Re: Tracing receive signal for low signal

Bob Lunsford <nocrud222@...>
 

Simple way to see if the probe's inductance, if there is any, has
any effect is to touch an oscillator's connections in the frequency
selection area to see if the frequency shifts.

Another factor is the loading and this means resistance. If you have
two forms of signal level reading, put one on at a later stage while
touching an earlieer stage. If the earlier stage is not at a gain that
is saturated, the later stage should not be affected by touching an
earlier stage.

It is common in metrology to use different pieces of test equipment
to check each other. In fact, this is precisely what is done. I worked
in test equipment calibration and certification at Fort Hood for six
years and we had A-level calibration there which is good as the
Pueblo, CO standards bureau. We were C-level calibration. We
repaired, tested and certified the equipment and it was then passed
to the A-level for their testing and certification.

Probes have considerations of both [residual] inductance and
resistance. The ideal is zero inductance and a high resistance. This
is why one wants a high resistance multimeter and why a 20K/volt
over a 1K/volt meter was wanted in the old days. Today, meters
are generally at least 20K/volt and no worries unless a needle
movement type is contemplated.

The idea is to use a probe that has the least effect on the circuit under
test. If this means using a X10 probe and doing some math or using
a different scale, this should be done to avoid affecting the readings
and coming up with something that has less real-world meaning.

Bob — KK5R

--------------------------------------------

On Sat, 1/12/19, jim via Groups.Io <ab7vf=yahoo.com@groups.io> wrote:

Subject: Re: [BITX20] Tracing receive signal for low signal
To: BITX20@groups.io
Date: Saturday, January 12, 2019, 2:43 AM


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=yahoo.com@groups.io>
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

--------------------------------------------
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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