[time-nuts] Fwd: HP5061B Versus HP5071 Cesium Line Frequencies

Donald E. Pauly trojancowboy at gmail.com
Mon May 29 14:13:37 EDT 2017


https://www.febo.com/pipermail/time-nuts/2017-May/105500.html

We recently did a partial alignment of the lock servo on our #2
HP5061B after replacing the beam tube. The previous owner had tried to
fix it by turning adjustments.  This made a big improvement in the
lock.  KB7APQ got the idea to use the audio spectrum analyzer in his I
Phone to measure the noise output of the beam tube.

We used the Beam I meter driver emitter follower for an audio source.
It provides about 0.4 Volts per 25 uA on the meter.  A 100 ohm safety
resistor was in series with Q6 emitter on the A7 board.  It was
followed by a 100 nFd condenser into the 100 k input impedance of the
I phone.  Low frequency cutoff is about 16 cps.

See http://gonascent.com/papers/hp/hp5061/waveform/spectrum.jpg .
Start frequency is 4 cps and each bin is 8 cps wide.  Center frequency
of each bin is 8 cps higher than the one before it.  Frequency and
amplitude are both logarithmic.  Amplitude is 12 db per division.  The
first three bands show the low frequency rolloff of the coupling
condenser.  Five harmonics of the 137 cps modulation frequency can be
seen.

For unknown reasons, a sharp null in the noise of about 2 db at 137
cps is seen.  The servo nulls the 137 cps there but I can't see how
the noise could be nulled.  The prominent second harmonic at 274 cps
is normal.  It measures -74 db below reference.  I calculated it at
about 0.15 V pp or 53 mV rms.  The third harmonic at 411 cps again
shows up as a 2 db noise null for unknown reasons.The fourth harmonic
at 548 cps cannot be seen. The fifth harmonic at 685 cps barely breaks
thru the lower limit of the spectrum analyzer.

It looks like rectifier pulse harmonics can be seen at 120 cps.  They
may be getting thru the mu metal shields of the beam tube.  That
frequency is right on the border of two bins.  360 cps third harmonic
of rectifier pulses can be seen.  It appears in the middle of a bin.
An unknown signal is seen at 564 cps.  This could be the +3500 power
supply frequency.

1 cps bandwidth noise in the 50 to 100 cps area seems to be about 20
db below the 274 cps second harmonic.  This will determine the
possible lock improvement with improved modulation methods.

πθ°μΩω±√·Γλ
WB0KV


---------- Forwarded message ----------
From: Bob kb8tq <kb8tq at n1k.org>
Date: Sat, May 27, 2017 at 6:01 PM
Subject: Re: [time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>

Hi

Having run a 5071A with a *very* good 10811 in it, the OCXO does
dictate what happens at 0.1 seconds. Once you get past that, you are
headed into a bit of a gray zone. You are partly looking at the Cs and
partly looking at the OCXO. Pushing out the crossover between the two
could help you at 1 second. The gotcha is that the “hump” will still
be there, just a bit further out. The net effect at (say) 100 seconds
could easily be worse
with the “fix”.

Bob


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