5. DHCPPA Experimental Implementation and Practical Results
5.4 DCHPPA Circuit Configuration and Practical Result
5.4.1 DCHPPA Stage 2: IF Signal Processing Circuit Configuration
The idea of the DCHPPA design, based on the super heterodyne principle, exists in
conventional RF/MW radio receiver, which is still the most popular technique since it was
invented in 1918. In the superheterodyne, dual, down-conversion mixing technique is used
as shown in figure 5.14.
Figure 5.14 System diagram for super heterodyne double down conversion
Also the heterodyne approach has been seen as a very attractive technique for an
optical SCM system, particularly in terms of a long haul application; it can also be more
applicable for a free space application, as seen in this chapter. In the SCM optical receiver,
as shown in figure 5.15 [75], the optical signal can be detected by the PD and then
amplified by a low noise amplifier LNA, which results in amplification of all the received
signal (i.e desired and non-desired signals), including the PD noise occurring due to the
optoelectronic signal converter. These output signals will feed to the mixer for down or up-
PORT1 P=1 Z=50 Ohm Pwr=-50 dBm PORTFN PORTF
FET LNA
Mixer-1
1st IF Filter
IF Amp
Mixer-2
LBF
Amp
LO-1
LO-2
PORT P=4 Z=50 Ohm BP SUBCKT SUBCKT NL_AMP2 NL_AMP2 DLPFC RF IN IF OUT LO MIXER RF IN IF OUT LO MIXER 10will feed again into the second up/down mixer, which aims to recover the original
baseband signals to achieve better gain and better receiver sensitivity. In general, the
multiple conversion technique has been shown to work well in many RF/MW receivers,
but it is more complicated compared to a simple photo-detection circuit.
Figure 5.15 System diagram for a microwave-multiplexing light wave system
The DCHPPA acts in a parallel manner to a conventional double superheterodyne
detector system, but without the noise penalty that normally occurs. Photoparametric
amplification is used at the first stage instead of a resistive/transistor-based mixer or
preamplifier front end circuit. In this section, the practical demonstration of the DCHPPA
will be presented. Figure 5.16 shows the circuit configuration of the DCHPPA receiver.
Each stage requires careful consideration of the choice of components. The whole circuit
was divided into three stages, each stage with its sub-circuits; the PPA circuit stage 1,
which includes passive LC band pass filter circuit and the photo detector circuit as
explained in previous section; IF Signal processing stage two, which includes the pre
selector cascading band pass filters followed by an IF amplifier circuit, followed by a
PINDD SDIODE
NL_AMP2
Microwave power combiner Subcarrier modulator
LO
Microwave receiver Single-Mode Fiber Baseband -1 fsc1 fsc N Analog or digital videoLASER PHOTODIODE FET LNARF IN IF OUT LO
MIXER
PORTFN PORT
second IF cascading bandpass filters; down converter mixer stage 3, which includes a
passive LC bandpass filter circuit and DBM circuit, followed by a low pass filter.
As illustrated in the graph, the same pump source (LO) was used for the up-
converter and down-converter via a 2-way, 0 degree power splitter (i.e. no phase shift)
device. The LO was injected via high quality band pass filters that provide a convenient
method of applying the pump to the PD ( i.e. mixer one) as well as the DBM circuit (i.e.
mixer two), whilst at the same time providing isolation, reducing LO sideband noise and
blocking dc components from passing through. The up-converter mixer works to convert
an RF signal (1MHz) to the first upper sideband signal IF (RF+LO; 433.92MHz), where
432.92MHz was used as the LO pump frequency. The down-converter mixer works to Figure 5.16 Experimental arrangement of the DCHPPA
Stage-1 Stage-2
recover the desired baseband channel (1MHz) from the IF signal (433.92MHz). A picture
of the final setup can be seen below in figure 5.17.
As mentioned before, the signal processing stage two required a considerable
amount of attention; in particular with respect to selectivity and sensitivity, the most
important in this stage is selectivity; however, sensitivity is also desirable. In contrast,
there are four frequencies in the PPA output spectrum (ωp,ωs,ωp±ωs) with their
harmonics (i.e third and fifth order; ωIF=mωs±nωLO) at various level of powers. As shown in figure 5.5(b). At 22dBm pump power, the LO signal level (i.e. 432.92MHz) was
measured over the PD to about 10dBm using a spectrum analyzer.
