domenica 28 settembre 2014
Test of ADA antenna amplifier with Siemens K2155
Following my previous post on the same subject, this video shows a "kitchen table" test of my build of ADA antenna amplifier by Claudio Re. During the test, the amplifier has shown an almost constant voltage gain of about 30 dB. Gain variations were within +/- 2 dB over the whole frequency range of Siemens K2155 selective level meter, that is 200 Hz to about 650 kHz, with only a slight decrease starting at about 400 kHz. No particular care was given to shielding, insulation, etc. so the results are to be taken as approximate figures only, expecially below 500 Hz where AC mains noise (50 Hz and harmonics) is dominant.
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Can you share some details how you built the balun between the output of your amplifier and the balance input of the level meter? I see no power supply cables so I assume it is not a OpAmp based design but rather a broad band ferrite transformer, right? Which material did you use and what number of turns?
RispondiEliminaThe label "BALUN 1:1" on the small plastic box on the left is misleading, my apologies for that. The box is simply a BNC-to-banana adapter. The level meter is used with unbalanced input and output. Consider this was a very simple kitchen-table test, with very little to share with a serious measurement.
EliminaBy reviewing the video on YouTube I have realized I had written "Tank you for watching" on the final screen. Of course I intended to write "Thank you for watching". Sorry for the mistyping. Thank you for reading :-)
EliminaAh, interesting. You did however use the balanced output of the tracking generator to feed into the balanced input of your amplifier, no? So if I understand correctly then you kept one end of the balanced input of the level meter at ground level (shield of coax) of the amplifier, right? I wonder if this makes the meter readings off by 3dB or so ... Not that this is terribly important for your demonstration but I'm currently restoring such a level meter and try to understand the various possible balanced to unbalanced tarnsformation options. Ideally I'd like to have two boxes similar to your video. One which transforms the low impedance balanced output of the tracking generator to an unbalanced BNC connector. The second box should be like your 1:1 balun box (unbalanced to balanced) in the video but with content which matches the label outside the box :o)
RispondiEliminaYes, you are right. Balanced output of the tracking oscillator is connected to balanced input of ADA amplifier. The "zero" impedance output is selected for the generator. Then the unbalanced output of the ADA amplifier is connected to the balanced input of the level meter, so one of the meter inputs is connected to the "ground" reference of the amplifier (though this is left unconnected from actual ground). The high impedance input (about 10 kohms, if I remember correctly) is selected on the level meter. About the "balun" box, it actually contained a small balun transformer originally, then I recycled the box for a BNC-banana-UHF connector adapter, quite handy when try to perform simple measures or tests. But I forgot to remove the label :-)
RispondiEliminaI know this is a bit off topic but would it be possible for you to oen test for me: Simply connect one of the balanced output sockets with a straight cable to one of the input sockets and leave the others open. When I try this then around 30 kHz and lower the meter readings drop dramatically. I do not see such a behavior when I make straight connections from both tracking generator sockets to both meter input sockets. In that case the readings are flat over the whole frequency range. I wonder if this is a defect of my particular unit or a general behavior ...
RispondiEliminaOn a related note: Meanwhile I built an active balanced to unbalanced driver which presents a balanced 2K Ohms impedance to the generator and transforms this into a 50 Ohms unbalanced output. I did it with a gain of 2 setup using a LM833. LTSpice simulations show that it is reasonably flat over the tracking generators frequency range. I am now working on the reverse line rceiver: a 50 Ohms unbalanced to as low output impedance as possible balanced. Then I should be all set for real measurements. BTW: Did you notice that the level meter has a 10V aux power socket at the backside? My plan is to use this as the power supply for my active baluns ...
Unfortunately I don't have the level meter anymore. I returned the testset to the fellow ham who I had borrowed it from. I could send to you the operating manual in PDF format, if you haven't it yet.
RispondiElimina