X-Band IF

• 1: Bridge12 X-IF System Overview
• 2: X-Band IF Backpanel Connections
• 3: Digital Demodulation
• 4: Circulator Attachment
• 5: Example Configurations
• 5.1: Example: Pulsed X-Band Operation
• 6: Manual Operation of the X-Band IF
• 6.1: Controlling the X-Band IF Synthesizer
• 6.2: DAQ Control

The Bridge12 X-Band IF system is at the heart of all Bridge12 EPR spectrometers. The system is highly modular. For X-Band EPR spectroscopy only a high-power microwave amplifier, digitizer and AWG is required.

The system can be completely controlled using SpecMan4EPR.

1 - Bridge12 X-IF System Overview

The Bridge12 X-IF system consists of four different sub-systems:

1. Synthesizer (SYN): - Two-channel microwave synthesizer.
2. Reference Clock (REF): - Internal 10 MHz reference clock.
3. Pulse Forming Unit (PFU): - IQ mixer based pulse forming unit.
4. Receiver Unit (RCVR): - IQ mixer based receiver unit with video amplifiers.

A schematic of the entire system is shown in the figure above. Most of the connections of the individual sub-systems are routed to the back panel of the system (shown as circles) to provide the user maximum flexibility. Only a few connections are made internally and cannot changed by the user (dashed lines). In the following section a brief description of each sub-systems is given.

Synthesizer (SYN)

The Bridge12 X-IF system has an integrated microwave synthesizer with two independent channels, LO1 and LO2. The signal of both channels is split into two signal paths. Both signals (LO1 and LO2) are available on the back panel. LO1 is also connected internally to the input of the Pulse Forming Unit (PFU). LO2 is also available on the side panel of the X-IF system, below the signal (SIG) input.

Both microwave synthesizers are locked to the internal 10 MHz reference clock. By default both channels are phase locked to each other. This can be changed through the software.

Reference (REF)

The Bridge12 X-IF system has an integrated oven-stabilized 10 MHz reference clock. Three 10 MHz outputs of the reference clock are available on the backpanel. This can be used to sync other devices such as an AWG or Digitizer. One channel of the clock is internally connected to the synthesizer for phase locking.

Pulse Forming Unit (PFU)

Pulses in the X-IF system are created by the Pulse Forming Unit (PFU). At the heart of the PFU is an I/Q mixer. Both channels, I and Q would normally be driven by an AWG. However, to create rectangular pulses at the LO1 frequency, these channels can also be driven by rectangular pulses created by a pulse programmer.

Internally, the LO of the IQ mixer is connected to LO1 of the synthesizer. However, a different LO signal can be supplied through the back panel connector AUX LO. If a rectangular pulse is applied to the I and Q channels, the pulse frequency will be identical to LO1. If a waveform is supplied (e.g. rectangular pulse at f(AWG) = 250 MHz) the output frequency at TX will be LO1 + f(AWG).

The output signal of the IQ mixer (RF) is amplified and sent to the TX back panel connector. The PFU has a blanking switch. The BLNK gate should be connected to a pulse programmer (or a marker/channel of the AWG if available). The gate is active HIGH. In between pulses, this gate should be LOW to minimize any LO bleed-through.

The microwave pulses generated by the PFU can be directly monitored on the TX MON backpanel connector.

An auxillary LO signal can be supplied to the IQ mixer by connecting the AUX LO input to for example the LO2or an external synthesizer. The rise/fall time for this switch is about 10 ns. This can be used for example to create a non-coherent microwave pump pulse in a DEER experiment. Alternatively, an additional signal can be supplied to the AUX IN connector.

PFU Control Signals

Receiver Unit (RCVR)

The Bridge12 X-IF is equipped with a IQ mixer based Receiver Unit. The signal from the probe is connected to the SIG side panel connector.

The input signal can be further amplified using an internal amplifier with about 20 dB gain. The receive signal can be monitored on the RECV MON back panel connector. This signal is sent to the RF port of the IQ mixer. The LO signal for the IQ mixer is supplied to the IF back panel connector. For X-Band operation, IF is typically connected to LO1, the same signal supplied to the PFU.

The down-converted signal is further amplified by a pair of video amplifiers. The gain of the video amplifiers is variable, and the video amplifiers can also be bypassed. The down-converted EPR signal is available at the I and Q back panel connectors. This signal can be either sent to a digitizer or lock-in amplifier for detection.

RCVR Control Signals

These signals

2 - X-Band IF Backpanel Connections

Below, please find a list and description of the backpanel connectors of the X-Band IF system. Please refer to the figure below for the location of connections.

RF Connections

Please refer to the table below for a description of the back panel connectors of the X-Band IF system. The function (I/O - input/output) of the connector is indicated in the second column.

Side Panel

Back Panel

Receiver (RCVR)

Pulse Forming Unit (PFU) and AWG Input

Synthesizer and Monitors

Reference Clock and External IO

Other Connections

Side Panel

Other Connectors

3 - Digital Demodulation

The Bridge12 X-Band IF system supports digital demodulation. Digital demodulation allows for easy removal of baseline artifacts and will result in a much cleaner signal detection. Instead of down-converting the EPR signal to DC level, the LO frequency is slightly offset and the signal is detected at a frequency of e.g. 200 MHz. The exact frequency depends on the sampling rate and input bandwidth of the digitizer (or oscilloscope).

The example below shows a digitally demodulated signal of a 2-pulse Hahn echo of a sample of BDPA in polystyrene, recorded at Q-band frequencies.

We recommend using an intermediate frequency of about half the bandwidth of the digitizer. For example, if the digitizer has an input bandwidth of 400 MHz, we recommend to choose an intermediate frequency of 200 MHz.

