With the right ferrite and a thick bobbin from PTFE or a similar low permittivity material it could be possible. I've never tried because no need but in medical devices with intracardiac patient contact we have to keep such capacitances super low. Single-digit picofarads are normal there and usually we must tranfer multiple signals plus power.
[...]-- Regards, Joerg http://www.analogconsultants.com/
And not all applications are above ground. Then there are the applications I deal with a lot, where the medium is acoustic but in the 3MHz to 30MHz range. For example, in order to get a useful aortic Color Doppler reading at 3.5MHz or 5MHz in a very obese patient every dB of noise counts.
-- Regards, Joerg http://www.analogconsultants.com/
Yikes! But not mine. I wouldn't have bought such a lousy receiver.
Aside from medical ultrasound diagnostics I use my HF receivers also to find wee traces of conducted noise, internal self-pollution and so on. If they had a NF of 15dB they'd be rather useless for that. For example, a whistler 5dB above the noise floor would show up as a noticeable line in a Doppler FFT and cardiologists would complain about that. So you have to find where it's coming from or at least how it gets in and then do something about it.
-- Regards, Joerg http://www.analogconsultants.com/
Single digit pf is pretty different from single digit tenths of pf, though.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
I've never had 10Mbps, and get by on ~1.5Mbps now (pay for 3Mb). In my last house, I was getting half that and I lived inside the city limits. Though they were threatening[*] to plumb the city for fiber to the homes.
[*] They got as far as collecting the taxes for it, though whether it ever happened is another issue.There is certainly a diminishing return. Faster than a video feed, give or take, it's probably not warranted.
It would be a matter of trying, I don't see a reason why a ferrite transformer shouldn't be able to get below 1pF. It's just that I've never needed to push things that far.
-- Regards, Joerg http://www.analogconsultants.com/
Surely at least a few times that to get several TVs and streaming music playing high resolution (if not quality) simultaneously. The digital cable here is only 720p real resolution (upsampled to 1080 to the HDTV by the cable box).
Unfortunately, the ISPs that control the "last few miles" infrastructure here are run for-short-term-profit and the average customer will put up with a lot of drops and hiccups on their internet and crappy 720p digital resolution on the video. "Almost third-world" internet doesn't cut it in 2014. Average in Canada is only 14Mbps (Kazakhstan gets 11Mbps average).
I'm pretty sure if we get decent penetration at anything like 300Mbps, it _will_ get used. I'm not sure that the money to improve the infrastructure will get spent anytime soon.
LTE might be a way out (75-150MBps) if it wasn't controlled by the same companies.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Do you watch several TVs at once? I sure don't. Any audio is a trivial use of bandwidth. I'm not a huge fan of HDTV anyway. I like the sets but am ambivalent about the source.
The point is, after 10Mb, or so. Who cares? IME, the jump from .7Mb to 1.5Mb was huge. 6Mb or so would be nice but I have no idea what I'd do with more than one video feed. I'd like Netflicks (to displace premium channels) but after that I don't have a use case for more.
For what?
That doesn't solve the infrastructure problem. It makes it worse.
In the _real_ Canadian hinterland there is no cable or DSL, no convenient cell phone towers with LTE (or even (ugh) 3G), and no roads. I think only one of the satellite constellations offers full coverage (Iridium), and data costs are like $10/megabyte at ~0.15Mbps.
But, yes, you get a couple hours out of any city center and things tend to slow down significantly. It's a big hassle for a friend of mine who lives in his lake house (cottage country) and does high class advertising work. Some of those files can be huge. They recently got about triple the speed, but it took several years.
Here installers are generally third-party contractors and they also tend to do the minimum.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
never
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Not really. The one hard part was getting the LO to track properly above the received frequency, the frequency ratio was smaller. This also contributed to using lower IF frequencies. But that had to balanced against image rejection.
?-)
Hey it's like that at my Western NY hinterland. (No cable, DSL) there is cell tower coverage, but the cost! We get internet over a satellite. (Hughesnet) I think about $80/ month for about 20 GB of data. Speed is OK, but there is a long delay so my son can't play any MMOG's. (Poor boy :^)
George H.
That's the kind of situation I mean. Back in the 90's it must have been really bad. I sometimes had to send large data files from Germany to Saskatoon (university, so in the city). It had to go via modem and phone line back then. Beaucoup bucks per minute because of monopoly strangleholds. To the US I could regularly sustain 4800bps and sometimes more. To Saskatoon it immediately dropped to 2400bps and sometimes
1200bps. That made the data transfer painfully expensive.
Here it's the cable company's trucks, mostly.
-- Regards, Joerg http://www.analogconsultants.com/
To some extent, we do. There has been some interesting work done lately, mostly in universities, in making poor and minority kids fit in better, and not drop out for psychological reasons.
