[time-nuts] How does sawtooth compensation work?

Bill Byrom time at radio.sent.com
Sun Aug 7 22:36:21 EDT 2016

A slight correction to a typo in the description below (sorry for the
long delay). The correct Tektronix model numbers of these counters start
with DC (not TM).

The Tektronix TM500 (manual control) and TM5000 (GPIB or manual control)
instruments which used the National Semiconductor MM5837 noise generator
chip were the following:
These models were manufactured from the early 1980's until 1995.

The first two digits of the instrument model number designated the type
of instrument. So:
DC = Digital Counter
DM = Digital Multimeter
SG = Signal Generator
PG = Pulse Generator
AA = Audio Analyzer
PS = Power Supply
TM = Test Mainframe (which includes the power supply for the plug-
in modules)

Typically the 10 MHz internal standard or proportional oven timebase
(or external 1/5/10 MHz rear edge connector timebase input) is dived
down with 7490 TTL /5 and /2 divider sections to 1 MHz. The 1 MHz
reference then drives a X100 PLL (using an ECL oscillator with varicap
frequency control and 4044 phase detector) to create the 100 MHz main
internal clock. The MM5837 pseudo-random noise generator phase
modulates the PLL in some instrument modes of operation so that time
interval averaging works correctly if the input signal is synchronously
related to the clock.

Bill Byrom N5BB
Tektronix Application Engineer

On Tue, Jul 19, 2016, at 01:52 AM, Mark Sims wrote:
> The Tektronix TM509/5009 (and I think the 5010) counter modules have a
> National Semiconductor noise generator chip in them.  It injects noise
> into the counter to get around counter oscillator/input frequency
> synchronization.  I was once given a TM509 with a bad noise generator
> chip...   Some Very Smart People spent ages thinking the
> problem was in
> the device they were working on until they tried a different counter.
> The Very Smart People (too smart to RTFM)  could never figure
> out why the
> counter was flakey and finally tossed it.  A little dumpster
> diving a few
> minutes with a scope yielded a very nice little counter.
> ----------------------------
>> Universal timer/counters and equivalent time sampling DSOs can have
> this problem when their timebase ends up synchronized with the signal
> they are measuring.
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