The following section describes how to tune the NMR probe to the correct spectrometer frequency. This requires disconnecting the NMR probe from the spectrometer.
Make sure all experiments have been finished and no pulses are transmitted anymore before disconnecting the NMR probe from the spectrometer.
Tuning the NMR Probe
To tune the NMR probe to the spectrometer frequency follow these steps:
Note down the spectrometer frequency. This does not have to be an exact value, but should be close (within 0.1 MHz of the operating frequency).
Switch on the nanoVNA and set the center frequency to the spectrometer frequency (set it to the frequency of the nucleus you want to observe, not the 2H lock frequency). Set the span to a value of 1-2 MHz.
For instructions on how to setup the center frequency, span, marker position, etc. see the page with the setup instructions for the nanoVNA.
If this is the first time using the nanoVNA you should calibrate the device. This calibration needs to be repeated every time you change the center frequency or span.
Disconnect the NMR probe from the Bridge12 SCN spectrometer and connect it to channel 0 (CH0) of the nanoVNA.
You should see a ‘Tuning Dip’ on the nanoVNA. If you do not see a tuning dip try one of the following things:
Increase the span of the VNA to scan a broader frequency range
Change the matching on your probe
Use the slider at the top of the nanoVNA to move the marker to the spectrometer frequency (within +/- 0.1MHz). The marker will tell you the absolute value of the S11 parameter at the spectrometer frequency (see image below, S11 = -7.8 dB).
Turn the tune and match capacitor of your probe to achieve a minimum reflection. In general, the tune capacitor will shift the dip up in down with frequency, while changing the match capacitor value will increase the depth of the dip. Optimize both capacitor values for a minimum in reflected power. The probe is tuned once you achieve the lowest value for S11 (see image below, S11 = -40.9 dB)
The Bridge12 SCN is delivered with the nanoVNA, a pocket-size vector network analyzer (VNA) to tune the probe to the transmitter frequency of the spectrometer.
The nanoVNA has two channel and has many different ways to display the (complex) S parameters. However, in the following we provide a brief set of instructions to setup the nanoVNA display to:
Only show the reflected power from the probe (S11)
How to calibrate the nanoVNA.
The nanoVNA will store these settings, so you only have to go once through these instructions.
To navigate through the different menus of the nanoVNA you can either use the wheel located at the top of the device or using the touch screen. The touch screen is best used with a stylus pen.
Setting up the Display
To setup the display:
Tap the touchscreen to show the menu
Tap the Display button (top right corner)
Tap the Trace button on the display
For convenience, select only Trace 0 by tapping on it. The selected trace is highlighted with a colored background (yellow for Trace 0). Unselect any other traces by tapping on the button.
To leave the menu tap somewhere on the touchscreen.
Setting the Frequency Range
Tap on the touchscreen to show the menu
Tap on the Stimulus button
Next, tap the Center button
On the next screen, enter the spectrometer frequency using the number pad and press the M button to indicate that the entered frequency is given in MHz.
Next, tap the Span button and enter the frequency range to observe the resonator tuning dip. Typically 1-2 MHz is a good range to start with.
Calibrating the nanoVNA
Important
The nanoVNA should be calibrated frequently. The calibration is only valid for a given set of parameters (e.g. span, center frequency, number of points ….). If one of these parameters is changed, the device needs to be recalibrated.
The device will still work even with a calibration that is “slightly off”. For example, the spectrometer frequency will not change drastically on a day to day basis and using the same configuration is ok. However, if you require accurate values for S11 we recommend calibrating the device before taking the measurement.
We recommend calibrating the device once and then not changing the measurement parameters. This should be good for a day-to-day operation.
The nanoVNA is calibrated using a basic OPEN-SHORT-LOAD protocol. Three SMA connectors are included with the nanoVNA, used for calibrating the device (see image below).
SHORT - Left, used to shorten the input of the VNA. This connector is all metal on the inside.
OPEN - Middle, used to create an open port. This connector does not have a middle pin.
LOAD - Right, used to simulate a 50 Ω load. This connector has a middle pin and a white teflon ring around it.
