I just bought an HP 105B Oscillator. It's doing something that may or
may not be a problem. I'm hoping that someone can help me out.
It's a newer unit that's based on the 10811 oscillator plus two
interface boards to adapt the 10811 to the rest of the 105B circuitry.
Does anyone have a soft copy of the manual for this configuration? Mine
has a serial # prefix of 2308. I have the manual from Didier's web
site, but it's for the older style with a serial # prefix of 808. Or,
does anyone have schematics & circuit descriptions for the 5061B Cesium
Standard? The more important of the two interface boards is part #
05061-6165. Pictures of the interior of the 5061B show a 10811-style
oscillator with two interface boards in the same location that the 5061A
had an older-105B style oscillator. It looks like both units had
similar upgrades.
My questions concern the performance of the 5 MHz output under different
loading situations. The older manual suggests that what I'm seeing
isn't normal.
First, all circuit check functions are good (80 +-7) with nothing
connected to the outputs.
My first question is whether its okay to use both the front and rear
connections simultaneously. The 1 MHz and 100 KHz outputs show the
expected drop in level if you have two 50 ohm terminations. The 5 MHz
output drops from 76 (3V2 p-p) with a single termination to 60 (2V2 p-p)
for the double termination which is fine since the spec is >1V rms i.e.
>2V8 p-p. But this drop in level crashes the 1 MHz and 100 KHz signals
completely. The older manual includes a stability test with the 5 MHz
output shorted and the test equipment connected to the 1 MHz output.
This unit would fail that test completely.
Second, if I attach a scope (1 Mohm impedance) via BNC cable to the 5
MHz output, the circuit check level rises from 86 to over 100. The
scope shows a clean sine wave of about 4V2 p-p. Even a 10 Mohm probe
with a BNC adapter gives a circuit check reading of 96 and the same 4V2
p-p signal. Adding the 1 Mohm scope to the 10 Mohm probe raises the
meter from 96 to over 100 and increases the amplitude to about 4V8 p-p.
AC or DC coupling has no effect. I checked for ground loops by plugging
everything into the same power bar. I ran the 105B on batteries. I
also tried a second scope. No change. I repeated the test with a 50
ohm termination on the scope and the circuit check shows 76 with a 3V2
p-p amplitude which is perfect. But adding the 1 Mohm scope to the
termination raised the meter to 82 with a 3V4 p-p amplitude. The same
tests on either the 1 MHz or 100 KHz outputs make more sense. The high
impedance connections don't affect the circuit-check reading and adding
a terminator drops the level by about half as measured by either the
meter or the oscilloscope.
In summary, should I be able to use both front & rear connectors
simultaneously without disrupting the other outputs and does it make
sense that high impedance connections cause the 5 MHz level to rise?
Sorry for the rather long message. Thanks for any help anyone can provide.
Ed
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