Measurement Techniques, Vol. 50, No. 3, 2007

A MICROWAVE ZERO RADIOMETER WITH COMBINED PULSE MODULATION

A. V. Filatov, N. A. Karataeva, and V. D. Dmitriev

UDC 621.396.9

A microwave radiation radiometer is described which uses a modification of the zero-reception method. Two input units are developed which enable the antenna signals in different ranges of variation to be measured, and their fluctuation sensitivity is analyzed. Key words: passive radar, microwave radiometer, remote sensing.

At the present time, not a single large-scale investigation of the Earth is being carried out without using passive and active radio-physical systems [1, 2]. The high information content of microwave methods has stimulated a considerable number of different specialists – meteorologists, geophysicists, oceanographers, geologists, and soil scientists – to participate actively in the process of understanding remote microwave data. When developing modern radiometric systems, designed for remote sensing (monitoring) of natural objects under conditions in which the parameters of the environment vary over a wide range, it is necessary not only to ensure the required accuracy but also to minimize the mass/size characteristics and cost. This requires the use of new radiometric system structures, which reduce the effect of destabilizing factors on the reliability of the results. In this paper, we consider a new type of microwave modulation radiometer, in which combined pulse modulation is used in order to establish and automatically maintain zero balance in the measuring channel. A block diagram of this radiometer is shown in Fig. 1. The input unit 2 and the receiver 3 determine the main characteristics of the radiometer, namely, the measurement range and the fluctuation sensitivity. The signals, modulated in the input unit, are amplified at low and high frequencies and are square-law detected in the receiver, which has high sensitivity and a broad frequency band of 100–200 MHz [3]. The choice of the receiver depends on the wavelength of the received signals and the problems being investigated. In the low-frequency processing unit, the filter 5 operates in synchronism with the output signals of the control unit 8 and provides preliminary filtering and storage of the constant component of the signals, which the filter 6 then eliminates. In the comparator 7, the signal is compared with the zero level. The control unit functions as a system with tracking feedback, using an algorithm which ensures that the zero method of measurement takes place. To measure signals in different ranges of variation, the radiometer is provided with two input units, block diagrams of which are shown in Fig. 2. Each unit contains two noise sources, which process the reference signals: the reference noise generator 2, the signal at the output of which, in units of equivalent noise temperature, is Tref, and an additional reference noise generator 3, which, together with the microwave switch 4, forms a “noisy” reference channel. The additional reference signal Dad is introduced into the antenna or reference channel via the directional coupler 5. The modulator 6 alternately connects either the signal Ta of the antenna or the signal Tref to the input of the receiver 7. In the input unit, shown in Fig. 2a, the signal Tad enters the antenna channel, since over the whole range of the measurements the condition Ta < Tref is satisfied. For the other unit, shown in Fig. 2b, over the whole range Ta > Tref and, consequently, the additional reference signal Tad is added to the signal of the reference channel. Tomsk State University of Control Systems and Radioelectronics; e-mail: [email protected]. Translated from Izmeritel’naya Tekhnika, No. 3, pp. 65–69, March, 2007. Original article submitted December 12, 2006. 0543-1972/07/5003-0337 ©2007 Springer Science+Business Media, Inc.

337

4 Ta

1

2

3

5

6

7

tw tmod Digital code Ta

8

Fig. 1. Block diagram of the zero modified radiometer: 1) antenna; 2) input unit; 3) radiometric receiver; 4) low-frequency signal-processing unit; 5) synchronous analog low-pass filter; 6) high-pass filter; 7) comparator; 8) control unit.

3

Ta

1

6 tw

4

Ta

Tad

6 2

Tref

a

Tn

tmod tw

4

5

7

5

tmod

Tad 1

2

Tref

Tn

7 3

b

Fig. 2. Block diagram of the input receiving unit of the zero modified radiometer with directional couplers: 1) antenna; 2, 3) main and additional reference noise generators; 4) microwave switch; 5) directional coupler; 6) modulator; 7) receiver.