When I was a kid, black kids couldn't go to my schools. Whole generations of families had no higher education culture, and some of that no doubt persists over generations.
The point about privilige is that everyone should help those
1st-generation-in-college kids to make it. Some things do seem to work.-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
People listen to Jesus radio even if they aren't religious. What's wrong with listening just to hear what they are saying? I'm very much the curious type although I think I've heard enough from Fox cable news to last the rest of my life. I did appreciate them airing Futurama though.
btw, is it really impossible for you to trim posts?
-- Rick
Note that the 15-20 dB is the overall noise figure, not that of the front end.
If you're using an HF receiver as an instrument, as you seem to be, a much better noise figure is required and is easily achievable. For example, the typical frequency selective voltmeter has a more sensitive receiver (and a broadband front end). For example, the HP3586A can be used as an HF SSB receiver. Same with the front end of a good spectrum analyzer.
However, if you attach a typical HF antenna to one of these, the weak signals that these can hear on the test bench will be buried in atmospheric (and man made) noise. For a communications receiver, it doesn't make any sense to offer sensitivity that can hear below the ambient noise level.
What some HF receivers do is add an attenuator at the antenna to reduce the sensitivity (and raise the NF) of the receiver. This way, they can claim really good numbers for the reviewers, and still make the receiver usable for high noise level applications.
That's fair. Any application that doesn't require connecting to an antenna justifies a more sensitive receiver.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I looked into Hughesnet when we moved here (slow DSL is the only land alternative and 3/4G is very spotty). It's about $60/mo for 10Mb/1Mb and 20GB/mo (though there are limits on time of day). Up to about $140 for 15Mbps and 40GB/mo. This is if you buy the hardware, otherwise add $10/mo. It is expensive.
Kid'z long gone. ;-)
Right, but the later stages contribute very little if they did the design correctly.
Again, I would not buy a comms RX with a 15-20dB noise figure and would consider it junk or at least a flawed design. Those receivers are not always connected to huge dipole antennas. They are often connected to small mag-loop, other direction finder antennas or vehicle whip antennas where the signals are much smaller.
Most have attenutaors but really good designs excel by not needing to engage that attenuator.
Sorry, but I have to disagree on that one. Any decent comms RX needs good sensitivity.
[...]-- Regards, Joerg http://www.analogconsultants.com/
Too much sensitivity is bad for the dynamic range of the receiver. A really good HF comm receiver (not a spectrum analyzer or freq selective voltmeter) should have a dynamic range of about 100 dB. Sensitivities typically run about -136dBm: Plug that receiver into a real antenna, where the atmospheric noise is
50 dB above the thermal noise or for a 2.2KHz wide SSB receiver: -174 dbBm/Hz + 10*log(2200) + 50 dB -174 dbBm/Hz + 33.4 dB + 50 dB = -90 dBm The dynamic range (linear range) of the receive starts at the base line noise level or about: -174 dbBm/Hz + 33.4 dB = -140 dBm (or about 0.02uV) Ignoring any noise contributed by the receiver front end, this radio would overload at about: -140 dBm + 100 dB = -40 dBm (or about 2.2mV) Add 50 dB of additional noise at the antenna, and the base line of the receiver is moved up 50 dB, resulting in only 100-50=50 dB of dynamic range.It's MUCH easier to simply build a receiver that doesn't operate right at the thermal noise floor, but rather targets the atmospheric noise levels at the operating frequency. The low end of the dynamic range would start at a much higher level, thus offering a much higher overload point for the same dynamic range. This can also be accomplished by just adding attenuation at the RF input, or disabling any RX RF amplifier. Instruments have this anyway in the form of the input attenuator, so they need not concern themselves as much with dynamic range.
If you're building a piece of HF test equipment, that will never be connected to an antenna, go for the best sensitivity. If you're building an HF communications receiver, think about dynamic range and atmospheric noise levels first.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I go for best sensitivity even if it is to be connected to an antenna. One of the receivers I have is the NRD-515 which AFAIR is around 5dB in noise figure. It behaves excellently in terms of large signal handling yet you can also "hear the grass grow". That is how it's supposed to be for a correctly design RX.
Why would I want a one-trick pony for an RX if I can get better?
-- Regards, Joerg http://www.analogconsultants.com/
For receiver applications you may have a point, but for amateur radio/military communication systems, in which you also have a transmitter and you really need to have a matched antenna, in order to radiate the maximum amount of transmitter power.
Thus we are talking about matched antennas and huge received antenna powers.
Those are receive only applications,
Throwing in a switchable low noise amplifier _after_ a selective front end makes sense.
Any half decent front end design has the option of getting rid of any excessive front end gain by bypassing it, not by inserting an attenuator in front of a fixed amplifier.
The real issue is the very fast build up of third IP3 (and higher order) mixing products. If there are no justification to a large front end gain, do not use it, since you end up with lots of problems.
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