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.
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.
Upon arrival, the first thing I did was open it up to check for transport damage or any other things demanding immediate attention. Finding none, I was absolutely stunned at the old-fashioned, but VERY clean way this device has been built, similar to HP/Agilent products. Since there are few useful pictures on the net, I will start to provide some along with any technical info I manage to develop for this fine piece of tech. Here are some quick shots done while inspecting, more will follow later.
After the checkup, I tried turning it on. It booted fine right away, with the sweet clicking sound of relays inside. I then hooked up the output to my HP analyzer and ran some tests. First up was the selftest, executed by pressing the conveniently labelled key at the bottom of the front panel. Pressing it again after reading the error message brings up the next, and the next, and so on until the IEC bus address is displayed last. The errors coming up in my case are:
- AGC DOUBLER (intermittent, might have been a bad connection)
- AGC OUTPUT STAGE
So there is something wrong that even the device itself can detect. It would be nice to find out how it determines the faults to get more detailed hints. Several signal lines labelled “SELFTEST” can be found on the doubler and filter/AGC modules, so these probably lead to the respective points of interest.
Moving on, next up is selecting arbitrary frequencies and checking the freq/amplitude values on the analyzer. Turns out that the generator seems to contain multiple circuits for different ranges. The main divisions I could make out so far are 0.1 to 479 MHz, 480 to 1079 MHz and 1080 to 2159.9999 MHz. At these points, relays click inside and the output behavior changes dramatically. In the first region, I get LOTS of spurs and harmonics, and the fundamental is always off by a certain amount. Funny enough, that offset is almost “binary”, since it results in 127 MHz instead of 100, 255 MHz instead of 200, and so on. In the second range, the frequency is almost right and the spurs vanish. Instead, the level drops and moves about. This might fit the AGC (auto gain control) error. The third range sometimes delivers something, but never quite right. This might be a problem with the doubler block which sits next to the attenuator. Have a look at the next pictures. The last one shows a complete recorded sweep over the whole band.
For measurements, I have already disconnected the doubler block and the attenuator from the generator and checked the signal from the generator directly. The harmonics can be seen there, too. A first visual inspection of the blocks did not yet bring any revelations, except that someone seems to have done some soldering work in what I believe is the Filter/AGC block.
Well, seems like I have some work to do. Next up is the removal and inspection of all the RF blocks to get a better feel for how this works, and developing a crude block schematic from what I find. I will probably dedicate a separate post for each module as I work my way up the line.