The signals are pulse modulated in the input units, one period of which is shown in Fig. 3. The signals Ta and Tref, on a control signal tmod from the control unit, are subjected to symmetrical pulse modulation with a duty ratio of 1/2, as in the usual modulation radiometer. These signals also contain the constantly acting inherent noise of the receiver, reduced to the input, which is characterized by an effective temperature Tn. In the first-period of the modulation in the antenna or reference channel of the input units of the radiometer (as was noted above, this depends on the range of the measured signals), an additional reference signal of fixed amplitude of variable duration is applied from the control unit via the directional coupler in synchronism with the controlling signal tw. As a result, for the input unit, shown in Fig. 2a, the noise temperature of the antenna channel at the input of the modulator is increased to Ta + Tad, while for the input unit shown in Fig. 2b, the signal of the reference channel increases to Tref + Tad. Zero balance is obtained by adjusting the duration of the pulse-width signal tw and consists of equalizing the energies of the signals arriving at the input of the receiver in different half-periods of the symmetrical modulation. The proposed principle enables one to judge when the energies are equal from a comparison of the images in Fig. 3 of the voltage-time areas of the positive pulse S1(t) and the negative pulse S2(t) at the output of the high-pass filter of the low-frequency signal 338

Period of the modulation 1st half-period Ta + Tad + Tn (Tref + Tad + Tn)

tw U+

S1(t) Tref + Tn (Ta + Tn)

2nd half-period

0′ t′ S2(t)

U– Ta + Tn (Tref + Tn)

tmod

Tn

tmod

0 t

Fig. 3. Time diagram of the signals at the output of the zero-radiometer receiver, which explains the principle of the modification of the zero-measurement method.

processing unit. As an example, we will consider the use of the input unit, shown in Fig. 2a, and we will determine the transfer characteristic of the radiometer. As follows from the time diagram, the amplitudes of the pulses of the input of the comparator of the low-frequency channel are proportional to the differences of the signals at the input of the receiver Tref – Ta and Ta + Tad – Tref. If the modulated periodic sequence of signals obeys the condition, according to which the voltage in the second half-period of the modulation is equal to zero and the time axis passes through the signal with level Tref, then the equality S1(t) = S2(t) will be satisfied for the voltage-time areas of the pulses in the first half-period of the modulation. Since the voltage-time area of the positive pulse S1(t) = U+tw and of the negative pulse S2(t) = U–(tmod – tw), we have the obvious equality U+tw = U–(tmod – tw). (1) The amplitude of the positive pulse is directly proportional to the difference of the signals Ta + Tad – Tref and is found from the expression [4]: U+ = Gk∆ƒ(Ta + Tad – Tref), (2) where G is the overall transfer factor of the whole measuring channel of the radiometer, including the gain at high and low frequencies and the transfer factor of the square-law detector, k is Boltzmann’s constant, and ∆ƒ is the bandwidth of the receiver in which the signals are measured. We similarly have for the negative pulse U– = Gk∆ƒ(Tref – Ta).

(3)

Substituting (2) and (3) into (1), we obtain Gk∆ƒ(Ta + Tad – Tref)tw = Gk∆ƒ(Tref – Ta)(tmod – tw), whence we obtain tw = tmod(Tref – Ta)/Tad.

(4)

Formula (4) is the mathematical model of the realization of combined pulse modulation, which enables the radiometer to operate under zero-reception conditions. An important property follows from this: the antenna signal can be determined 339

not by the direct method, but indirectly in terms of the duration of the pulsed “noise,” which balances the energy of the noisy signal at the input of the receiver in the symmetrical half-periods of the modulation without converting the signals in the low-frequency channel. Equation (4) does not contain the transfer constant G of the whole measuring channel, which indicates that the radiometer is operating using the zero method. From (4), we obtain the antenna signal: Ta = Tref – Tadtw /tmod.

(5)

Carrying out a similar analysis for the radiometer with the input unit shown in Fig. 2b, we obtain tw = tmod(Ta – Tref)/Tad,

Ta = Tref + Tadtw /tmod.

