I am NOT a principle or anything else for AirSpy except a user. I am a 3/4 century old retired engineer with a background in both RF (high dynamic range receivers, exotic frequency synthesizers, GPS, and SatCom) and software (device drivers, broadcast live video effects/channel branding, and some GUI work pushing boundaries of MS supplied libraries.) Um, in the '70s I was building fast Hedy Lamar transceivers, various segments of the GPS satellites, and such exotica as a martini olive equivalent for satcom for an unnamed government agency. (The famn dool thing actually came together and worked! I did the RF part of it.) My ham license is about .6 centuries old.
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My name is Joanne. I've been cyberstalked (one of the very first) in the 1985-1986 online era. I used to include my ham call letters with my online information. I learned not to. I received repeated VERY graphic threats of rape and dismemberment. I took down all but vague references to where I live (Southern California sort of near KONT). During the episode I learned to use and care for a .38 police special which I ended up illegally carrying while the threats were coming in. (My gun was seconds away while the police were 10 minutes or more away and could admire the dead and dismembered body.) As soon as the threat was over (that in itself is a long story) I stored the gun. It's my reminder of some nasty days, over 365 of them.
If you cannot guess my last name from the ID, well, you need some help with solving simple puzzles, right?
Note that Moonbounce problems' relationship to HF problems is at an exceptionally tenuous generic level.
On 20191017 07:40:33, Wes Stewart via Groups.Io wrote:
On Wed, Oct 16, 2019 at 10:43 PM, jdow wrote:
Well, for what it is worth I will reiterate what Youssef said. In general the
input match on the receiver does not matter.
In specific, however, there is an issue with filters attached to the input of
the receiver. Their passband shape will be distorted. If this is a problem for
you then put up a little more wire and add an attenuator to the receiver front
end. The attenuation won't matter and the attenuator will make the filter happy.
For what it is worth for most circuit configurations (Leif appears to have
exception) an input match gives you an absolute minimum noise figure of 3 dB.
However, if you mismatch the inputs creatively you can get a noise figure well
below 3 dB. All devices have an input noise current in shunt with the input and
an input noise voltage in series with the input. The optimum match is
(obviously) En/In, which isn't a neat 1:1 SWR at any impedance. You can,
however, do startling things with extreme mismatches so that En or In are
immaterial. You do not get the optimum possible noise figure. But, you do get a
very very good one.
All that said, what matters in the end is the signal to noise power ratio at
demodulator. And at HF the single largest source of noise is external to your
system. This is why you can put in a 30 dB attenuator from a 160 meter dipole
and still copy all the weak signals you can find. That is why I declare if
match is critical to a filter you have designed, then put in a 10 dB pad to
it happy. I learned this the expensive way as a teenager. I was on 15 meters at
the time in about 1960. I was displeased with SNR on received signals. DowKey
made a super low noise preamp. I scraped up the money (never smoking helped me
save the money) and bought it. If anything the results were worse due to
increased IMD. In college some extra-curricular reading led me to an excellent
book, long out of print, on noise figure. And the old Terman
found used filled in the galactic and other noise sources picture for HF
2 meters where it becomes nearly negligible. Noise figure cannot be reduced
what your antenna presents your radio. It's something about thermodynamics, the
"you can't win" law. It was some years later that I learned about IMD, the
slab of bread surrounding the signal sandwich. Gain can help front end noise
mask noise from succeeding stages; gain also puts you closer to overload. (And
the really stinky part of this is that AGC systems may reduce signal level;
most ways you can do it also reduce the large signal handling ability.)
So, Wes, at the risk of being overly blunt, for the most part your excellent
efforts were a misinformed waste of time. Sorry for hammering the so many nails
in this coffin; but, this ex-cathedral pontificating finally got to me.
Well, let me be blunt and see if my comments get past the moderator. First, I feel a distinct disadvantage because I don't know who I'm discussing this with. Two of you hide behind screen names but I assume that you are the principals in Airspy. My name, and amateur callsign, is in the clear and you can look me up on QRZ.com if you wish and get a clue about my capabilities. Note that I was communicating via moonbounce way back in the 1970s and was building my own equipment. I know a bit about noise figure, so don't need any lessons. I also did this stuff professionally but I won't go into that here.
The reason that I chose to measure input matching is because that is the only thing readily accessible to me; I have one coax connector to look at. If I'm allowed, I can post screenshots of my unit operating in my local environment which would make evident my concern about the performance of the input filtering.