[R-390] Filters

[Bob Camp]

Hi

We're not dealing with silly 50 ohm systems here. With zero power gain, 1 uV in 50 ohms becomes 44.7 uV in a 100K ohm system. One volt RMS is only 10 micro watts in a 100K ohm system (E^2/R). Hitting the filters with milliwatts is not a good idea, micro watts had better be ok….

If you put a couple of volts RMS into the filter you should get a couple hundred mili volts out of the filter. If you use a scope, that's quite possibly 10 volts p-p in and a bit under a volt p-p out. Scopes with lots of tubes in them can handle those sort of levels at 455 KHz.

Bob
 
On Feb 19, 2013, at 6:45 PM, quartz55 <quartz55 at hughes.net> wrote:

> Thanks for the ideas, yes, I've got the filter innards set in a trough, and the wire I've put into a 'sled' composed of a Teflon center with rolled up tape as the outer cover, I've thought about hemostats, but they crinkle the wire and are too large, I have a small spring clip that has flat 'teeth' so it doesn't crinkle the wire, but it's so small it's hard to deal with, it's too small to keep my fingers on it reliably, alligator clips are just too clumsy and the teeth don't meet that good, the tweezers are also clumsy and too hard to hold together.  I'm trying to come up with some sort of clamp/clip that I can slide in the trough while holding the wire on the end of the 'sled'.  I thought I could use the end handle of a voltmeter lead, but that has the pointed end and nothing good to attach the wire with, plus it's not exactly centered either.
> 
> The other problem is that all this has to go into the copper cup about 1/8" which doesn't seem like much, but we're dealing with 1/10ths of inches here.
> 
> Here's the end of the flat clip, the voltmeter probe and the 'sled' with a test wire in it.  I've found my close up lens for the camera.  That sled is about 1/4" dia.
> http://i251.photobucket.com/albums/gg287/DogTi/R390A/filterwiresled_zpsb2481c56.jpg
> 
> That clip with the flat ends seems to be the thing, but it's unmanageable it's so small, maybe I can solder some handles on it and make it so it slides in the trough along with the sled.
> 
> In the meantime, I may test the 2K filter, 20logV1/V2=-25(db)  That's 17.7(or .0564) for V1/V2 if I calculate right.  Now I need to know roughly what voltage is going into the filter.  It's probably in the order of 100-200uV.  My old Motorola RF milivoltmeter will read down there.  Hmmm, maybe not smallest scale is 1mV.  That means if I put 200uV in I should be getting 11uV out.  Hmmm.  Where's that 20db gain amp.
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