The first thing I wanted to do before starting this project was to get a general idea of the system layout, hence the “part 0” thing above. Also, this will serve as the master post, linking to the other pages when the time comes.
Extracting a simple connection diagram always helps to simplify things later on when tracing and verifying signals, and also when a sudden space-time-vortex comes along and throws all those neatly organised cables into a messy heap. Might happen to be a cat, too. So let us first cycle through the different modules and their approximate functionalities. Click the thumbnails to get a better view and more detailed explanations in the picture subtext. Also, be advised that my interpretation of things is not necessarily consistent with the original service documents.
Oven-controlled crystal oscillator (OCXO)
Fig. 1a: OCXO top.
Contained in the bottom right module block, consists of a styrofoam-encapsulated 10.000 MHz temperature-controlled oscillator and some clock distribution buffering. It delivers the main TTL clock that is also available on the backside ports as an instrument reference. The picture shows the whole top side of the module block, but the actual OCXO and distributor PCB are on the right. The rest of the circuits belongs to the next module, the RG.
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.
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]
As I already mentioned, the highest priority fix is the input RF attenuator. To get access to this, the control panel must be taken out since the connector is placed at a slightly inconvenient place – the bottom of the motherboard. Unfortunately, right beneath this is the aluminum carrier plate that contains all the RF circuits consisting of semi-rigid coax and clunky metal-jacketed modules. I first unmounted the top, bottom and right side panels, which was an easy job:
- Loosen the single screws at the back of top and bottom covers and pull them off towards the back.
- Remove the two screws holding the carrying handle and pull off the side panel along with it.
- Also remove the top and bottom plastic inlays from the front aluminum frame, these cover up the panel screws.
- Remove all visible screws from the top of the frame that seem to belong to the right panel (should be 3) and also from the bottom (should be 2). Take care not to remove the 2 rightmost screws on the bottom, these hold the front connectors and are best left in.
Now pull the control panel right out.
Fig. 1: Control panel removed from case.
If it sticks, the points to watch out for are the N-type RF connector and the PCB edge of the sweep time selector. Gentle pulling while wiggling the panel up and down some will bring it out. Remove all connectors from the backside. Don’t worry, the plugs can’t be interchanged. Set the panel on a flat surface, front side down (Fig. 1). [Read More]
Originally I was looking around a well known auction house for a digital Spectrum/Network analyzer from one of the older Tektronix 2715 or HP8566 series to extend my measurement rack to higher frequencies, but they are hard to find for a reasonable price in relation to the risk of buying a device in unknown condition. Still, I wanted one of the older models, because of their excellent design and repairability. Maybe without the exception for some very special, custom parts – but we’ll just hope that those don’t break.
Also, in my understanding a slightly older system from the top series at the time still outperforms most more expensive, modern, all-digital-hey-we-compensate-all-the-errors devices. That is not to say that digital processing and compensation of systematic errors is bogus, of course! But at the same time, any weak measurement hardware can be made to appear top-class by taking several thousand complete sequences and averaging. Getting it right on a single try is an art for itself, and designing a combination of precise hardware and just the right amount of post-processing is the reason for the price. Or maybe I’m just a sucker for retro tech, with all its edges, heavy metal and shiny parts.
Fig. 1: HP 8565A running, showing a weak signal in the GSM area.
So, I finally got a fair deal on this 8565A unit. I might have wanted to choose its bigger brother 8569B instead, which has a wider external mixer span of up to 115 GHz and a digital control interface, but the LED readout certainly adds a special flavor to the set. The seller had informed me that the device would be uncalibrated and the sweep time selector didn’t work anymore. Usually such estimates contain some tolerance, so I already expected some other things that maybe nobody noticed. Since my original plan was to recalibrate whichever device I got anyway and fix all the problems over time, that would be okay. The fixes will be documented here. [Read More]