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LOW OISE AMPLIFIER DESIGN CHAPTERS

Table (8.1) compares the measured s-parameter results with that of the simulation (after optimization) at the 20Hz centre frequency.

S-parameters Simulation results (after optimization)

Measured results

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Table S.l: Comparison of simulated and measured s-parameter results

As can be seen in table (8.1), there is good correlation between measured and simulated s-parameter results, with the exception of the input reflection parameter

( I

Sill). Nevertheless, an input reflection parameter of - 15.9dB is reasonable.

The gain of the device was measured to be 13.1 dB. The 3dB bandwidth was computed as 520MHz (Q factor of 3.85) which exceeded the minimum specified

IOMHz bandwidth. A broadband design has thus been achieved.

8.6.1 Noise figure measurements

Procedures and techniques for noise figure measurements are described in [91], [92]

and [93J. The technique described in [91], utilizing a spectrum analyzer, has been deemed the best measurement choice for the following reasons:

a) The noise figure can be measured at any frequency within a spectrum analyzer's frequency range. This enables measurement at the device's operating frequency without changes in the test set-up.

b) Due to the frequency selectivity of spectnIm analyzers, noise figure measurements are independent of device bandwidth or spurious responses.

However, as with any other measurement, the analyzer's sensitivity and accuracy become limiting factors in noise figure measurements. The technique outlined in [91]

has a limitation: the noise output of the device under test (DUT) must be greater than

8-13

LOW NOISE AMPLIFIER DESIGN CHAPTERS

the analyzer's internal noise level so that it can be measured. If the noise output of the OUT is below the analyzer's sensitivity level, its power must be raised by a low-noise.

high-gain preamplifier. Then the noise level measured by the spectrum analyzer is greater than the device's output noise by the preamp's gain.

Due to inadequate gain of the designed amplifier and the inavailability of a preamp at the desired frequency, this technique (utilizing a spectrum analyzer) could not be used. lnstead, another straightforward technique utilizing a HP346B noise source and HP8970B noise figure meter.

The basis of noise figure measurements using a noise source and noise figure meter depends on the noise linearity characteristic of linear two-port devices. The nOise power out of a device is linearly dependent on the input noise power or nOlse temperature (Ts) as shown in figure (8.14). No is the noise power added by the OUT.

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Fig. 8.14: Noise linearity cbaracteristic of a linear two-port device

I f the slope of this characteristic and a reference point is known, the output power corresponding to a noiseless input power, Na can be found. From _Na, the noise figure F can be calculated using the well-known relation,

F = N. +kT.BG kT.BG This equation appeared in chapter S.

(8.1 )

LOW NOISE AMPLIFIER DESIGN CHAPTERS

From figure (8.14), for a source impedance with a temperature of absolute zero, the power output consists solely of added noise Na from the OUT. For other source temperatures the power output is increased by thermal noise from the source amplified by the gain of the OUT. The noise slope is determined by applying two different levels of input noise and measuring the output power change. A noise source is used

(0 provide these two known levels of noise.

The HP346B noise source consists of a low-capacitance diode that generates noise when reversed biased into avalanche breakdown with a constant current [941. When the diode is biased, the output noise will be greater than kTeB (thermal noise) due to avalanche noise generation in the diode. When unbiased, the output will be the thennal noise (kTeB) produced in the attenuator of the noise source. These levels are called Th and Te corresponding to the tenns "hot" and "cold".

The HP346B produces nOIse levels approximately equal to IOOOOK when on and 290K when off. To make noise figure measurements a noise source must have a calibrated output noise. The excess noise ratio (ENR), expressed in dB is the ratio of the difference between Th and Tc. divided by 290K,

EN _{• RdB = I} 0 1 og-~T',-:-:-,T-,-,

T. ^(8.2)

It should be noted that a OdB ENR noise source produces a 290K temperature change between its on and off states. 11 is often erroneously believed that the ENR is the "on"

noise relative to kTB. However, this is not the case.

Te in equation (8.2) is assumed to be 290K when it is calibrated. Noise sources are supplied with an ENR table giving the ENR versus frequency values. The noise figure meter uses ENR and the Y-factor method as the basis of noise figure measurements.

Using a noise source, this method allows the detennination of the internal noise in the OUT and thus the noise figure. With a noise source connected to the DUT, the output power can be measured corresponding to the noise source 011 (N2) and the noise source off (N 1). The ratios of these two powers is called the Y-factor,

8-15

LOW NOISE AMPLIFIER DESIGN CHAPTERS

y = output power with noise source on = N 2

output power with noise source off N1 ^(S.3) or in dB units, Y'B = 10 log Y .

The V-factor and ENR can be used to find the noise slope of the DUT that is depicted in figure (8.14). Since the calibrated ENR of the noise source represents a reference level for input noise, an equation for the internal noise (Na) of the OUT can be derived. In a noise figure meter, this is automatically detennined by modulating the noise source between the on and off states and applying internal calculations,

N ~ ^{kT nG( ENR -I )}

a " Y-l (S.4)

From this an expression for the noise figure can be derived. The noise figure that results is the total system noise figure, F_sys- It includes the noise contribution of all the individual parts of the system. In this case the noise generated in the noise figure meter has been included as a second stage contribution. If the gain of the OUT is large, the noise contribution from this second stage will be negligible. By substituting equal ion (S.4) into equation (S.l),

(S.5)

When the noise figure is much higher than the ENR, the device noise tends to mask the noise source output. In this case the Y-factor will be very close to I. It is difficult to measure small ratios of the Y-factor accurately. For this reason, the Y-factor method is generally not used when the noise figure is more than IOdB above the ENR of the noise source, depending on the measurement instmment.

Table (8.2) shows the noise figure results for the designed amplifier at three frequencies. These results were obtained using the HP346B noise source and HP8970B noise figure meter. Measurements were restricted to a frequency of 1.8GHz which is the maximum operating frequency of the noise figure meter. At 1.8GHz the simulated noise figure (after optimization) from figure (8.5) was 2.2dB while the noise figure measurement yielded 2.7dB. At 1.7GHz and 1.75GHz, noise figures of 3.2dB and 4.97dB respectively, were measured. Simulated noise figures (after

LOW NOISE AMPLlFrER DESIGN CHAPTERS

optimization) of 2.8dB and 2.5dB at l.70Hz and l.75GHz, respectively were obtained. Unfortunately, no correlation/comparison between simulated and measured noise figure (at the desired frequency of2GHz) could be made.

Frequency Noise figure

1700MHz 3.2dB

I 750MHz 4.97dB

1800MHz 2.7dB

Table 8.2: NOise figure measurements uS. lDg the HP346B nOise source and

H P8970B noise figure meter

8.7 C onclu sio n

It was the intention of the author to specify, design and construct an LNA with parameters that would provide acceptable performance of the HPSK RF front-end of chapter 9. However, due to the reasons cited in section (8.6.1), the noise figure of the

LNA could not be measured. Nevertheless, a low-cost, in-house 2GHz broadband low-noise amplifier (with 13.ldB gain and 520MHz 3dB bandwidth), for possible use in the adaptive CDMA system, was developed.

Relevant theory focusing on amplifier stability was presented and a suitable design technique, discussed and implemented. The topology of a low-noise high-frequency amplifier seems very simple. Tuning, shielding, proper grounding techniques and the design of a good layout plays an important role in the design of a fully functional amplifier. To conclude, a firm theoretical and practical basis on microwave low-noise amplifier design has been laid down in this chapter.

8-17