The duration of the pulse-width signal tw is regulated by the control unit after analyzing the output signal of the comparator. The latter is used as a feedback signal. A result of the functioning of the tracking circuit is the establishment and maintenance at the input of the comparator of a zero voltage in the second half-period of the modulation. The tracking mode of operation of the control unit occurs in real time. In each modulation period, a digital code of the antenna signal is generated in the control unit and is transmitted to the dynamic integrator for further signal storage. The dynamic integrator receives digital codes, adds them arithmetically and then averages them over a specified time interval, thereby obtaining a single result of the measurement. The fluctuation sensitivity of the radiometer with the input unit shown in Fig. 2a is expressed as

∆Ta =

Tref (Tref + Tad + 4Tn ) + 2Tn2 − Ta (Ta + Tad − 2Tref ) 2∆ƒ τ R

,

(6)

where τ is the time constant of the synchronous filter of the low-pass channel, and R is the number of digital codes of the antenna signal stored in the dynamic integrator of the control unit during a single measurement. It follows from (6) that the sensitivity depends on the specific antenna signal being measured Ta, which, according to (5), may vary from Tref – Tad to Tref when the duration tw changes from tmod to zero. The dependence of the fluctuation sensitivity on the antenna signal was pointed out in [5]. However, in the present investigation we considered the case of a high antenna signal, when it is comparable with the inherent noise of the radiometer. Because of the availability of modern super-low-noise amplifiers with levels of inherent noise of the order of tens of Kelvins [6], the inherent noise of the whole radiometer system was reduced considerably, and which became comparable with the signals being measured. The fluctuation sensitivity of the radiometer with the input unit shown in Fig. 2b, is defined in the form

∆Ta =

Ta (2Tref + Tad + Ta + 4Tn ) + 2Tn2 − Tref (Tref + Tad ) 2∆ƒ τ R

.

(7)

In (6) and (7), the divider 2∆ƒτ R shows how the signal/noise ratio is increased in the low-frequency section when measuring the antenna signal. In the radiometer sample constructed, the band of received frequencies ∆ƒ = 100 MHz. For a chosen modulation frequency of 1 kHz, the time constant of the synchronous low-pass filter τ = 30 msec, and the number of storages R = 1000 corresponds to a signal storage duration of 1 sec (in the literature the value of the sensitivity threshold is often normalized to a measurement time interval of 1 sec). Then 2∆ƒτ R is a dimensionless quantity equal to 78383.7. Graphs of the fluctuation sensitivity of the radiometer for different ranges of measurement and different input receiving units, constructed from (6) and (7), are shown in Fig. 4. As follows from the graphs, for a radiometer with an input unit as shown in Fig. 2a the sensitivity remains almost unchanged over the measurement range, while for a radiometer with the input unit shown in Fig. 2b, it varies almost linearly over a considerable range. It has a minimum value for signals with a 340

∆Ta, mK 4 3

5

6

2 1

8

7

Ta, K

Fig. 4. Theoretical curves of the fluctuation sensitivity of the radiometer with different input receiving units as a function of the antenna signal (curves 1–4 are for the input units shown in Fig. 2a and curves 5–8 are for the input units shown in Fig. 2b) for different inherent noise temperatures Tn of the receiver: 1, 8) 200 K; 2, 3, 6, 7) 30 K; 4, 5) 150 K.

low effective antenna noise temperature Ta and a maximum value for signals with a high value of Ta. Consequently, by using the input unit shown in Fig. 2b in the radiometer one can obtain a higher sensitivity than when using the input unit shown in Fig. 2a for the same measurement range. As an example, in Fig. 4 we will consider two curves 3 and 6, representing the same measurement range of the antenna signal of 150–900 K by a receiver with the same Tn = 30 K, but for different combinations of the modulated signals. Curve 3 corresponds to adding the additional reference signal Tad to the antenna signal Ta, while curve 6 corresponds to adding an additional signal Tad to the main reference signal Tref. In both cases, the additional signal introduced in the channel via the directional coupler is equal to 750 K. Hence, Tref = 900 K for graph 3 and 150 K for graph 6. When measuring an antenna signal close to the lowest limit of the range, the sensitivity is increased by a factor of 5–6. This can be explained by the different mechanism by which the reference signal Tref is formed. Higher sensitivity at the lowest edge of the measurement range is due to the fact that different noise generators are employed to generate Tref: the reference signal of 150 K is generated by an active “cold” noise generator [7], while the reference signal of 900 K is generated by an active semiconductor generator based on a noise diode. Minimum sensitivity of the radiometer with the receiving input unit shown in Fig. 2a (a local maximum of the expression (6)) is achieved in the middle of the measurement range for an antenna signal Ta = Tref – Tad/2. The minimum sensitivity in the measurement range for this signal will be ∆Ta max =

