: Jim Thompson <jim@netgate.com> To: John Nielsen <lists@jnielsen.net> Cc: freebsd-wireless@freebsd.org, freebsd-embedded@freebsd.org Subject: Re: looking for suggestions for a small router/appliance board/SoC Message-ID: <CFB78FF8-F8F1-4AEF-ABF7-FEAEF4364C14@netgate.com> In-Reply-To: <7D3FEA1A-0EAF-4B0A-867D-49DCEF34D0CC@jnielsen.net> References: <2EB47812-4744-48B5-BEBF-B2074D9EEA8F@jnielsen.net> <ED6B0DC8-7325-4054-A4CF-ED62A14FD39F@netgate.com> <7D3FEA1A-0EAF-4B0A-867D-49DCEF34D0CC@jnielsen.net>
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> On Oct 22, 2015, at 2:39 PM, John Nielsen <lists@jnielsen.net> wrote: >=20 > On Oct 22, 2015, at 12:19 PM, Jim Thompson <jim@netgate.com> wrote: >=20 >>> On Oct 22, 2015, at 12:57 PM, John Nielsen <lists@jnielsen.net> = wrote: >>>=20 >>> Hi- >>>=20 >>> I=E2=80=99m working on a proof-of-concept for a kind of networking = swiss army knife. Can anyone suggest a board that meets the following = requirements? CPU arch doesn=E2=80=99t matter as long as it will run = FreeBSD (Atom, ARM, MIPS, etc). >>>=20 >>> - Small form factor (SoC, probably) >>> - Can support at least 2 802.11a/b/g/n adapters, prefer 3 (any = combination of chip-integrated and mini PCI-e slots. Prefer to avoid USB = if possible) >>=20 >> I openly question your need or the desirability for 3 802.11 = adapters. It can be made to work, but you=E2=80=99re going to have some = intermod. >=20 > I don=E2=80=99t mind being questioned. :) I haven=E2=80=99t yet had to = worry much about intermod; can you educate me? One or two of the radios = would be in the 5GHz band at any given time. One scenario (out of = several) where I envisioned having 3 radios is taking a wireless uplink = (STA in either 2.4 or 5 GHz band) and repeating it (HOSTAP) on both 2.4 = and 5 GHz. Totally crazy? Many people believe that there are =E2=80=9Cthree non-overlapping=E2=80=9D= channels for use with 802.11b/802.11g/802.11n in 2.4GHz. While the transmit masks don=E2=80=99t overlap, the selectivity of the = receiver (especially after the industry turned to direct conversion = architectures around the advent of 802.11g) is not sufficient to operate even two radios in = any given bad (2.4GHz, etc.) It=E2=80=99s all been covered before. You=E2=80=99ve unknowingly hit = =E2=80=9CJim=E2=80=99s favorite point of banter=E2=80=9D. = https://lists.freebsd.org/pipermail/freebsd-wireless/2013-December/004158.= html http://seclists.org/interesting-people/2009/Oct/77 http://www.ietf.org/mail-archive/web/manet/current/msg05757.html http://lists.shmoo.com/pipermail/hostap/2004-April/006524.html Even where you can operate in separate bands, there are mixing products = that can greatly interfere with correct reception. Receivers live under constant bombardment of signals which enter through = the antenna port. Some of these signals are immediately attenuated due = to front-end filtering, (aka pre-selection). When the remaining signals = reach a non-linear element, such as a detector, mixer or amplifier, = harmonics of the signals are generated. Most of the harmonics are well = outside the pass band of RF and IF filters and cause no problems. However there are some frequencies where the mixing products =E2=80=93 = intermodulation products =E2=80=93 of the various signals fall on or = near the desired receive frequency range. The intermodulation products = that tend to cause the most problems are the so-called odd-order = products. This is true because odd-order products of signals near your = desired receive frequency also are near your receive frequency. 802.11 = systems tend to suffer more from these issues due to the uniform spacing = of the channels. (Remember that OFDM separates the signal out into many = sub-channels.) With DC receivers, the intermediate frequency is zero and the image to = the desired channel (for all but single-sideband signals) is the channel = itself. This means=20 only one local oscillator (LO) is required, which means only one phase = noise contribution, and as such, the need for the bulky off-chip filters = is consequently removed. Filtering now only occurs at low frequencies = (baseband) with some amplification, which means less current consumption = than