Hi John,
After much experience swapping ROM's and EPROM's in commercial
equipment, I've found that you can never be sure of using a new part.
The address and control timing on many designs is not all it should be,
often the system is only working due to marginal (or even totally
incorrect) timing. Changes in propagation delays can wreak havoc. In
particular I've seen problems were the supposedly identical new device
(same part number, same manufacturer) has been migrated to a smaller
die. This reduces propagation delays (the specs give a maximum delay not
a minimum!) causing system failure. At least with UV-EPROM's you can see
the difference in die size, sometimes it's huge.
I guess that not many equipment manufacturers measure their timing and
ensure adequate margins, unless it doesn't work first time. Shrinking
die size's can also be an issue with analogue devices with reduced
output drive capacity and poorer stability / noise rejection being most
common.

Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of John Miles
Sent: 19 October 2006 17:54
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] HP8568B firware eproms anyone ?

Heh, that's great.  Interesting that at least some CMOS parts will work.
I
may have had some bad chips, but the same ones that failed in an 8566B
worked fine in a 494AP.

Not knowing what happens if an address line toggles in the middle of a
read
cycle, I wonder if it'd be a good idea to tie those floating address
lines
to ground or Vcc...

-- john, KE5FX

luck

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Dave Brown wrote:

Why anyone would bother to create something like this escapes me.
Just another example of unintelligent engineering.
If one is foolish enough to insist on using a non timing GPS receiver
then one cant really expect too much in the way of stability.
A good GPS timing receiver isn't that much more expensive and the
improved performance more than justifies the extra cost.

Since it is not clear how the frequency was measured any conclusions
must be somewhat tentative.

1. The frequency stability over a few hours appears worse than that of
   the crystal alone.
2. The short term DAC stability needs to be very high -low pass
   filtering the PWM output of the PIC is inadequate the switching levels
   of the PWM output need to be tightly regulated.
3. The DAC resolution needs to be very high (> 20 bits) for the
   frequency stability to approach the limits set by a good crystal and GPS
   timing receiver.
4. Using an RS485 transceiver with its built in input attenuator will
   degrade the SNR of the 10MHz  clock.
5. Using the same buffer IC to buffer different frequencies will produce
   undesired phase modulation of its outputs.
6. Using a ripple counter without resynchronising its output signals
   will phase modulate the divided down outputs.
7. The apparent absence of a ground plane should allow the circuit to
   effectively radiate RF noise and harmonics.
8. The bypass capacitors on the supplies will be relatively ineffective.

There are GPS disciplined crystal oscillators available which have a 1s
Allan deviation of 2E-11(8E-11 @1000s, 1E-12 @ 1day), the performance of
this circuit falls woefully short of this.

No allowance appears to have been made for weeding out spurious
measurements. What happens if a PPS pulse is missing or has an
abnormally large timing error?

Degrading the resolution by ignoring sawtooth timing corrections and
using a 100ns resolution timer to measure the PPS pulse position
relative to an internal (10/2^16) MHz clock just throws away the
inherent timing precision (~10ns or better) of a good timing GPS receiver.
GPS timing receivers that internally correct for the sawtooth error are
available.

There are no Allan deviation plots for either the GPS derived PPS signal
or for the OCXO.
These are necessary for intelligent design.

Bruce

Tks Bruce

I did wonder why on earth he didn't use a timing rx-and that raised
questions re the remainder of the design, but I've not had a chance to
read it through completely as yet. My initial impression was that too
many corners were being cut. Said's comment re the 7805 tempco was
another one I hadn't thought about.  Should be able to better that
without too much effort.

DaveB

----- Original Message -----
From: "Dr Bruce Griffiths" bruce.griffiths@xtra.co.nz
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, October 21, 2006 5:00 PM
Subject: Re: [time-nuts] The VE2ZAZ GPSDO

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