A popular solution down under is to use a 555 timer driving a charge pump to
generate negative rails from a single positive supply.
Example here: http://www.vk2hmc.net/blog/?p=970
Hi all - I'm back :)
--marki
-----Original Message-----
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf
Of Stewart Cobb
Sent: Thursday, 31 October 2013 5:12 PM
To: time-nuts@febo.com
Subject: [time-nuts] powering Trimble Thunderbolt with -5V rather than -12V
Executive summary: you can power a surplus gold Thunderbolt using a -5V supply
in place of a -12V supply, and it will probably work just fine.
Details: The manual for Trimble Thunderbolts specifies power supplies of +5V,
+12V, and -12V. It turns out that power supplies that provide
+5V, +12V, and -5V are easier to obtain locally. I began to wonder
what circuitry in the Thunderbolt required -12V, and whether it would run just
as well on -5V. So I took one apart and started probing.
As far as I can tell, the -12V supply goes to only two places. One is the
negative supply pin for the quad op-amp (LT1014) in the DAC circuit for the
OCXO. The other is a strange little circuit involving a 2N3904 (SOT-23 marked
1A) near the "232" driver chip, right next to the serial port. This circuit
seems to be comparing the -12V input with one of the charge-pump pins on the
232 chip. Its output (?) connects to a test point labeled "MON". I assumed
this was non-critical and decided to ignore it.
The LT1014 op-amp is rated for operation on supply voltage as little as 5V and
as much as 30V (+/- 15V). The spec sheet says the output saturates about (1V
typical / 3.5V max over temperature) above the negative supply. Presumably, if
the op-amp is not asked to generate output voltages lower than -1.5V, it should
run fine with a -5V negative supply.
The only negative voltages I could find, probing around the op-amp circuit,
were generated by AC-coupling digital square waves. None of the op-amp outputs
were negative. (My DAC steady-state value was around +300mV, which appeared
many places in the circuit. Presumably a slightly negative DAC value would
also appear in many places, but as long as it's greater than -1500mV, it won't
matter.)
Armed with theoretical and practical confirmation that this should work, I
tried it. And, oddly enough, it appears to be working. Two different
Thunderbolts have been powered by +5/+12/-5 supplies, and both have settled
down and started tracking exactly as one would expect. For one, the "settled"
DAC voltage was within a few millivolts of the value it had on the specified
power supplies, shortly before the change. The other had not been powered on
for a while and is still settling, but it seems happy.
There is a subtle possibility for concern, in that the sensitive DAC signals
near ground are now about 3.5V away from the center of the op-amp supply range.
This could theoretically cause increased distortion, offset, or offset drift
due to the larger common-mode voltage on the op-amp inputs. In practice, it
does not appear to be an issue.
This note applies to the common surplus Thunderbolts in the gold-anodized box,
with the Trimble-branded OCXO. All of those I've tried seem to settle with DAC
voltages near zero. If you try this with another style of Thunderbolt, you're
on your own.
Cheers!
--Stu
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