Gain Chain DCHPPA System

The current proliferation of optical wireless devices result in large diversity of

designs and most of the future devices need better sensitivity, and have a requirement to be

aware of energy scale-down. Significant attention was paid to increasing the sensitivity and

reducing energy consumption, such that, the PPA works in equilibrium mode (i.e. no bias

source), employed passive components, low power components and low loss components

in designing the OW receiver.

All the receiver components can have a direct affect on is the noise figure (NF),

according to Friis formula for NF calculation. An important consequence of this formula is

amplifying stage (i.e. PPA stage). Subsequent stages have diminishing effect on SNR and

do not have a drastic affect, but everything must be done in maximizing the gain and

dynamic range. The overall receiver NF can be expressed as:

PPA rest PPA receiver G 1 F F F   

whereFrest is the overall noise factor of the subsequent stages and GPPA is the PPA gain;

the overall NF of the receiver (Freceiver) is dominated by the noise figure of the PPA if the

gain is sufficiently high.

According to the above, the schematic circuit diagram of an optical gain chain

DCHPPA system is shown in figure 5.23. The diagram below shows typical components

used to build the receiver, as mentioned in the previous section, in addition to a new stage

added to the IF signal processing stage, which includes the IF amplifier and other SAW

BPF. The main advantage is in the superior sensitivity that designers almost take for

granted; additional gain in the IF stage makes the desired IF signal levels high enough for

noise sources at the IF signal processing stage so as to have a negligible effect on the SNR.

As mentioned earlier, all the receiver components were built in individual PCBs

and were connected by 50Ω coaxial cables. In the gain chain DCHPPA circuit

configuration, two Mini-circuit IF LNA amplifiers (TAMP-72LN+ operates at 5 volts)

were built in a cascade, with each amplifier having a 20dB gain and a very low noise figure

(i.e. 1 dB). This type of amplifier can improve a system spur-free dynamic range, which is

often the critical driver in many receiver applications. Moreover, it helps subsequent stages

(i.e. stages two and three) to enable greater sensitivity for receiver applications.

Experimentally, the gain chain DCHPPA technique overall subsequently exhibited

a 56.25 dB baseband signal gain over the modulated optical signal, as shown in figure (5.1)

Figure 5.23 Gain chain DCHPPA circuit diagram

Figure 5.24 Frequency spectrum of 1 MHz recovered baseband signal using chain gain DCHPPA showing 56.25dB gain over the modulated optical signal.

The technique can be seen as a promising approach to achieve high gain at low

cost, compared to other optical amplifiers designed for free space or a long-haul

environment, such as EDFAs and PSA amplifiers. Although the practical implementation

for the whole receiver as individual PCBs circuits performed well and exhibited desirable

on only one PCB circuit, as shown in (Appendix B12), can provide better performance

with respect to noise and conversion gain; this because all the components are

implemented in one BCP with short transmission lines (e.g. as coaxial cable at high

frequency has high signal attenuation and long signal delay), this can also increase the

compactness and reduction of parasitic effects, and makes the system easy to isolate using

a die-cast box. Effort was made to consider all the PCB technical requirements,

particularly at high frequency; however the performance regressed, as compared to

employing individual BCP circuits. Both passive and active surface mount components

were employed, which can cause a ground loop problem. Moreover, cascade amplifiers

may oscillate when mounted together, or due to not being physically separated, which can

cause coupling feedback that might also need extra care to decouple the dc power supply

lines that feed the two stages.

5.6 Summary

To summarise, a novel approach to the design of an optical wireless receiver has been

presented, based on the superheterodyne principle but using photoparametric amplification

at the first stage instead of a resistive/transistor based mixer. The designed OW receiver

acts in a parallel manner to a conventional double super heterodyne detector system, but

without the noise penalty normally incurred. DCHPPAs have properties that make them

potentially attractive for use in future optical wireless communication systems. In

particular, they can provide a very high gain with high selectivity, combined with very low

noise operation. The experimental work described in this chapter includes the design and

implementation of wide band test-bed which showed that, the gain frequency variation was

using a PD in equilibrium mode, leading to potentially greater conversion gain at lower

penalty (i.e. in power and noise); the PPA is better at quite a low load impedance, which

can provide a better GBP; as has also been shown, the gain is related to the pump power

over the reactance impedance, the value of reactance impedance at applied frequency, and

the idler frequency over the source frequency. In addition, the junction should exhibit high

nonlinearity, with very low parasitic resistance.

The tests on the second stage (i.e. IF signal processing) indicated a promise, and

can be implemented satisfactorily using SAW filters and very low noise amplifiers that

lead to potentially high selectivity and sensitivity, with additional gain at an early stage

also leading to a cost-effective solution. Tests on the third stage (recovery baseband)

indicate good results, and are implemented by using passive and low loss components (i.e.

DB mixer); alternatively, an active mixer can provide better overall conversion gain but an

additional attenuation circuit and power source is needed to accomplished the work. The

DCHPPA technique overall subsequently exhibited a 34.9 dB baseband signal gain over

the modulated optical signal. In addition, it seems that employing a chain gain DCHPPA

technique to be preferred, as is subsequently exhibited by a 56.3 dB baseband signal gain

over the modulated optical signal, which can maximize the SNR at signal frequency; this

technique can bring up the signal gain to certain levels required for effective utilization.

Optoelectronics mixing in DCHPPA appears very promising as means of linear

amplification and frequency conversion of optical signal that offers the prospect of

significant benefits to OW and FSO, as well as offering improved performance in fibre

access networks (i.e. wireless and long haul applications). The next chapter is concerned

with performance analysis as well as analysing the noise performance of the amplifier, as

Chapter 6