2 /4 2(Tref + Tn )2 + Tad

2∆ƒ τ R

.

(8)

To obtain the necessary sensitivity of the radiometer, one must choose the parameters τ and R of the low-frequency signal-processing channel. As regards the choice of the time constant of the synchronous filter τ, it is governed by the dynam341

∆Ta, mK

1

2

3

4

8

7

5

6

Ta, K

Fig. 5. The same as in Fig. 4 but with the same temperature of the inherent noise of the receiver Tn = 100 K.

ics of the operation of the zero-balance self-regulation circuit. Hence, only the parameter R remains, by changing which one can establish the necessary signal-detection threshold. The number R, which gives the number of digital signal codes in the dynamic integrator for storage, is related to the measurement time. To achieve the necessary sensitivity threshold in the radiometer with the input unit shown in Fig. 2a, we obtain from (8) τR =

2 (Tref + Tn )2 + Tad /8

∆ƒ∆Ta2max

.

Similarly, for the radiometer with the input unit shown in Fig. 2b, ∆Ta takes the maximum value if Ta = Tref + Tad. By substituting this value of Ta into (7), we obtain a formula for determining the minimum radiometer sensitivity: ∆Ta max =

2 2(Tref + Tn )2 + Tad + 4Tad (Tref + Tn )

2∆ƒ τ R

.

(9)

If the radiometer sensitivity is specified, we have from (9) τR =

2 + 2Tad (Tref + Tn ) (Tref + Tn )2 + Tad

∆ƒ∆Ta2max

.

In Fig. 5, we show curves of the fluctuation sensitivity of a radiometer with a receiver having a noise temperature Tn = 100 K, calculated from (6) and (7). It follows from Fig. 5 that one can obtain a small increase in sensitivity for a “shorter” measurement range. Thus, in the case of a “noisy” reference channel for the 50–900 K and 150–900 K ranges (curves 5 and 6, respectively) when measuring an antenna signal in the region of 150 K the sensitivity increases by 5%. The natural noise temperature of the receiver has a similar effect on all the units considered. An increase in Tn causes a proportional increase in the minimum detectable antenna signal and a reduction in the sensitivity. 342

During tests of a radiometer model at a wavelength of 21 cm, we carried out an experimental check of the fluctuation sensitivity and of the long-term and temperature stability, and we determined the linearity of the transfer characteristic over the whole dynamic range. The minimum signal detection thresholds obtained confirmed the theoretical conclusions regarding the nature of the change in the sensitivity of the modified radiometer as a function of the antenna signal. The maximum spread of the data, obtained theoretically and experimentally, amounted to about 20% for a standard deviation of 5–6%. The modified zero radiometer was compared with the usual modulation radiometers in tests on the stability of the characteristics. The metrological parameters of the modified radiometer turned out to be higher. Thus, the temperature stability was higher by a factor of 22 and the long-term stability was higher by a factor of 2.2. The maximum nonlinearity of the transfer characteristic of the radiometer in the 50–350 K measurement range was ±0.54 K, which corresponds to an error of 0.35%. The radiometer has found practical application in remote investigations of fresh ice sheets, frozen grounds and soils. The results obtained show that these measurements are useful for geology in that they reveal subsurface structures, and in limnology for determining the biological activity of reservoirs. An investigation of the dynamics of seasonal freezing and thawing of cryogenic systems of different scales is important for preventing the development of cryogenic geological processes. This research was supported financially by the Federal Agency for Science and Innovation, State Contract No. 02.438.11.7046 of March 20, 2006.

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