Rohde & Schwarz SMS Teardown


This is a quick tear-down of a Rhode & Schwarz SMS frequency generator, with the 1040MHz option installed.
I had to replace all the electrolytic capacitors, as they are failing all over the unit. The PSU ones blew up and filled the unit with electrolyte vapors.

The photos are quite poor and the colors are a little off – i have some warm LED lights that look good but mess up photo cameras.
I had to take the photos at F2 -> the DOF is low and there’s a lot of fringing. I’ll try to improve them as much as i can ( sharpen, etc )


The first photo has the instrument’s front panel.
The layout is quite good, all buttons well spaced out and with a nice feel.
The 7 segments display is bright and big enough to read from across the room.
One of the front pane screws is stuck – I could not remove it .

Removing the top and bottom covers reveals some shields and the power supply on the back. Between the shielded section and the power supply there is a mechanical attenuator and the frequency doubler that pushes the range to 1GHz.
All RF connections inside are made with hard coax.

The back of the unit comes apart as one piece and contains the heavily shielded toroidal transformer, the power supply regulator board, power switch, brush-less cooling fan + driver module ( it’s potted so no tear-down of it), all back-side connectors and all power related stuff – regulators, power transistors and radiators.
The control board connect to the power transistors via some strange socket pins that are a pain to re-align.
I had to remove all power devices and reinsert them one by one to get them back in place after removing the control board.
Everything is bolted down with a ton of screws and washers – takes 15 minutes to locate and remove all screws for this section.
The black caps are new – replaced after the first power up. The brown ones are the ones that got replaced this time – they look potted, some of them where discolored or cracked
A third of the unit’s weight is in this section, due to the power transformer and heat-sinks.

There are 2 modules that are unshielded: the cpu board and the modulation control.
The cpu board contains a Intel 8049 masked rom controller, Intel 8355 io expander, an unpopulated slot and a HEF4738 gpib controller
Some of the connectors are covered in a white residue – maybe something leaking from the plastic.
The modulation control board is mostly analog, with a I8343 expander controlling a lot of analog switches that change the modulation depth, rate, etc

Removing the top-side cover of the shielded sections reveals the “motherboard” – nothing interesting
All the good stuff is on the other side
Both RF section covers have a lot of screws, the metal pieces are somehow twisted – you need a lot of force to align them and under each aluminium shield there is a thin hard metal foil ( maybe for magnetic shielding ? )

The first 3 RF modules are:

100Hz oscillator
– a PLL loop locked to the 10MHz main oscillator with a 25-34MHz VCO
– 100-135KHz output, divided from the VCO
– the cpu controls the PLL via a I8243.
– the phase comparator is made from a 4046 flip-flop
– all off-the shelf parts

50KHz oscillator
– a PLL loop with a 19.9 – 21.9 MHz VCO, locked to 10MHz reference
– a PLL loop with a 20-22MHz VCO locked to the 100Hz oscillator, that controls it’s VCO mixed with the first VCO
– a 2-2.2MHz output divided from the 20-22MHz VCO

reference oscillator
– an 10MHz oven controlled crystal oscillator
– a 80MHz oscillator controlled with a hard-wired PLL
– the 80MHz oscillator is also controlled by the FM modulation signal.
– the board outputs 40 or 80MHz from the VCO, 10MHz from the xtal oscillator and 1MHz divided from the 10MHz source.
– the 80/40MHz output is sine-wave, filtered after some freq. dividers.

following those modules:

the “converter” board
– 380MHz VCO locked to the 1MHz source, hard-wired PLL
– a mixer with the 40/80Mhz signal from the reference board
– band-pass filters and diode switches for 300,340,420 and 460MHz mixer products

global PLL board that receives all reference signals and controls the main oscillator
it receives the 2-2.2Mhz reference oscillator, a 20-60MHz divided signal from the main oscillator and a lot of control signals from the CPU board. It outputs a 0-26V tuning voltage

main oscillator board
– 260-380MHz VCO
– 380-520MHz VCO
– diode switches controlled from the CPU board
– mixer with the 300-340 or 420-460MHz oscillator
– 260-520MHz and 20-60Mhz outputs

the last 2 board are

fine attenuator
– pin diode attenuator
– switched filters and signal paths for the different output ranges
– amplitude modulation

output amplifier
– main amplifier
– AGC block
– multiple signal paths, band-dependentfine_attenuator

The unit is generating it’s output range by mixing the 260-520MHz VCO’s with the fixed frequency ones.
The range from 520 to 1040MHz is covered from a frequency doubler module inserted after the output amplifier.

The fine step attenuator is paired with a mechanical one that’s using some custom made attenuator pads and custom relays.
This unit also got the output protection module, that forces the mechanical attenuator open when power is fed back into the output.

Most of the parts are off-the-shelf – normal op-amps, digital logic, etc. There are some hybrid amplifier modules that are no longer produced, but the documentation provides useful information to locate replacement ( gain, freq. range, etc)

The main PLL module got some ECL chips that act as a HF divider – also no longer in production, but they should be replaceable.

Most of the chips and transistors are mounted in sockets. Everything is heavily shielded with double-shields – thick aluminium outer shield and a thin / elastic metal interior shield. Most of the shields are clip-on, some of them covering the entire board. The converter board and output amplifier have an extra set of screws – over the filter section and output transistor heat-sinks.
Keeping this unit running for the next 100 years should be simple.

You can find high resolution schematics, scanned from the user manual at the following link:
manuals on google drive
You can also find these document on BAMA – last time i checked the .djvu files where in good order, the .pdf fiels have some problems.
manuals on BAMA

Comments are disabled