S5i - RF Generator


The S5i module is a RF source ranging from 40 MHz to 4 GHz, initially designed for reflectometry. The output power of the fundamental frequency ranges from -14 dBm to 20 dBm and can be controlled by both the front panel and via software. The RF output can be muted by an external source, which gives an on/off ratio of larger than 60 dB.

As this module was initially designed for use in reflectometry, it also has a secondary output which can be used as the LO of a mixer. This output is always set to the same frequency as the main RF output and is unaffected by the mute input. Its output power is fixed at 0 dBm. Optionally two more outputs can be made available by hardware modifications. These outputs will also have the same frequency as the RF output.

The module has the possibility to run on either an internal or external reference. If multiple S5i modules are placed in one SPI Rack, it is recommended to set them to the external reference. This reference is shared by all modules via the SPI Rack backplane. The connector for the external reference is found at the back of the SPI Rack.

Below is a block diagram of the inner workings of the module. It is based around a frequency synthesizer chip, of which the frequency can be set via the controller module. A variable attenuator (controlled by a DAC) is placed in the RF path to allow for a 30+ dB output power tuning range. The mute switch just before the output switches the signal between the output SMA and a 50 Ohm internal resistor. If the switch is set to ‘mute’, the output will be switched to a 50 Ohm resistor to keep an impedance match at all times.

S5i block diagram


As can be seen in the block diagram, the frequency synthesizer outputs square waves. To achieve a CW/sinusoidal output, an external low pass or band pass filter needs to be added by the user. As the output is a square wave, the first harmonic to be filtered it the third one. A smart filter choice still leaves a tuning range of a factor two. Therefore this module should not be used for broadband frequency sweeping, unless a square wave is not an issue. The figure below shows a measurement of a 300 MHz RF output signal without filter (yellow) and the LO output with 400 MHz filter (pink).

Output: the modules outputs a square wave

S5i output


To use this module, it is important to know how the frequency generation works as this imposes certain restrictions on the module settings. The chip at the heart of the module is an integer Phase-Locked Loop (PLL) frequency synthesizer. A PLL is a control system with feedback: it compares the phases of two signals and produces an error signal proportional to the difference in phases. This is the task of the Phase Detector. The error signal is then low pass filtered and is used to control a Voltage Controlled Oscillator (VCO). The output of the VCO is the output of the chip. It is also fed back to the phase comparator through a divider. If the output of the VCO drifts, it gets corrected by the output of the phase detector: the output is locked to the frequency of the other input. This input is called the reference. In this module it can be either the internal 10 MHz or an external reference frequency. This reference frequency can be further divided to increase the resolution of the output frequency. In the software this is called the stepsize, it determines the steps the output frequency can take: $\text{stepsize} = \frac{\text{reference}}{m} $.

PLL Synthesizer Diagram

Example Assuming the reference frequency to be 10 MHz, we set the step size to 1 MHz. This requires $m=10$. The output can now be configured in multiples of 1 MHz: the phase comparator will only output zero error signal of the two input signals are equal. If we set $n=1$, the VCO will be set to 1 MHz. For $n=2$, the VCO will be set to 2 MHz. Note that you can have only whole multiples of the reference frequency.

Stepsize: A higher resolution (smaller step size) does come at a price: increased phase noise

The figure below shows the single side band phase noise of a 300 MHz RF output signal for different step size settings. As the step size decreases, the phase noise goes up. This is because the phase noise is multiplied up from the reference frequency: the smaller the reference frequency, the larger the multiplication. Therefore the highest reference frequency should always be used (largest possible step size).


Parameter Value Units Conditions
Frequency Range 0.04 to 4 GHz
Frequency Resolution 10 kHz 40 MHz ≤ f ≤ 655 MHz
100 kHz 40 MHz ≤ f ≤ 4.4 GHz
Frequency Accuracy < 1 Hz
Setting time 4 ms
Internal reference 10 MHz
External reference 1 to 200 MHz MHz
Level Range -14 to 20 dBm Power of fundamental frequency
Resolution of setting 1 mdB
Level Error ± 0.5 dB 40 MHz ≤ f ≤ 2 GHz
± 2 dBm 2 GHz < f ≤ 4 GHz
Setting time 300 μs
On/off ratio >60 dB
Rise/Fall time 12 ns
On/off delay 5 ns Delay to mute on/off
Spectral Purity
SSB phase noise 10 kHz from carrier -108 dBc f = 100 MHz
-103 dBc f = 500 MHz
-98 dBc f = 1 GHz
-92 dBc f = 4 GHz