Análisis comparativo de las OPAs elegidas

The previous asymmetric FM design procedure modifies the waveform nominal instantaneous frequency by the same amount at / , ( ± T / 2 ) . The superposition o f an even and an odd phase functions expressed as polynomials o f terms o f even and odd powers

allow a different control o f the modulation function for positive and negative values o f time. The nonlinear phase polynomial is defined as:

N

I t\ n N + l

'' n=4

I t

n=3 ₍₉₎

where are the coefficients o f the polynomial o f odd powers, k^ the coefficients o f the polynomial o f even powers and N the odd powers polynomial order. The signal combined instantaneous frequency is given by:

N

I t n - \ r, N + \

n=3 n=4 ₍₁₀₎

where k^ = nk^ and k^ = nk^ . The polynomial coefficients are again calculated in order to attain determined and independent values at the edges o f the signal instantaneous frequency: f i B 2 N N + \ B n=3 n=4 (11) (12)

where and ^ 2 coefficients which determine the desired different increments (or decrements) o f the nominal signal instantaneous frequency. In order to simplify the design procedure, the polynomial coefficients are considered to be as follows:

kn —nk^

kn — nkg

(13)

(14)

The relations between k^, k^, q^, ^ 2 and the maximum polynomial order N (of odd powers) are now given by:

BT ke — i[n ^ + 2 N - ' i ) __________ 5 T __________ 2((7V + l f + 2 ( i V + l ) - 8 ) f e + ^1) (15)

A symmetric compressed pulse is achieved by passing the signal through its true matched filter, regardless any asymmetries which may be introduced by the phase polynomial. The compressed pulse can be formulated as:

1 r _{r o} — rect — rcct T J [t J I T

J

However, and in order to produce a symmetric frequency modulation, the condition qi = ^ 2 must be imposed on the instantaneous frequency design, obtaining a phase polynomial formed by terms o f even powers. Figure 6.8 shows the nonlinear FM compressed pulse range sidelobe level as a function o f phase polynomial order for different symmetric instantaneous frequency increases. A combined 10^^ order phase polynomial appears to be the optimum choice before the range sidelobe level is degraded by a raising sidelobe adjacent to the mainlobe. The instantaneous frequency and symmetric compressed pulse o f a nonlinear FM signal designed with the described method are shown in Figure 6.9(a) and Figure 6.9(b), and its characteristics listed in Table 6.Ill (nominal linear FM TB

o f 300). Although the peak range sidelobe level o f -35.5 dB is far above the spaceborne meteorological requirement, the range sidelobe pattern is compressed below -60 dB for most o f the duration o f the compressed pulse (with no required filter mismatch). A further increase in the order o f the phase polynomial yields a non optimum peak range sidelobe level, but reduces the range in which the range sidelobe pattern is above -60 dB.

p h a s e p o ly n o m ial o rd e r

Figure 6.8. Nonlinear FM compressed pulse range sidelobe level as a function of phase polynomial order giving instantaneous frequencies of ±0.75 B, ±1.0 B and ±1.5 B at fi{ ± T !'i)

with respect to a linear FM nominal TB of 300.

nonlinear frequency modulations as a function o f Doppler frequency offsets. The ambiguity function (displayed in Figure 6.9(c)) shows the rapid degradation o f the range sidelobe pattern for very relatively small Doppler shifts, reaching a peak range sidelobe level value o f approximately -5 dB for a Doppler offset of 150 kHz.

Table 6.111: Symmetric nonlinear FM compressed pulse characteristics

RSL h 4 A?L3dB 1.50 B -35.5 dB 0.0 dB 2.0 dB 0.71 tim e ( ^ s ) (c) - - 4 0 tim e (u s) do p p le r (kHz)

Figure 6.9. Nonlinear FM signal (10^'’ order phase polynomial giving an instantaneous frequency of ±1.5 B at /j ( ± r /2 ) with respect to a linear FM TB of 300): (a) Instantaneous frequency; (b) Compressed pulse, RSL = -35.5 dB; (c) Ambiguity function.

Finally, it should be pointed out that ‘continuous’ linear FM predistortion functions may be generated by the previous design method. For a very large phase polynomial order, the overlapped phase function resembles the linear FM predistortion function described in Chapter 5, and gives better results than in the case o f predistortion functions consisting o f short portions o f different linear modulation rates. The nonlinear FM signal can be compressed in reception by a linear FM mismatched filter, and its compressed pulse is given by:

îo('r) = ÿ J ( jl K B /2fV' Af+] I T n = 3 7^^^+E C IF + E ^/thF_{{ T} n = 4 2t j A t - x Y dt ₍₁₈₎

where w{t) is the mismatching function. Ih e instantaneous frequency and compressed pulse for a nonlinear FM (phase polynomial of order 1042, for a nominal linear FM TB of 300) are shown in Figure 6.10(a-b) and its characteristics listed in Table 6.IV.

Table 6.1 V: ‘Continuous’ predistorted linear FM compressed pulse characteristics

RSL 4 Af_3dB 1.50 B -64.4 dB 7.5 dB 5.1 dB 1.85 (a) (b) 2 I (C ) (cl) d o p p le r (kHz) tim e (MS) -61 -62.5 - 6 3 SO 150 d o p p ler freq u en cy (kHz)

Figure 6.10. Nonlinear FM signal (phase polynomial of order 1042 giving an instantaneous frequency of ±1.5 B at f i[±T/2) with respect to a linear FM TB of 300, minimum 3-term Blackman-Harris mismatching function): (a) instantaneous frequency; (b) compressed pulse, RSL = -64.5 dB; (c) Ambiguity function; (d) Range sidelobe level as a function of Doppler frequency.

level improvement o f approximately 2 dB (-64.4 dB) with identical loss factors with respect to the predistortion functions described in Chapter 5. The ‘continuous’ predistortion function improves the cancellation o f the compressed pulse constituent terms in a significantly longer time range of the compressed pulse.

The ambiguity function o f the ‘continuous’ predistorted linear FM is shown in Figure 6.10(c). The Doppler tolerance o f the waveform is excellent up to Doppler frequency offsets o f 80 kHz (1.6% o f the signal bandwidth). However, larger shifts rapidly distort the range sidelobe pattern adjacent to the mainlobe. The range sidelobe level is degraded 3.4 dB for a Doppler frequency offset o f 150 kHz (Figure 6.10(c)).