I think I basically understand how amplifiers, filters and mixers work, including how intermodulation products arise and how noise gets amplified by subsequent stages; however I've been struggling to get an intuitive grip on things like noise figures and 2nd/3rd order intercept points.
I've now scanned through this article and I think that if I sit down one evening and work through it properly, things might just start to go "click" for me.
To those with more experience: does it look sound? Is it still relevant today, 33 years after it was written?
Nice article, makes noise power very clear. That will always stay valid.
Reading further down the pages..
2 meter transceivers are better now I'd think. We now live in the days of RTL-SDR USB sticks and software defined radio...
My RTL-SDR stick has no problem receiving GPS signals at 1.5 GHz, but outdoors only with a GPS mouse antenna.
Large scale integration...
Even the 10 GHz LNB PLL and mixer in one chip:
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So.. where will it go? hard to DIY, more dependency on special chips. war, large EMP, all your Baofengs and LNBs, and back to tubes?
I used to have a metal box for EMC protection with some spare RF transistors in it.
Now I have one to protect my FLASH USB sticks and cards.
Imagine archaeologists digging up what's left shortly after trumpy canceled the nuke treaty, ... Look, in the 21st century they already made GHz chips? How did they do it?...
Like now we look at the pyramids.
Or it will be just ants and mosquitos that survived.
It is still a very valid article which essentially appeared in RSGB VHF/UHF manuals for decades.
However, the RF-environment has changed during the last decades.
- For ground based communication, in which part of the antenna lobe sees the "hot" (300 K) earth, the noise figure or noise temperature requirements are modest as in the article.
- In satellite (and EME) communication, the signal source is against the "cold" (above 4 K) sky, getting the ultimate preamplifier performance makes sense
- Current semiconductors have a decent noise figures even at slightly higher collector/drain currents, making it possible to adjust between noise figure and strong signal handling.
- These days, the spectrum is full of strong (nearby) signals up to at least 6 GHz. If a preamp has, say a 10 GHz bandwidth, it will amplify all this crud, causing potentially thousands of intermodulation products.
- For best performance, the input spectrum should be band limited to the band of interest. Unfortunately a bandpass filter has also some passband attenuation, which adds directly to the noise figure. A bandpass filter with very high unloaded Qu (such as silver coated 1/4 wave resonator) will have a low attenuation.
- Mixer intermodulation performance has been a problem, forcing a simple low loss filter between antenna and first preamplifier and a sharp high loss bandpass filter between the preamp and the mixer.
- In SDRs with direct (or sub)sampling seems to tolerate big signals better. However, if there are mixers in front of the SDR, there are still these strong signal issues.
To get the full performance of SDR, front end filtering is still needed, see FunCube Dongle vs. FunCube Dongle Pro+.
Strange, I use it for 2 meters and 70 cm, wrote a simple spectrum analyzer for it, band is further clean:
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Antenna type matters a lot for selectivity, a simple rod or wire will pick up strong unwanted signals. I use slim-jims for AIS (ship traffic) and 70 cm. Those are basically a short on other frequencies. Have several of these sticks, main one has a temperature controlled oscillator:
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I designed and build several receivers from the sixties upwards, best one with cascode JFET pre-amp stage, low noise, good selectivity, for short wave and 10 meters. Tuners for VHF TV.. dual gate MOSFET.
So now I play with that LNB stuff for 10.4 GHz... Not much I can do to improve that, only to make it more stable. I am sort of out in the country, small village, so far away from big transmitters, OTOH some years ago I called my cross the road neighbor on CB and told him I have a LED connected to my GPA antenna and it lit up every time he pressed 'transmit', asked him how much power he was running... eeeh So YMMV RF bands are polluted completely due to all the switchers around.
I build a down converter for in my drone to send it up with a slim-jim hanging under it, to get real antenna height, thin coax cable with 100 kHz few hundred volts to power the drone and for the down converted signal. Antenna height matters. Have not done much testing with it yet, 'they' declared the area a no fly zone after that, close to mil airport.
Oh well. I am not into hundred element yagis and that sort of stuff. I Eshail 2 sat is working, and I can RX it, then maybe start on the 2.4 GHz upload, will need a bigger dish, already have the power amp.
I tried to do that with my LimeSDR thing. It didn't work very well, and I don't honestly understand why - I suspect that (a) the input and output ports aren't really 50+j0 ohms, and (b) it has some kind of AGC that I can't switch off in software.
I've had some great success building antennas.
Sadly things are different for me. There are pagers, DAB radio broadcasts, and cellphone signals, all being chucked out at enormous strength from the top of the water tower at the end of my road.
I prefer to see it as a challenge, rather than a setback.
At my place I have an official restriction, courtesy of the local planning department, that I'm not allowed to build an airport. LOL!
It's because I'm too close to the local USAF bases.
How much liquid nitrogen (or even liquid helium) does it use daily ?
I might believe that some radioastronomy telescope site might be using superconducting band stop GSM filters, but why use such filters on cellular towers ?
Much has been posted about it here in the past, I bought the thing for that cooler, not for the filer. Make liquid nitrogen, liquid oxygen, etc.. Cool electronics (cameras especially).
The actual filter is in the dewar, the cooler sticks into it:
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The cooler runs on 110 V 60 Hz (mechanical resonance, so in the US it could be run from a variac, but the machine has a large more precise frequency driver board.
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but.. I have driven it from an audio amp with step up (old mains transformer in reverse with sgen in Linux as signal generator.
It is a band-pass, so only cellular signals are passed, and all that interference is stopped. I do not use the filter.
Using device 1: Generic RTL2832U OEM Found Rafael Micro R820T tuner Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6 Sampling at 2048000 S/s.
Yes, sure and the neighbors have WiFi (I have it off), and I have cellphones...
Yes, I like these USB sticks, there is plenty you can do with it. Curiosity is maybe more my thing.
What also works great is decoding my outside weather station once a minute using rtl_433 and planes traffic with dump1090
We have a national 70 cm repeater network that it picks up no problem, and also one for the NE part of the country, same, all on slim-jims. Sometimes I hear Germany,..
Also have an old TV UHF bowtie antenna in the attic that works with it. I must admit I have a 1 transistor preamp upstairs and then 15 meter RG58 to downstairs to compensate for cable losses.
That preamp may or may not improve noise performance.
Am 27.10.18 um 13:38 schrieb snipped-for-privacy@downunder.com: nd noise).
If your antenna points to a cold region of the sky, 0 dB can be much lower than -174 dBm/Hz. In this case, having less antenna gain is not as bad as having side lobes.
If you have a signal generator with known output (possibly checking with an oscilloscope), a step attenuator (to avoid overloading the receiver/SA) you should get a peak on your spectral display. Note the peak level value and you can calculate the absolute value for the "0 dB" line.
The displayed spectral curve suggest a stone deaf receiver, so no intermodulation problems expected.
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