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How to set up low frequency RF system on platform
In order to generate the RF frequencies used in BNMR/BNQR, the signal synthesized by the PSM must be amplified to the required power, but these amplifiers only work over a specific frequency range. There are two amplifiers: a high frequency which operates between 250 kHz-80 MHz, and a low frequency amplifier which operates over 10-500 kHz. There is also the DAC attenuator on the platform (the blue box located at "a" in picture below) has its own operational band width different from the amplifiers. This causes this attenuator to behave non-linearly at low frequencies resulting in less power than expected being delivered to the RF circuit (cct). It has been observed that this effect is taking place for frequencies around 630 kHz (the Larmour frequency for an external field of 1 kG). The blue box is an attenuator that protects the RF cct at the sample from receiving too much power, and therefore one should take caution when removing this element as the cct is no longer protected without it.
The first step in going to low frequency is to start a 1f "test run" at high frequency (say f=21-21.01 MHz) with maximum power (ie: PSM=255, DAC=1540). Once the run is started measure the peak to peak amplitude on the oscilloscope ("b" in the picture) from the FWD output of the "bidirectional coupler" (located at "c" in picture, and shown in detail below).
This peak to peak amplitude is the maximum that can be sent into the RF cct without damaging it. Once this is measured it is a good idea to stop the test run to stop the RF. Now, disconnect the 50 Ohm load(located at "d" above and shown below) (which is normally connected to the output of the RF cct), and connect to the "output" of the bidirectional coupler (the output is normally connected to the RF cct input in the spectrometer). This is to disconnect the RF cct while bypassing the DAC attenuator.
Now, connect the output of the amplifier, "e" in above picture, (which is normally connected to the DAC attenuator input) directly to the input of the bidirectional coupler (which is normally connected to the output of the DAC attenuator). Now connect the PSM (wire that is labeled "PSM", and will be connected to the amplifier input) to the input of the "clicker" manual attenuator (located at "f", and shown in detail below), and the output of this attenuator to the amplifier input. By doing this you have bypassed the DAC attenuator, and added manual attenuation.
Now, start another 1f test run over a range of low frequencies (ie: 300-310 kHz) and PSM=255. Again, measure the peak to peak amplitude on the oscilloscope from the FWD of the bidirectional coupler, and add attenuation (by flipping the switches on the manual attenuator) so the the peak to peak amplitude does not exceed the amplitude measured at high frequency. Now you can reconnect the RF cct by reconnecting the its output to the 50 Ohm load, and the input to the output of the bidirectional coupler. To go to frequency regimes lower than the 250 kHz limit of the high frequency amplifier (for fields on the order of 1kG, for example), you must replace this amplifier with the low frequency generator from the BNQR side. The rest of the procedure is the same, but "amplifier" will refer to the low frequency amplifier. The key in this procedure is to error on the side of caution with attenuation, as burning out the RF cct in the spectrometer will severely disable the experiment for the rest of the run, not to mention you will be severely disabled after Syd gets ahold of you!
Page last modified: 07/23/09 02:45 by Andrew MacFarlane.