at higher frequencies (to drive device parasitics), fewer = components and lower cost. This is all good, and contributes the much = lower cost for today=E2=80=99s 802.11 radios, though the largest = contributor here is the absence of SAW filters at the IF in a superhet = receiver. Practically, however, strong out-of-band interference or blocking = signals may need to be removed prior to down-conversion in order to = avoid desensitizing the receiver by saturating subsequent stages, as = well as producing harmonics and intermodulation terms which will then = appear in the baseband. =20 In direct conversion, as the signal of interest is converted to baseband = very early in the receive chain, without any filtering other than RF = band-selection, various phenomena contribute to the creation of DC = signals, which directly appear as interfering signals in the band of = interest. The LO may be conducted or radiated through an unintended = path to the mixer's RF input port, thus effectively mixing with itself, = producing an unwanted DC component at the mixer output. Worse still, = this LO leakage may reach the LNA input, producing an even stronger = result. This effect presents a high barrier against the integration of = LO, mixer and LNA on a single silicon substrate, where numerous = mechanisms can contribute to poor isolation. These include substrate = coupling, ground bounce, bond wire radiation, and capacitive and = magnetic coupling. I=E2=80=99ve seen LO signals cross over the PCI bus = lines. (Vivato had a design with 14 802.11b (superhet) radios on the = same PCB. I improved that to 6 802.11g (DC, Atheros) radios on discrete = cards.) Conversely, a strong in-band interference signal, once amplified by the = LNA, may find a path to the LO-input port of the mixer, thus once again = producing self-mixing.=20 Some amount of LO power will be conducted through the mixer and LNA (due = to their non-ideal reverse isolation) to the antenna. The radiated = power, appearing as an interferer to other receivers in the = corresponding band, may violate emissions standards of the given system. = It is important to note that since the LO frequency is inside the = receive band, the front-end filters do nothing to suppress this LO = emission. Additionally, the radiated LO signal can then be reflected by = buildings or moving objects and re-captured by the antenna. This effect, = however, is not of significant importance compared to the aforementioned = LO self-mixing and blocking signal self-mixing.=20 The leakage of LO or RF signals to the opposite mixer port is not the = only way in which unwanted DC can be produced. Any stage that exhibits = even-order nonlinearity will also generate a DC output. Suffice it to state that many people have this idea, but few actually = endure the engineering to make it a) work and b) be legal in a given = (set of) regulatory domain(s). >>> - Has or supports at least 2 1GbE ports. Prefer 3-5 ports with = switching functionality >>> - Storage not super constrained. Built-in storage (if any) can be = small (which I=E2=80=99m arbitrarily defining as less than 128MB) if = there is also an SD card slot or similar. USB storage will do in a = pinch. >>> - Has at least 2 free USB ports after meeting previous requirements >>> - Serial port or header (or GPIO pins that can be used as one? Not = too familiar with that) >>> - Low power consumption (within reason taking the above into = account) >>> - Low cost (again, within reason) >>>=20 >>> I may just start with a PC Engines apu1d, but if there are boards = that are smaller, cheaper, have lower power requirements and/or have = integrated wifi or switch capabilities I=E2=80=99d like to look in to = them as well. >>=20 >> you can probably get APU in low-volume, but quantity is very = constrained these days (it=E2=80=99s been true all year). >=20 > Good to know. >=20 >> We do have the RCC-VE and RCC-DFF units available. >>=20 >> http://store.netgate.com/Desktop-Systems-C83.aspx >=20 > Thanks for the link! You could also look at Minnowboard Max (quite difficult to get) or = Minnowboard Turbot. http://www.minnowboard.org/meet-minnowboard-max/
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