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Principles of the Direct Diode Detection Base-Band Q (3D-BBQ) Measurement are sketched in Fig. 1.

Fig. 1. One (vertical) channel of the 3D-BBQ-Measurement System.

The system consists of a beam position pick-up (PU), diode detectors connected directly to PU electrodes and front-end electronics, located close to the detectors. The front-end electronics in turn consists of input differential amplifier, a filter and an amplifier. The front-end is connected by long coaxial cables to an acquisition hardware, located in a place further from the beam, to limit radiation doses for its sensitive components, such as memories, CPUs, ADCs.
Individual parts of the system have the following purposes:

The detectors, installed directly on PU electrodes, convert short beam pulses into a saw waveform, with the teeth amplitude conserving betatron modulation. The large DC part of the signal is related to beam pulse amplitude (revolution frequency energy).
The detector storage RC network is connected to the amplifiers by a series capacitor, to cut out the DC detector voltage.
The detectors from opposite PU electrodes are connected to a differential input amplifier, to subtract detector voltages and remove an offset related to the beam average position. For a beam which is close to the pick-up centre, the differential input improves revolution frequency rejection. It suppresses also potential interference signals, likely to appear on both inputs in a similar way.
The filter, in addition to the detectors, is the most important part of the system. It attenuates the revolution frequency and its harmonics, as well as very low frequencies. The revolution frequency, which is the largest and hardest component to suppress, is attenuated by some 5 orders of magnitude (i.e. 100 dB) with respect to the betatron signal. See the SPS front-end characteristic in Fig. 7.
The output amplifier brings the signal to the level of a few Vpp, suitable to be transmitted over long cables.

The 3D signals can be acquired with a spectrum analyzer, or with a PC sound card.
The signals were also used to lock a lock-in amplifier on the betatron tune and then to feed back onto the beam to keep it excited continuously. The excitation was applied to a small audio amplifier (delivering a fraction of a W into 50 Ohms), driving a position pick-up working as a kicker.

Please find below a few photographs with parts of the system, showing how simple it is. At the bottom of the page there are shown examples of the corresponding final parts. More information on the LHC BBQ system can be found here.

Fig. 2. The very first detector built. The input, connected directly to a PU output, is on the left. The right connector is connected through semi-rigid coax cables to the front-end box. The detector consists of a coaxial line high frequency transformer (the thing with the toroid), an inductor and two Schottky diodes. The storage network is formed by the capacitance of the cable connecting the detector to the front-end box, as well as a small value capacitor. The discharge resistor of a very large value is located in the front-end box. These few components seen in the picture are the most important in the system.		Fig. 3. The very first prototype of the front-end electronics. It had only one processing channel (the left compartment). On the bottom there are two differential inputs and at the top - the output. The right compartment accommodates the power supply and control logic. The prototype was almost as powerful as the final version with serious PCBs. After a very good service on the SPS, the circuit shown had been modified and installed on the PS for 3D studies.
Fig. 4. The SPS 3D installation. The detector boxes, connected to a standard position stripline PU, are put inside heavy copper braids, together with semi-rigid coaxial cables connecting the detectors to the front-end lying upon the pick-up base. The braids improve shielding of the input signals. This is extremely important due to enormous gain of the front-end, exceeding 100 dB.		Fig. 5. The SPS 3D installation (cont.).The braids are carefully tightened on the front-end N input connectors.
Fig. 6. A PC sound card as an excellent acquisition system. The 3D signals had been acquired with a 24 bit, 96kS/s, USB external sound card Sound Blaster Audigy 2 NX.		Fig. 7. The frequency characteristic of the SPS front-end. One of the advantages of the 3D method is that the filter has to be done for relatively small frequencies, for which active components have great parameters. The notch is at the revolution frequency. The filter does not contain any adjustable elements.
Fig. 8. For a comparison, the final version of the LHC detector with an input 450 MHz low-pass filter.		Fig. 9. For a comparison, the final version of the 3D analogue front-end. Versions for each machine are built on the same PCBs. Only values of some components are speciffic for each machine.
Fig. 8. For a comparison, a photograph of the operational BBQ installation in the LHC tunnel. Two front-end boxes are seen, containing four independent channels, for both H and V planes of each LHC beam.