[time-nuts] HP5061B Modulation Adjustment
Donald E. Pauly
trojancowboy at gmail.com
Wed May 17 20:15:43 EDT 2017
The first time that we worked with this HP10514A mixer we had an HP105
oscillator to provide the 5mc comparison against our #1 cesium clock.
Stability was not good enough to get closer than a few hundred
picoseconds. We expected some diode drift but not at that level. We
could not see a hot finger on a diode in the mixer. One millivolt out
of the mixer is about 31 picoseconds phase change near quadrature at
the drive levels used then. (±1 V peak dc) The theoretical dc output
is sin θ where θ is the phase angle between inputs. Two volts pp of
10 mc rides on that dc. We were operating unterminated on the output
of the mixer.
As long as the diode drops are stable, we can still see phase changes
to a fraction of a picosecond. A picosecond is the delay in an 8 mil
length of RG58 cable. (1,500 ps per foot) We have one DVM with a
resolution of 1 μ V dc. The data sheet on the mixer is at
http://gonascent.com/papers/hp/hp10514a.pdf . We would appreciate
anyone who has experience on diode drift on these types of mixers to
post accordingly. We built a 400 kc five pole low pass Butterworth
filter yesterday that removes the 10 mc from the mixer output.
(CLCLC) Attenuation is about 10 million to one at 10 mc which reduces
the rf to about 0.1 μV peak to peak. We can now see phase changes on
the oscilloscope of a few picoseconds at 1 mV per division.
The modulation deviation is hard to see at 5 mc and there is no handy
output there to drive a mixer. We therefore drove our mixer with the
90 mc outputs of each HP5061B. We used 6 db 50 Ω attenuators ahead of
the L and R inputs to avoid mixer overload. In phase dc out was about
0.5 V and 180° out of phase was about -0.5 V dc and was terminated in
50 by the low pass filter and the scope. We ran both HP5061B's in
open loop with modulation on one of them on and the other one off.
See http://gonascent.com/papers/hp/hp5061/waveform/90mcmod.jpg . This
shows the 137 cps modulation. Note that there is a fair amount of
noise present. This contributes to noise in the final lock.
Modulation from cesium #2 was 16.9 mV pp and #1 was 13.9 mV pp. The
#2 is about ±0.968° lead and lag or ±29.8 ps at 90 mc. Modulation is
at 137 cps. Phase change in a half cycle of the modulation is 2x 29.8
ps in 1/274 sec or 16.3 parts per billion average rate of phase
change. Average phase change for a half cycle of a sine wave is 2/π
(0.636) times peak phase change. Peak phase change is therefore ±25.7
ppb or ± 2.31 cps at 90 mc.
The 90 mc is multiplied in the A4 mixer by 102 to 9180 mc. At that
frequency the deviation becomes ± 236 cps or 472 cps peak to peak.
This is roughly the 550 cps distance between the cesium atomic
resonance at 9,192,631,770 cps and the valley either side of it. We
have not disturbed the modulation adjustment on our #1 machine which
is 82% of the deviation of #2. We will correlate the HP modulation
adjustment method with our measurements. It seems to me that over
deviation of the modulator will increase the noise of the system. Has
anyone experimented with lower modulation and higher gain on A7 board?
We can easily see any improvement with our mixer.
We are familiar with the 2nd harmonic gain pot but have decided that
the instrument needs a complete alignment after the beam tube change.
We do not know how many knobs were turned trying to get it to work
with a bad beam tube. It needs an alignment like it would have gotten
after it was built in the factory.
---------- Forwarded message ----------
From: Bob kb8tq <kb8tq at n1k.org>
Date: Wed, May 17, 2017 at 4:05 AM
Subject: Re: [time-nuts] HP5061B Modulation Adjustment
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Cc: "rward0 at aol.com" <rward0 at aol.com>, "Donald E. Pauly"
<trojancowboy at gmail.com>
Keep in mind that the mixer has millivolt level offsets and millivolt level
drift (over a wide range).
There is a gain pot that “sets” the second harmonic. It can be low and the
device will still lock.
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