Products > Test Equipment
HP 5316A Teardown and Frequency Standard
JoeD:
Received a board from amc184 and have built and fitted to my 5316a. Working well, have used rubidium reference to set it up and it's very stable. The Morion OXCOs are absolutely rock steady and the fourth picture shows the oscillator output reading on a Racal 1998 in overflow mode. That's 10.000000010 MHz ie 10 parts in a billion out...
Nice board Anton, thanks very much
JoeD
nixxon:
--- Quote from: JoeD on November 29, 2014, 12:21:32 pm ---Received a board from amc184 and have built and fitted to my 5316a. Working well, have used rubidium reference to set it up and it's very stable. The Morion OXCOs are absolutely rock steady and the fourth picture shows the oscillator output reading on a Racal 1998 in overflow mode. That's 10.000000010 MHz ie 10 parts in a billion out...
Nice board Anton, thanks very much
JoeD
--- End quote ---
Very nice results and heat-sink. Did you do the math to calculate the minimum size of the heatsink for the LM1085? With a Vin to the LM1085 of ~9V and a 5V output, the voltage difference is 4V. Multiply that with the < 600 mA power consumption of the OCXO, and it seems that the LM1085's output section must dissipate some 2.4W during OCXO warm up. After a couple of minutes the power consumption of the OCXO is <200mA, so the LM1085's output section needs to dissipate up to 800mW. In addition to the output dissipation, the LM1085 also has to dissipate the power from the control section. This is possibly negligable compared to the power dissipation?
The next step would be to calculate the thermal resistance between Junction and Ambient. I guess any value < 30 deg C/W would be safe...
nixxon:
I forgot to order a heatsink for the LM1085 voltage regulator when I ordered all the parts for the OCXO assembly. I found a disposable serving tray that was made by thin (~0.3 - 0.4mm) Aluminium and used scissors to cut out two identical shapes that could be folded into a suitable heatsink.
Then I could finally finish the last part of OCXO assembly by installing the LM1085. Little did I know that the OCXO would fail miserably... :palm:
nixxon:
--- Quote from: nixxon on December 23, 2014, 08:33:59 am ---Then I could finally finish the last part of OCXO assembly by installing the LM1085. Little did I know that the OCXO would fail miserably... :palm:
--- End quote ---
When I slowly increased the voltage to the 004 equivalent assembly, I noticed that the power supply seemed to be faulty. The voltage didn't go up as much as expected. But the amps did, rising to ~3A. And I could tell that the heatsink I had made was getting really warm. So I measured the resistance between the assembly voltage input pin and GND, and it was above 100 Ohms and not very low/shorted. However capacitor C105 was shorted. And so were the other capacitors between the 5V rail (LM1085 OUT) and GND, C104, C106 and C107. Then I looked at the LM1085 datasheet and noticed that the mounting tab, where the 3mm mounting bolt goes through, was not GND, but voltage output. Therefore I removed the bolt going through the LM1085, the heatsink and the PCB. I was not convinced this would make a difference, as the PCB ground plane was not exposed around the hole for the LM1085 on either side. And everyone else that have built his thing have just put a metal bolt straight through. Nevertheless, the OCXO assembly was now reacting very differently as I applied power to it. I could see the 10 MHz output on the scope as well, as I noticed that the current, that this time had started out at ~0.6A, was decreasing. Success!
With the assembly running, I reinserted the 3mm bolt, and all was OK initially. I had to tightened the nut a little bit before the output of the OCXO was affected. Maybe the designer, amc184, or someone else that can read the gerber design files in reply #12 in this thread can figure out if the bolt hole is drilled through the copper ground plane of the PCB. Maybe it barely makes contact with the ground plane edge in the hole as the nut is tightened?
Now I only have to find a way to fasten the LM1085 to the rest of the assembly. It is odd that everyone else that have done this job before me seem to have had success by simply putting a plain bolt straight through, without electrically insulating it.
By the way: the 20 turn trimpot is a very good design choice (made by amc184) to adjust the frequency output of the OCXO. No need to have the right tongue angle, as one full turn makes less than 0.1 ppm (1Hz) change.
nixxon:
5316A option 004 equivalent install (in 5315A) update:
To avoid the problem with shorting the TO-220 style voltage regulator to ground, I replaced the metal screw with a nylon screw. I ordered a bag of pretty nicely finished M3 nylon screws from dealextreme.com. I orded rather long screws and trim length as needed with side cutters.
At the same time, I ordered a bag of M3 nylon standoffs for the 004 equivalent board. The nylon threads will fasten ok and not damage the 6-32 nuts which are riveted to the counter's main board. The standoffs were too long, so I had to trim the lenght.
Just like amc184 suggested previously, I had to modify the 004 equivalent board (I used a Dremel sanding drum) to accommodate the large plastic pillar in the 5315A. Because of the mod, I also had to route the RF output trace with a (bodge) wire, as you can see on the picture.
I calibrated the OCXO just before Christmas, and it is still spot on, as seen in the picture. In the picture, the most significant digit ("1") is missing due to overflow mode. The display should be read as 1.00000000 Hz. (I used a Rubidium Standard FE-5680B with 1 pps output and long gate time to calibrate and check calibration).
Thanks to amc183 for making this mod possible.
I also checked how accurate my Siglent SDG1020 signal generator is with a Vanguard 0.1 ppm TCXO installed. It is exactly 0.1 ppm off. A 10 MHz output signal is measured to 9.9999990 MHz on the 5315A. (This is 180 times more accurate than the standard SDG1020 crystal that was 18 ppm off).
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