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 <[email protected]> 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 <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
