Products > Test Equipment
HP 5316A Teardown and Frequency Standard
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David Hess:

--- Quote from: nixxon on November 19, 2014, 12:44:59 pm ---Is there any reason why the 5V OCXO can not be powered directly by the existing +5.00V supply (pin 4)?

To me it seems that the +5.00V (pin 4) is powered by the unregulated +7,5V that is also present on pins 1 and 6.

The Morion MV85 OCXO has a peak power consumption of <600 mA during warm-up.
--- End quote ---

The way it was originally designed, the XO and TCXO use the +5 volt logic supply but the OCXO instead has a discrete low dropout regulator to provide +5.0 volts from the +7.5 volt unregulated supply.  Offhand I can think of two reasons HP did it this way:


* This distributes the power dissipation across two regulators and avoids overloading the discrete +5 volt logic regulator.  Doing it this way would be less expensive if a majority of the counters did not include the OCXO option because the +5 volt logic regulator would not need to be as powerful.
* Noise and drift on the logic supply may compromise the OCXO performance.
I probably would have used a separate regulator even for the XO and TCXO.
nfmax:
The OCXO is powered from the permanently on 7.5V supply, so it is kept running in standby. Crystal oscillator drift is approximately proportional to the square root of time, so the longer it has been powered on the (relatively) more stable it gets.
David Hess:
That is an even better reason that I should have remembered.  My best counter does the same thing even with the TCXO.
amc184:
Yeah, you guys have got it.  The two main reasons are:
- To keep the OCXO powered and up to temperature at all times so the counter can be turned on and used without delay.  An SC cut crystal won't even put out the right frequency when it's cold, let alone stability.
- To make sure the 5V rail isn't overloaded.  An OCXO draws a lot of current.  The MV85 is actually quite frugal, drawing up to 600mA on start up and 200mA steady state, many are much worse.  You can't assume a regulated rail can provide this much extra current.

Besides all of that though, when I designed the board I just wanted to duplicate the function of the original as closely as possible with more modern parts that are easily available now.  I didn't put much time and thought into changing anything, this was just a quick side project.  I mainly do audio.
HiTech:
Very nice work/detail on your mod -- AMC184.   :-+
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