Schlumberger 4002 signal generator

While working up some extra circuits for the spectrum analyzer, I managed to pick up an old signal generator from eBay.

I heard a lot of positive things about the German (actually French origin, please look at comments below. Thanks to Rohit for pointing this out!) brand “Schlumberger” before, even though there is no relation to any personal experiences with their equipment. Seems like they also ran some kind of subcompany outfit called “Solartron” or “Enertec” which would today sound more than fishy, what with all those copycat-brands out there. But when an auction came up for a reasonable price I decided to go for it after some short research on the net.

Fig. 1: Schlumberger 4002 signal generator, already opened up.

Fig. 1: Schlumberger 4002 signal generator.
Don’t mind the tearing effect on the LED displays, not visible to the naked eye. I already pulled all the side panels.

What I got was a Schlumberger 4002 signal generator. It ranges from 0.1 to 2160 MHz with 10-20 Hz tuning accuracy, selectable output amplitude from -138.9 dBm up to +13 dBm in 0.1 dB steps, auto-sweeping and several extras like an OCXO for stability, 20 dB of linear attenuation range without using the step attenuator, an internal modulator and IEC bus remote control. If you looked at the photo closely, you will have noticed that the frequency range is written as “0.1…1000/2160 MHz” on the front panel. The reason for this is the optional doubler module included in this instrument. If the module is installed and detected, the software switches over to extended range without any further changes. Else, 1000 MHz is as far as it goes. More detailed specs will follow as soon as I can decypher the bad scan of a manual page that cropped up on Google. Judging from the inventory labels on the backside, the device must have been used in the manufacturer’s own lab. Unfortunately I have not yet managed to find any service info even though the manuals seem to be sold sometimes, for rather terrible prices. [Read More]

Wacom digitizer screen

Anyone who has already tried to use some kind of tablet device for writing should know that there are fundamental differences between screen types.

The most common is the capacitive type, where you use a finger or some kind of conductive pen to write on a glass surface, while the touchscreen device captures movement of the capacitance change through a grid of transparent electrodes on the backside of the glass. This works, but it sucks for writing precise text or drawing sketches. You can find these screens in almost every modern smart phone, tablet PC or kitchen appliance. They are cheap!

Next is the resistive touchscreen, where a small, hard point presses down on a plastic surface. The touch element is composed of two pieces of clear foil, coated with a conductive material. While the two layers stay isolated when there is no pressure applied, the pen forces them together in a certain point, forming a conductive path. By knowing the specific resistance of the surface coating, the circuit can determine the position of the pen tip by measuring path resistance from different edges of the screen. This type was pretty popular in PDAs (which have by now been fully replaced by smart phones, what a shame 😉 ) as well as the almost equivalent navigation assistants – and is not that common anymore. Writing performance is fair but not exceptional, though.

The third kind is the most interesting one. Real tablet PCs (the ones with the flip-over display) have this normal-looking pen with the nylon tip, which you can use to accurately write on the glass/plastic display surface. Many even feature some buttons on the pen, some kind of eraser on the backside – and they are damned accurate! They have another thing in common: Most of them use technology by a company called Wacom, also producer of digital writing and drawing pads for artists.

This type is called a “digitizer screen”, and it uses a sensing panel *behind* the actual display to recognize and track the pen. The digitizer panel contains an amazing set of surface coils to provide an alternating magnetic field through the screen. Inside the pen, there is a resonant circuit which uses the field energy to transmit the button states and even pressure on the pen tip back to the coil. By monitoring the strength of the resonance through different surface coils, the digitizer then calculates the position of the pen above the surface. In other words, you get a high-res info about the pen position (easily above 25.000 points resolution along the surface edge, depending on the digitizer type!), you know the pressure applied, button presses on the pen and even where the pen is when it is not yet touching the surface.

I recently disassembled a trashed tablet PC (Toshiba Portege) with a broken motherboard for interesting parts, and came across this:


Fig. 1: Backside of Toshiba Portege LCD module. Wacom digitizer (red/orange foil) and control logic (PCB in top left corner) are visible. LCD control board on the bottom.

The LCD panel is a LTM12C328T type. Attached to the backside is a SU-010-X01 tablet pen digitizer, and the marking on the ICs clearly suggests that it is made by Wacom. This would make a fine graphics tablet – but how to attach it to any other PC? [Read More]