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    Low Frequency of the High Field Spectrometer

    Setting up the High Field Spectrometer for Low Frequency Sweep


    Note: Before you start make sure the the Berkeley Nucleonics Corp AFG and/or the magnet are set up correctly in Camp. The correct setting are saved in "bnmr_05042004", which can be loaded by opening a camp window, press on the "Tab" button, choose "configure", then "Open config file" and "bnmr_05042004". This will make the AFG available under the path "/frq" and the magnet under "/Magnet".

    Instead of using the "Freq" mode button - we use the "Camp" mode. However, there are two different camp devices that may be used in a camp sweep experiment,
    Buttons on the DAQ page
    1. The arbitrary function generator (FG)
    2. The magnetic field (MG)
    the correct one must be setup in the DAQ in "PPG1c". Change the value of  "Camp Sweep Device" to
    1. FG - for a frequency sweep
    2. MG - for a magnetic field sweep
    then go back to the "Status" page, and from there to
    1. For frequency sweep: "Expert" -> "CampSweepDev" -> "frequency generator" and set the value of "Camp path" to /frq
    2. For magnetic field sweep: "Expert" -> "CampSweepDev" -> "magnet" and set the value of "Camp path" to /Magnet
    For example the frequency sweep options would look like:
     
    Camp Device Parameters for the frequency generator

    In the frequency sweep we use the Berkeley Nucleonics Corp to generate the RF. "Telulex Model SG-100/A" frequency generator. In CAMP, this is known as the bnc625a. Its front panel looks like this:

    frequency generator front panel

    This unit needs to be
    1. powered: is obvious, and when it's on the screen is yellow.
    2. cabled to the serial interface of the camp MVME162 computer (bnmrvw), the cabling is done via a telephone-style patch cable. On the platform it looks like this - the cable going into the connector in the top-left here (/tyCo/2)
      Platform MVME serial ports
      The patch cable requires a "crossover" box shown below
      crossover and patch cables
    3. have its output sent to the low frequency RF power amplifier. A bnc cable is required to go to the input of the low frequency power amplifier.
    The low frequency power amplifier we are currently using is the EIN model 1040L which is normally in the low field spectrometer rack. It has a normal single phase three-pronged power plug, but it is very heavy, so be careful if you move it around to the platform.

    The front panel is quite obvious and it looks like this.

    Low Frequency Amplifier EIN

    Note this "push-pull" amplifier is quite nonlinear at higher amplitudes. Practically the amplitude of the linear region is limited to 50 mV on the camp amplitude control for the bnc frequency generator - unless you can tolerate significant distortion.

    We usually place the Werlatone directional coupler in series with the high power output. Note directional is real. There is an input and an output of this unit. Both N-type connections. There are also bnc connections for monitoring the reflected and transmitted power (through 40 dB attenuation - which makes it safe to attach to a scope). We do in fact usually monitor the RF system by doing this.

    Note the range of this coupler is 10 kHz - 250 MHz, so you may need to remove it for the lowest frequencies.

    The Directional Coupler looks like this:

    Directional Coupler

    Once all this is cabled together and powered (actually leave the amp off til you are really ready), one has to set up CAMP to talk to the device. Once the interface is setup, you should now be able to set the device "online". The camp device looks like this

    CAMP Frequency Generator Parameters

    Note - for linear region keep the amplitude parameter no greater than 0.05 V. Unless you know what you are doing, you may choose one of two possible functions, "Sine" ot "Sine_1_3". Do not use the "other_func" option to set the waveform. The maximum frequency is determined by the num_pts value in the "setup" screen. A lower number allowes higher frequencies. The "setup" screen looks like this.

    Camp FG setup parameters

    Note that the low frequency amplifier is limited to frequencies below 500kHz.

    Have a look at the output of the frequency generator check that it sweeps over the range, then power up the amplifier.

    Note there is no safety interlock for the low frequency RF system, so be careful not to apply too high voltages, and be sure the RF coil is terminated with the high power 50 ohm load.


    Page last modified: 07/23/09 02:45 by Andrew MacFarlane.