Inhaltsverzeichnis
Rohde & Schwarz SMAF (BN 41409)
Signal generator for AM and FM, type SMAF, BN 41409, manufactured by Rohde & Schwarz, Munich.
The receiver measuring transmitter SMAF BN 41409 is the latest development stage of the legendary R&S signal generator SMAF. The set provides AM, FM and video signals in the 4 - 300 MHz range; the signal can be modulated internally at 1000 Hz, from an external signal source or mixed.
Technical data
- Frequency range: SW, VHF (4-6.5 / 6.1-10.3 / 10-17.5 / 17-31 / 30-55 / 54-100 / 95-175 / 170-300 MHz)
- Frequency display: Analogue dial
- Frequency memory: none
- Output power: 0.05 μV - 50 mV
Power supply
- Mains operation: 115, 125, 220, 235 V
Dimensions
- 525 x 320 x 380 mm, weight 30 kg
Accessories
Operation
In a first step, the correct mains voltage must be applied; the sets used in German-speaking countries are usually set to 220 V alternating current. The set can be set to an alternative mains voltage using the voltage selector on the mains transformer, which can be accessed by removing the perforated cover at the rear of the cabinet.
The main switch (9) is used to select the corresponding operation mode, in most cases this will be AM or FM. The SMAF can also generate a video signal, but the picture blanking synchronisation signal must be fed to the set via the television modulation socket.
Frequency modulation is possible with a 1000 Hz tone (position 5 „FM 1000 Hz“) or external modulation (position 6 „FM external“), amplitude modulation is also possible with a 1000 Hz tone (position 3 „AM 1000 Hz“) or with external modulation (position 2 „AM external“). Various combinations of AM and FM signals are also possible.
In the frequency modulation operation mode („FM 1000 Hz“), the frequency deviation can be set with the control (12) and read on the left-hand instrument (13); the „Display times“ switch switches between a display range of 0 - 8 % (left-hand position x 0.1) and 0 - 80 % (right-hand position x 1); in broadcasting, a frequency deviation of 75 kHz is usually used.
In a similar way, in AM operation mode („AM 1000 Hz“), the modulation depth is set with the control (12) and read on the left-hand instrument (13), the „Display times“ switch now switches between a display range of 0 - 10 kHz (left-hand position 0.1) and 0 - 100 kHz (right-hand position 1); in broadcasting, a modulation level of 50 % is used as standard.
The operating frequency is selected with the range switch (3), the BN 41409 covers the ranges 4-6.5 / 6.1-10.3 / 10-17.5 / 17-31 / 30-55 / 54-100 / 95-175 / 170-300 MHz, i.e. the shortwave, FM broadcast band and the VHF range. The large centre frequency dial (1) on the frequency scale is used for coarse adjustment of the operating frequency, while a fine tuning knob (2) on the right with a log scale and adjustable zero is used for fine adjustment.
The output level can be varied; the level is set with the RF control (4) so that the instrument pointer is at 1. The correct output voltage levels are then read at the voltage dividers A and B. The control A (6) can be used to regulate the output voltage between 0.5 and 5 by, and the level can be switched using the step switch of voltage divider B (7). Depending on the range, the output voltage in the 0.1 range can then be selected between 0.05 and 0.5 μV, in the 1 range between 0.5 and 5 uV and so on up to the 10^4 range between 5 and 50 mV.
The output signal is feed to a typical Rohde & Schwarz Dezifix connector with an impedance of 60 Ω. These special connectors are no longer easily available.
Inspection
After a long period of storage, my set is carefully powered up using a regulating transformer. With the main switch (9) in the „AM 1000 Hz“ position, the set needs a few minutes to warm up, for good frequency stability Rohde & Schwarz recommends a warm-up time of 30 minutes.
Fortunately, i found a Dezifix to BNC adapter to connect the Dezifix B RF output to the oscilloscope.
To make a test with an output power of 10 mV, the output regulator A (6) is set to 1.0, the output voltage switch B remains in the 10^4 position fully clockwise, here the maximum level is selected.
The main tuning is set to 100 MHz in the 78 - 135 MHz range.
The oscillator valve voltage is increased with the RF control (4) until the pointer is at the 1 mark of the instrument. Now set the X deflection on the oscilloscope to approx. 5 mV/cm, the trigger responds and a stable sine wave appears on the screen when the time deflection is set to approx. 10 nsec/cm (and my oscilloscope goes up to 100 MHz).
The output voltage can now be varied using the output voltage regulators A and B and the change in level can be observed on the oscilloscope. If the frequency is increased, the sine wave becomes more narrow, with lower frequencies it becomes wider. Thanks to the measuring function, the duration of a sine wave can be measured to 9.99 nsec when tuned to 100 kHz. The SMAF, which is now sixty years old, runs stably and with astonishing precision with virtually no contact problems. The modulation is clearly visible with amplitude modulation at 1000 Hz and 50 % modulation depth.
After switching to 5 MHz, a period duration of 200 nsec is measured. At this frequency, the modulation with amplitude modulation 1000 Hz modulation level 50 % and with frequency modulation frequency deviation 50 kHz can be displayed even better.
Technical principle
RF oscillator (Tube 1), the frequency modulation is generated via a crystal diode, the FM modulation level is displayed on a AF tube voltmeter (Tube 5) with two ranges. The internal modulation of 1000 Hz is generated in an AF oscillator (Tube 6).
A diode voltmeter is used to display the RF oscillator valve voltage using the control (49).
Two tubes 2/3 form a broadband amplifier for the RF signal, the capacitive voltage divider A and the capacitive and resistive voltage divider B allow the calibrated setting of the RF output level.
The power supply is electronically stabilised.
Components
- Tube 1 E88C, oscillator
- Tube 2 E810F, broadband amplifier
- Tube 3 E288CC, broadband amplifier
- Tube 4 E88CC, cathode amplifier FM modulation
- Tube 5 E88CC, display amplifier I2, modulation level
- Tube 6 EF80, AF oscillator 1000 Hz
- Tube 7 E810F, amplifier oscillator voltage
- Tube 8 PL81, current control tube
- Tube 9 EF80, control tube of tube 8
- Tube 10 85A2, stabiliser tube for grid bias voltage Tube 9
The emission of the final tubes 2 / 3 can be checked by pressing the „final tube check“ button, the right-hand instrument must be in the red sector. According to the manual, it must be checked whether the red test sector is reached after every tube change, as the accuracy may not be satisfactory after a tube change and the set may have to be recalibrated.
