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.

this problem when their timebase ends up synchronized with the signal
they are measuring.

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:
DC509
DC510
DC5009
DC5010
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 DC510 and DC5010 phase lock a 320 MHz varicap oscillator to the 1
MHz reference giving them a 3.125ns single shot timing resolution.

The DC509, DC5009, DC510, and the DC 5010 are reciprocal counters.

The universal timer/counters in the 2236, 2236A, 2247A, and 2252
oscilloscopes phase lock 100 MHz varicap oscillators to their
references and are reciprocal counters which is important since they
can gate their timer/counters which is incredibly handy on a dual
timebase oscilloscope.  The 2247A and 2252 use noise modulation but I
do not think the 2236 and 2236A do.

On Sun, 07 Aug 2016 21:36:21 -0500, you wrote: