Question

In TDM, what do you think are some of the effects of increasing the number channels while holding the sampling rate constant?

Answer 1

On this paper, the TDM approached for simultaneous collection of information from all elements is mentioned in element, building on the system confirmed in [26]. There are two key areas of focus, the multiplexing electronics which might be located with the transducer, and the direct digital demultiplexing design which makes up the receiver.

A minimal TDM transmit system requires handiest an analog multiplexer and a buffer for each and every channel and so requires so much much less house to put into effect than the opposite channel discount schemes considered. On the receiver end the indicators then ought to be demultiplexed which requires extra hardware. If completed in the analog area this may require the excessive frequency multiplexed signals to be passed through a synchronous demultiplexer, filtered, after which handed into an ADC dedicated to each and every channel. Every of these stages would add noise to the procedure, would present further complexity in matching the circuits across all channels, and would require a colossal number of ADCs.

As there's now more and more popular availability of very excessive speed ADCs with amazing number of bits on the order of 10 or extra [28] [29], and also FPGAs capable of performing difficult DSP functions on many channels in parallel, a lot of the receiver hardware complexity will also be eradicated with the aid of moving all of the demultiplexing standards from the analog domain into an FPGA and permitting demultiplexing to be achieved straight in the digital area.

The clock frequency of the multiplexer will be so much greater than the sampling frequency for each channel, in an eight channel 25 MSPS design as an instance, the clock frequency could be 200 MHz. The analog alerts are modulated onto pulses which have to rise and stabilize speedy sufficient that the amplitude will also be extracted by means of the ADC correctly. The cables connecting the multiplexer and ADC need ample bandwidth to limit the quantity of distortion of the excessive frequency TDM signals.

In Fig. 1 an summary of the simplified TDM scheme is proven. The receiver end consists most effective of an LNA and ADC within the analog path, which is then adopted by using an FPGA to perform the digital demultiplexing. The transmitter finish contains an analog front finish (AFE) followed by using a sample and keep (S/H) capacitor for each channel, every of which is then related to an analog multiplexer. The Time-gain Compensation (TGC) in the AFE may also be managed via a single cable to digitally cycle by way of fixed levels, or however controlled making use of an analog reference signal for finer adjustment.

Determine 1
Analog TDM Scheme utilising Direct Digital Demultiplexing - AFE CMOS IC placed with Transducer Array to Multiplex indicators

The multiplexing is managed through digital circuitry which generates pattern clocks for every channel, and manipulate indicators for the multiplexer. This circuitry is managed by a clock signal generated by using the receiver and transmitted over another cable. The ADC clock is synchronized to this same clock signal to be certain that every ADC pattern corresponds precisely to one channel in the multiplexed data. Accordingly of this synchronization, demultiplexing theoretically turns into a easy task of separating the info into businesses consisting of each nth sample.

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4 combined sign Multiplexer Design
The multiplexing circuitry is to be placed with the transducer to obtain the alerts and attach them with a reduced number of connections to the again end electronics. To directly interface with the transducers an AFE is required which is proper to the technological know-how. For illustration a Transimpedance Amplifier (TIA) situated AFE would be suitable for CMUT designs [22]. Additionally required is an anti-aliasing filter prior to the alerts being sampled. Extra electronics equivalent to integrated TX/RX switches can be needed within the entrance end.

Following the AFE is the sampling circuitry. This consists of a pattern and hold buffer, an analog multiplexer, and sequencing good judgment. An summary of this is proven in Fig. 2. The sequencing common sense generates sample
When a given channel shouldn't be being driven on to the cable, the S/H capacitor is hooked up via a move transistor to the output of the TGC. This gives time for the capacitor to match the signal voltage and pattern the signal (Fig. 3a). As soon as it's time for that channel to be linked to the output, the S/H capacitor is disconnected from the input, and after a short useless time, is then related through a buffer and the multiplexer to the cable driver (Fig. 3b).

The S/H circuitry has been incorporated in the transmitter to be certain the alerts are properly sampled and transmitted. Sampling within the transmitter ensures that during whenever period on the cable, a constant price is sent. Because of this the excessive velocity cable driver can settle to a regular price after a short switching transient alternatively than having to precisely follow a time various sign for the period of each and every period. This must make certain that an accurate sample is quantized by means of the ADC for that time interval.

A key part of the TDM scheme is alignment between the ADC and the multiplexer. Two types of alignment are required to be certain that the samples are properly digitized: channel alignment and section alignment. The former is required to make sure the sample for channel 1 on the multiplexer is famous to be channel 1 when demultiplexing â with out this alignment the factors would get mixed up in the course of demultiplexing. The latter is used to make sure that the TDM and ADC clocks are correctly segment aligned to account for propagation delays within the cabling. This phase alignment is primary to make sure the ADC is taking samples when the signal on the cable has stabilized instead than for the period of switching transients.

Both alignment necessities are satisfied in the course of an initialization sequence which includes a training pattern being generated by means of the multiplexer and analyzed within the FPGA. During the training sequence, the inputs to the multiplexer are internally tied to bias voltages, with the primary channel connected to 1 voltage stage, and all other channels related to a second voltage stage as proven in Fig. Four. By way of inspecting the converted information it is feasible to appropriately align the procedure.

Figure 4
(a) link coaching Waveform to locate First Multiplexer Channel (b) area Detection Scheme Used for section Alignment

The alignment starts with channel 1 being identified by way of the quantized codes being greatly distinctive in a single pattern than the others as illustrated in Fig. 4a. After this the phase of the TDM clock is adjusted to search out the most suitable section shift between the ADC and TDM clocks. The TDM clock segment will also be adjusted making use of a section-Locked Loop (PLL) within the FPGA to investigate the most suitable alignment, as proven in Fig. 4b. In the beginning the phase shift is accelerated to locate the factor at which the difference between channel 1 and the others is lowered by using half of, ΦL. The phase change is then decreased except the point at which the change drops to the equal amount, ΦR. These two facets determine at which phases the switching transients are happening, so the top of the line segment the place the sampling must be carried out, ΦO, have to be the midpoint.

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5 Multiplexer Implementation in CMOS
A prototype TDM multiplexer stage for eight channels, comprised of a single 8:1 multiplexer, has been fabricated in 0.35 μm 2P4M TSMC system and operates from a 3.Three V provide. The carried out circuitry, shown in Fig. 5, occupies an area of zero.80 mm x 0.26 mm (without the bond pads). The chip consumes on common three.9 mW of vigour, assuming a ten% responsibility cycle on time. The circuitry was once designed to run at 200 MHz, sampling each channel at 25 MSPS. In addition, as it was designed exclusively for testing the TDM scheme best, there is not any AFE present within the design. Alternatively the input stage is a solidarity reap buffer.

The enter buffer drives the sign into the S/H capacitor by way of a pass transistor which allows the enter to be sampled when related, and then disconnected to preserve the worth in a position to be multiplexed. The S/H capacitor has a capacitance of four hundred fF and is carried out as a Poly-to-Poly (PIP) device, 19 μm x 19 μm in dimension. Each and every input buffer also has a controllable resistor, both pull-up or pull-down, which might be enabled when the sequencing common sense is switched into hyperlink training mode in order to generate the specified pulse sequence.

Subsequent, the S/H capacitors are buffered and linked to a analog multiplexer which contains a pair of go transistors for each and every channel. A digital grey code counter is applied which generates manipulate signals for these go transistors to sequence resolution of each and every channel. Lifeless time is generated between channels utilising digital prolong chains in the sequencing good judgment to prolong the rising fringe of the manage signal individually from the falling area. The final output stage is a current suggestions buffer adopted from [30] which has an output bandwidth of 450 MHz when riding a 7515 pF load.