Tektronix DC 509: Simple Mod for Adding an External Reference Clock

 

I recently needed a portable counter for some measurement work.  Looking at my shelf of test gear, I found a Tektronix DC 509 Counter Module that I could plug into one of my TM 503 Mainframe for transportation.

However, when I went to test it, I discovered, when measuring a 10 MHz signal from my HP 8640B (that is locked to my GPS frequency standard), that the counter was off by about 20 Hz, and that this error would change as the counter warmed up.

Very annoying.  If only I could synchronize the counter to my GPS frequency standard...

TM 500 Series Mainframe Requirements

Per the DC 509 manual, the counter does support an External Reference Clock via its rear connector pins, and this clock can be either 10 MHz, 5 MHz, or 1 MHz.

To provide, for example, an External Reference Clock to the counter, the TM500 series of Mainframes has an option (Option 2) to add BNC connector(s) to their rear panels, as shown below (image from eBay):

I have five TM 503 mainframes, but none of them have these BNCs.  Instead, their rear panels all look like this:

Rather than purchase a TM 503 from eBay with the appropriate BNCs, could I instead somehow make a simple modification to let me add a connector to the DC 509 for an External Reference Clock?

DC 509 External Reference Clock Circuitry:

The answer to the previous question is "yes," and it's actually a pretty simple modification.

First, let's look at the DC 509's clock circuitry:

I've annotated the schematic to show how to set the on-board jumpers to (1) select External or Internal Reference Clock, and to (2) select the frequency of the Reference Clock.  Note that the external clock's amplitude should be roughly about 4 VPP (into an open load), at a minimum, from my own measurements.

Adding an External Clock Connector to the DC 509:

The External Clock comes into the DC 509 via the following pins on the Rear Connector P1625:

     P1625 pin 14A:  External Clock Input

     P1625 pin 15A:  Ground

There are two side-by-side vias for these signals, next to the connector, as shown in the image, below:

These two vias are slightly more than 0.1 inches apart.  Never the less, a 2-pin header with 0.1 inch spacing can be inserted into them (after removing the solder in the vias) and soldered without too much difficulty:

A matching two-socket ribbon cable can now be attached to this header for the External Reference Clock connection to the DC 509.  In my case, I made a cable that adapts a short 2-pin socket cable to an SMA connector cable (all parts from my junk box).  See below:

Note that, assuming a 10 MHz clock signal, the length of the 2-pin ribbon cable is not too unimportant, given that a 10 MHz signal's wavelength is 30 meters long (in the ribbon cable this wavelength will be shortened somewhat from 30 meters due to the the cable's velocity factor).

The DC 509 can now be inserted into a TM 500 series mainframe and the External Reference Clock cable brought out the front of the mainframe (as long as the mainframe has at least one slot that can be left empty, to route the clock cable out the front of the mainframe), 

Setting the Jumpers:

On the DC 509 PCB, the 3-pin Header, J1621, selects whether the Reference Clock source will be Internal or External:

    Jumper J1621 pin 1 to pin 2 for Internal Clock (this is the default factory setting).

    Jumper J1621 pin 2 to pin 3 for External Clock

The 5-pin Header, J1511, should be set to match the Reference Clock frequency:

    Jumper J1511 pin 1 to pin 2 if the Reference Clock is 10 MHz (this is the default factory setting).

    Jumper J1511 pin 2 to pin 3 OR pin 3 to pin 4 if the Reference Clock is 5 MHz.

    Jumper J1511 pin 4 to pin 5 if the Reference Clock is 1 MHz.

The image below shows the 3-pin Header, J1621, set for External Clock:

Operation:

As a quick test I hooked up my "External Clock Header-to-SMA" cable to the DC 509, and then attached this cable to an HP 3335A signal generator set to 10.000000 MHz (itself locked to my GPS frequency standard).  The HP 3335A's amplitude was set to +10 dBm (i.e. 2 Vpp into a 50 ohm load, or 4 Vpp into an open load).

I connected the DC 509 to the output of my HP 8640B generator (the latter with its frequency display expanded to show resolution of  1 Hz), and compared the frequency displayed on the generator with the frequency displayed on the counter.  They matched (as shown, below)!

Standard Caveat:

As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations.  If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.

And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.