Repairing Rohde & Schwarz SMT03 RF Signal Generator – part one

The R&S SMT03

The R&S SMT03 is a RF signal generator which can generate signals from 5Khz up to 3Ghz, while the signal level can be from -144dBm to +13dBm with 0.1dBm resolution.

A datasheet of the R&S SMT03 can be found [here]

Signal generators like the SMT03, and other (for example the HP ESG series like the E4421B) are mainly used to test communication devices and a like. Signal generators like this, are not arbitrary signal generators (however they may have an option to generate LF (Low Frequency) signals like square, sine, sawtooth wave-forms.

RF Signal generators generate sinus signal, and can generate different modulations (FM,PM,AM etc). These signal generators are a specialized piece of equipment.

An R&S SMT03 with problems

The SMT03 I’ve got in the LAB has some problems:

    • A few lines are visible on the LCD screen
    • When the unit is on for some tine, above 1.8Ghz the error: 110 Output Unleveled ALC failure is displayed .

The lines on the LCD screen are somewhat annoying, they don’t influence the working of the unit. The ALC Unleveled error does. When I try to measure the signal, the level is all over the place, and has a low level.  I noticed that in the short time this error is not present I can loose up to 4dBm when I generate signal above 1.8Ghz.

The challenges

When trying to fix an error like this, it’s important to understand the error. At the time this error presented itself, I didn’t had access to a Service Manual. And from the User manual I didn’t got any wiser. So after some thinking I concluded that ALC must stand for: Attenuation Level Control

This leads me to the conclusion that a possible cause could be a PLL circuit which is unable to establish a successful lock.  The bad news is: this may be due to a lot of other problems:

    • The input frequency may be of, or the signal is to low
    • Power problem (low / not working power rail
    • Mixer problem
    • And a lot of other related problems

Trying to diagnose the problem futher

So to diagnose such a problem, a block diagram and a schematic would be very helpful. But I got neither of them.

Another challenge is that the unit consists of modules, which plug into a back-plane. Operating the modules outside of the  unit seems only possible with a “service kit” which I don’t have access to, and cannot find any information about.

Despite the fact I don’t have the above items, I can observe the error, and take a look at how the physical signal path. So I opened the unit by removing the covers. Which is a simple and straightforward process: By unscrewing the bumpers on the back of the unit the top and bottom covers come off.

R&S SMT03 on the inside

Tracing from the output connector the signal path back I noticed that:

  • From the output connector, the signal is going through an attenuator, and then straight into a 3Ghz module (W154).
  • From the 3Ghz module the signal is entering an 1.5Ghz module (W104).
  • From this module the signal is going to a synth module, and a signal generator module.

And this tells me that the 3Ghz module has some kind of “pass-through”, so that signals up to 1.5Ghz are passed through the 3Ghz module (not sure at this point if some additional filtering is done). When signals are above 1.5Ghz the 3Ghz module generates the frequencies from 1.5 till 3Ghz.

This could mean that the ALC Unleveld error is somewhere in the 3Ghz module. Which means I could leave all the other modules alone.

For a moment I was thinking about a possible power rail problem, however this is most unlikely, since I aspect that for every frequency the ALC Unleveled error should be displayed, or that other errors would pop-up.

To summarize

    • Working on these units is not easy
    • A Service manual would be a great help

In [part 2] I’m going to try to repair this R&S SMT-03