Re: Tracing receive signal for low signal

jim
 

To amplify a little bit ..Just measured my Tektronix P6100 probe ..100 ohms dc resistance at X1 and 9 Meg at X10 ...In the theoretical world, an ideal voltage source has Zero ohms impedance ..reality ..maybe 1 or 2  ohms ..they (the manufacturer of the test set) stuff some kind of network on the output to make it 50 ohms for rf ...used to be 600 ohms for audio ..dono if thats still true ..so...if your probes are 100 ohm at X1 then :   Rf Gen 50 ohm>>scope probe 100 ohm>>> terrminated at 50 ohm load ..equals 150 ohm in series with 50 ohm ..voltage drop across 150 ohm will decrease generator "ideal" output... 75% dropped across the generator/test lead resistance ..25% across the load

To test ..first measure rf gen output without it connected to circuit ...connect to circuit ,,measure at point where connected

Different?

Jim



On Friday, January 11, 2019, 11:04:54 PM PST, jim via Groups.Io <ab7vf@...> wrote:


Oscilloscope probes may have high impedancer  ..try measureing the dc resistance of the probes ..in both x1 and x10 positions ..really best to 'somehow' try to match the output impedance of the rf generator to the input impedance of the circuit under test

use a dc blocking capicitor (.01 uF is fine) between the rf generator and the circuit under test ..You will need to tune the rf gen for max output thru the filter ..may not be exactly 11.997 or whatever m Xhz ...depends on the crystals and the caps in the filter

Jim
On Friday, January 11, 2019, 5:56:14 PM PST, Dave Space <davesspacebar@...> wrote:


Hmm..I am rather new to this... I'm using the oscilloscope probes for both the signal generator input (set to 1x) at tp17 and oscilloscope probe at tp20 also set to 1x (as I believe I should be able to see the ~1khz or so signal at that point right? In the MHZ range I'd have to have it switched to 10x but figured I'm looking for low frequency 1x gives more granular voltage down to 2mv)  I tried using a small cap but I think it offset the voltage... might have to try it again with that ac coupled maybe.




On Fri, Jan 11, 2019 at 05:37 PM, Bob Lunsford wrote:
The type tip/cable you use on the tp may also be a factor.
Maybe you use an isolated tip but if not, the tip/cable's
input impedance and resistance may be affecting the
signal. Just a reminder. I'm also assuming it's an RF tip.

Bob — KK4R

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On Fri, 1/11/19, Dave Space <davesspacebar@...> wrote:

Subject: Re: [BITX20] Tracing receive signal for low signal
To: BITX20@groups.io
Date: Friday, January 11, 2019, 8:21 PM

I'm wondering about
the crystal filter.  I input a signal at tp17 11.996 or
around there to get a tone at 290mv it only comes out at
<10mv peak-peak that's like -29db does that sound
right? I'm measuring with the oscilloscope at tp20. 
Seems like an awful lot of loss or I'm detuning the
filter or something with the test.

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