Digital Demodulation (Recommended Operation)

To acquire the EPR signal at a This offset can be achieved in two ways:

1. Offsetting the AWG Frequency

The simplest way to use digital demodulation is by offsetting the frequency for the AWG generated microwave pulses. Instead of generating a rectangular pulse at DC level, the user can create a pulse at a frequency of e.g. 200 MHz. To make sure the frequency of the microwave pulse is within the bandwidth of the resonator, the microwave frequency needs to be lowered by the same amount. For example, to generate a microwave pulse at 9.8 GHz with an offset of 200 MHz:

1. Set the LO1 frequency of the synthesizer to 9.6 GHz (9.8 GHz - 0.2 GHz)
2. Set the AWG frequency to 200 MHz
3. Set the demodulation frequency in SpecMan4EPR to -200 MHz.

2. Offsetting the Receiver Frequency

If pulses are not created by an AWG but for example by a pulse generator, digital demodulation can still be used. However, in this case, both microwave synthesizers, LO1 and LO2, have to be utilized.

To use digital demodulation with DC pulses follow these steps:

1. Connect the RCVR IF to the LO2 output.
2. Set the LO1 frequency on-resonant with the resonator frequency, e.g. 9.6 GHz.
3. Set the LO2 frequency to 9.4 GHz. That way the echo signal has to be demodulated using a frequency of 200 MHz (9.6 GHz - 9.4 GHz = 0.2 GHz).

Demodulating the Signal (Demodulating the Signal)

Once the signal is digitized it has to be digitally demodulated.

In SpecMan4EPR

Demodulating the EPR signal in SpecMan4EPR is very straight forward and simple. Simply make sure the Demodulation Frequency is set to the correct value and the spectrum is automatically demodulated when the data is acquired.

Manually

If the X-Band IF is used manually, demodulation must be performed by the user during post-processing of the EPR data. This can be conveniently done by using the demodulate function in DNPLab.

DC Detection

To down-convert the EPR signal to DC level, connect the LO1 output of the synthesizer to the IF input of the receiver and make sure the frequency offset for the AWG pulses is set to 0 MHz.

4 - Circulator Attachment

The Bridge12 X-Band IF does not have an integrated circulator/pre-amplifier, instead an attachment is connected to the side panel of the X-IF system. This attachment will look slightly different for different frequency bands (X-Band, Q-Band, etc.). For some other frequency bands such as S-Band, or W-Band, the circulator is integrated into the frequency extension.

X-Band Circulator

The X-Band circulator attachment is directly connected to the side panel of the X-IF system (see figure above) and is held by two SMA connections. A schematic of the circulator attachment is shown in the figure below.

The circulator attachment has three connectors:

1. Input: This connector needs to be connected either to the output of the high-power microwave amplifier for pulsed experiments, or to the TX or the LO1 (LO2) output of the X-IF (back panel connector).
2. Probe: The EPR probe is connected to this connector.
3. SIG: This is the signal output which is connected to the side panel of the X-IF system. The SMA connector below is only for support and is internally terminated with 50 Ω.

5 - Example Configurations

The Bridge12 X-Band IF system can be used to operate EPR spectrometers from S-Band (2 GHz) to the millimeter regime (> 400 GHz).

Below, please find example configuration for different operating frequencies.

5.1 - Example: Pulsed X-Band Operation

A typical configuration of the X-Band IF system for pulsed X-Band spectroscopy is shown in the figure below. Connections that are not required are greyed out.

6 - Manual Operation of the X-Band IF

To manually operate the X-Band IF system please install the following software:

• Microwave Synthesizer: Please download and install the Software Control GUI distributed by Windfreak Technologies.

• Digital/Analog Controls: Please download Kippling distributed by LabJack.

Once these two software packages are installed, the X-Band IF system can be completely remote controlled.

6.1 - Controlling the X-Band IF Synthesizer

Please make sure the control software for the synthesizer is installed. If you haven’t done so, you can download it here.

6.2 - DAQ Control

Please make sure the control software for the DAQ interface is installed. If you haven’t done so, you can download it here.

Connecting to the DAQ Interface

To connect to the DAQ interface follow these steps:

1. Make sure the X-Band IF USB interface is connected to the host computer.
2. Start Kipling
3. In the GUI click the green USB button (see figure above, left). The USB button will turn red, indicating that Kipling is connected to the DAQ interface (see figure above, right).

Digital Input/Output Control

To control the digital input/output lines click on the menu item Dashboard (see figure above). Each of the lines can be configured as an input or output by selecting the control from the drop-down menu. If the line is configured as an input the indicator to the left will show whether the line is logic high or low. If the line is configured as an output, the status can be changed by selecting the desired status from the drop-down menu.

Below is a list of the different digital channels used by the X-Band IF system.

Pulse Forming Unit (PFU)

Receiver (RCVR)

Analog Output Control

The value of the various analog outputs can be controlled from the Register Matrix panel. The values for TDAC0 - TDAC3 can be set between -10 and +10 V, with a resolution of 16 bit.

Below is a list of the different analog channels used by the X-Band IF system.

Receiver

Miscellaneous Other Controls

The X-Band IF system has several spare digital and analog input and output lines connected to the IO and EXT connector located on the back panel. These lines can be configured by the user.

EXT Connector (9 pin sub-D, back panel)

The external connector is most commonly used to control an extension to the X-IF system, such as a frequency extension.

All lines are configured as outputs in SpecMan.

IO Connector (9 pin sub-D, back panel)

VCA Connector (SMA, back panel)