Figure 5.17 Final DCHHPA system set-up
At high pump, the LO signal over the PD can lead to serious drawbacks if its PD output is
connected to the following stages without drastic consideration. For example, it can
saturate the DB mixer (mixer two) due to a high level of input power, as well as forcing the
SAW BPF to an unstable condition or damage. Therefore minimizing the LO level of
power at the PPA output is one of the key considerations; a limiter circuit was used to
reduce this effect; however, due to both frequencies (i.e. 432.92MHz and 433.92MHZ)
being close to each other (e.g. almost commensurate frequencies), the desired IF harmonic
was affected as it is weaker than the LO.
In addition, the preselector IF SAW filter should have two primary functions, one
being to accept a high input power, such as LC passive filters and ceramic filters.
Secondly, it must provide high selectivity with low insertion loss, such as a Crystal filter
[142]. A high-Q commercial Crystal filter was used in a previous published paper [143] at
the VHF frequency range. The filter was from Filtronics INC (FN-3809) with a 240MHz
centre frequency and 100 KHz pass bandwidth. The input power level was up to 5dBm,
and it exhibited a good frequency response as shown in figure 5.18a. Although it showed
high selectivity, it can be unstable at high pump power and economically, it is very
expensive (i.e. 250$) (see Appendix B7 for schematic circuit). Most of the present-day
crystal filters start being unstable when the input power exceeds zero dBm, and may result
in poor performance, as reported in [81] .
Ambitiously, a Surface Acoustic Wave (SAW) filter may overcome the previous
issues with respect to selectivity, rejection, low loss, input power and cost (i.e. less than
2$), employing such components offering substantial yet affordable benefits in the access
network. Also it was shown to work well in both analogue and digital transceivers (i.e.
AMPS, GSM) with a very good performance for frequency/phase noise, and it exhibited
long term stability [144, 145]. Several SAW filters were implemented and tested for better
selectivity performance. Two narrow band commercial SAW BPF filters with 120KHz
between the filters to compensate for the capacitive part of the filter impedance (see
Appendix B8). The designed ultra narrow band pre IF filter aims to provide greater
steepness to the filter edge and high selectivity, while maintaining low insertion loss, as
shown in figure 5.18b. Moreover, it provides high stability, cost effectiveness and a better
frequency response performance compared to the Crystal Filter, as illustrated in figure
5.18a.
The pre IF SAW filters were used and performed as a good preselected to pass only
the desired IF signals, remove the noise outside its bandwidth, and also eliminate other
odd- and even mixing products from breaking through. Moreover, these provided high
isolation for unwanted frequencies, such as RF and LO, which could cause additional
distortion products, and which are reduced in severity. High selectivity and low insertion
loss are more desirable at this stage.
The IF amplifier in stage two is responsible for providing additional gain to the
receiver to render incon sequential any noise introduced in the subsequent stages, as well
as providing isolation by rejecting the reflect power form subsequent stage to pass back to
the pre IF SAW filter. A mini-circuit variable LNA (ZFL-500LN) was used with 24db gain
and 2.9dB noise figure. This amplifier can be used as what is known as an Automatic Gain
Controller amplifier (AGC) at IF stages, which consists of a variable-gain amplifier and
automatic gain controller mechanism that keeps the output swinging constantly over a wide
range of input swings, and which seems to be more desirable for diffuse and quasi-diffuse
optical links, particularly for mobile wireless receivers where the incident detected power
signal may vary, due to mobility. In general, any appreciable amplitude with very low
noise figure is desirable at this stage; two mount surface mini-circuit IF amplifiers (TAMP-
and successfully implemented and tested; each amplifier has almost about a 20dB gain; the
cascaded IF amplifiers resulted in 40 dB gain overall (see appendix B9).
a) crystal filter (4.9dB insertion loss);
In the third step in stage two, the LO signal is still the main cause of the distortion
products, even if it has been minimized in the previous stage; therefore a cascading of two
ultra narrowband SAW filters (Epcos-B3790, +5dBm input power) in series were designed
and connected in series and matched to 50Ω impedance to be used as a post IF filter (see
Appendix B10). The high ultra narrowband post IF filter aims to provide greater steepness
at the filter edge and high selectivity, as shown in figure 5.18c; and pass only the desired
IF signal whilst suppressing all other LO, harmonics and noise signals, as seen in the next
practical results section.