OPTIMISATION OF LIMITER CIRCUIT

Amplitude modulation will present only a minor limitation on achievable temporal resolution; the high power applied to the deflection plates implies a large region of linearity in scan and hence such fluctuations will have only a minimal effect on streak velocity. The amplitude modulation is effectively removed from the carrier by the limiter in the drive circuit. Streak camera performance is much more sensitive to phase fluctuations in the deflection signal. Since when the laser was showing this solitonic effect the third harmonic of its repetition rate gave a constant beat frequency with the crystal oscillator it was concluded that the phase fluctuations (ie, jitter in the pulse train) were minimal and the degradation in streak camera performance had its source in the synchronisation circuitry. It became our belief that the action of the limiter itself actually induces phase modulation to appear in the deflection signal since the outputs of the FET amplifiers within the limiter are driven into different levels of saturation, dependent upon input level. This will generate a varicap effect within the amplifier causing phase/frequency modulation to be present in the carrier which has some relation to input amplitude fluctuation. To provide some control of the limiting action the system was modified so that the first pair of the 560C amplifiers had

a separate, variable rail voltage compared to the output chip (Fig. 3.8). To enhance gain a i! fourth final stage amplifier was incorporated with a 300 MHz tuned output. It was then

found that adjustment of these rail voltages could reduce the level of sidebands on the limiter output and have a marked effect on achievable resolution.

300 MHz Tuned Amplifier 3.1 - 8.8 W Vi 2.1-5 3 . 1 - 8 . 8 V 5oa Input H H InF InF _Output 560C 560C _560C 560C O y _o InF InF ±: InF = : InF

Figure 3.8 Redesigned limiter circuit

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Assuming that all of the sideband content after lim iting is entirely due to frequency modulation - a reasonable assumption since no discernible amplitude modulation was ever present - its effect on synchroscan performance was then considered. It is useful to view the effect of frequency modulation sidebands on a carrier employing the rotating vector model [1^3. Referring to Fig. 3.9 the phase of the main carrier having a signal level Vc will be deviated through an angle A<|> equal to the vector addition of the various sideband

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signal levels. The maximum phase deviation A(j)max will occur when all the sideband components are in phase with each other and in quadrature with the carrier. In this case:

A<}>niax = +^V+, +V+2 J -eq(3.1)

% Assuming that this maximum deviation corresponds to the limit of the synchroscan %

resolution the expected temporal resolution will be:

% = -«q(3.2) -I

fo!t j

where fo is the deflection frequency.

There is an additional effect of the Q of the system which will act to amplify any frequency modulation thereby increasing the value of A(j>max- For the case of a basic resonant circuit 1^33, the phase shift for a frequency component f at the output of simple LC resonant filter with resonance fg is given by:

tan 6<t) = ± Q( ^ -eq(3.3)

For small frequency deviations around fg, tan 6(|> « 6(j> and thus:

-eq(3-4) After the limiter we have phase/frequency modulation defined by the sideband content which generates a phase deviation A(|). Thus the drive signal can be represented as:

V(t) = V gsin( %%t + (|>g+ A(j).cos w^t ) -eq(3.5) The frequency content of the drive signal is defined by the derivative of the argument of the sine term in the above expression and thus the instantaneous frequency of the drive signal,

f = — A<j) tt^.sin ). 2tc

Hence the maximum frequency deviation for a phase deviation A(|> is given by

Afmax — -eq(3.6)

Combining the above expression with equation 3.4 the frequency deviation induces an additional phase deviation at the output of the filter given by:

S(j> = ^ f m A<j)nniax -eq(3.7)

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where fm is the sideband frequency separation. Insertion of this into equation 3.2 yields the expression:

’ _q(3.8)

By adjusting the limiting level of each FET amplifier the degree of saturation was varied which produced an alteration in sideband content, Vsb* Figure 3.10 shows how measured values of CÇ vary with SVsbA^c as measured on the Photochron IIA streak tube operated at fo= 300 MHz. Also shown on the figure are the theoretical values of % calculated from eq(3.8). Good agreement is seen between theory and experiment which indicates that the above analysis represents a reasonable model and demonstrates the importance of phase noise in limiting synchroscan performance. (The fact that the measured values of % are slightly greater than those predicted by theory probably implies the presence of additional deliterious factors associated with the RF signal which is expanded upon in the next chapter.) From this analysis it becomes apparent that in order to achieve subpicosecond temporal resolution it is necessary to reduce the sideband content to less than 80 dB below the carrier power level (dBc).

% vs Vsb_Vc

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Relative sideband content (xio-S)

Figure 3.10 M easured temporal resolution (points) and theoretically- predicted (solid line) temporal resolution values as a function of sideband content in deflection signal.

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With the laser in this solitonic mode and limiter rail voltages adjusted to minimise sidebands to around 70 dB below the carrier the best resolution achieved realtime was 2.8 ps and with a 1ms mechanical shutter, optimum resolution of 2.4 ps was achieved as is shown in figure 3.11.

^--- 16.2 p s ^ -1 6 .8 ps

2.8 ps 2.8 ps 2.4 ps 2.4 ps

(a) (b)

Figure 3.11 a) Optimum realtime and b) 1ms shuttered streak records with laser operating in soliton mode.

3.5 S y n c h r o s c a n o p e r a t i o n a t 200 MHz

Because of the inherent difficulties in operating at 300 MHz streak frequencies (mainly due to RF heating effects in deflector feed-through pins) it was decided to scan the Photochron IIA at twice the laser repetition rate ie) 200 MHz. This required the retuning of the final stage tuned amplifier in the limiter, the construction of a 200 MHz power amp (Mullard BGY45C) and the modification of the can filter. The power amplifier was supplied with a adjustable rail voltage thus allowing its gain to be controlled. To bring the can filter to the lower resonance of 200 MHz required its end capacitance to be increased by reducing the air gap. Low resonant transmission loss of 2dB was maintained by pushing the inductive loops closer to the centre rod to improve coupling. Inductive coupling to the streak tube deflectors was modified; the inductance loop area was increased and the double

plate wide air gap tuning capacitor replaced with a variable 8 plate narrow gap capacitor stack.

Initially results were very similar to those achieved at 300 MHz operation but camera handling was much easier since settling times and detuning due to RF heating were much reduced. It became fairly clear that if better temporal resolutions were to be achieved the phase noise in the drive signal had to be improved and, since its origin was in the main part due to amplitude fluctuations in the laser output, time was taken to improve the CPM laser performance. This involved the inclusion of a 4-prism sequence in the laser cavity to control group velocity dispersion (GVD) and the development of realtime intensity and interferometric autocorrelation monitoring equipment These are discussed in some detail in Chapter 9.

Control of the dispersion in the cavity enhanced laser performance significantly. Pulse durations were reduced almost threefold and amplitude fluctuations were largely removed apart from a few percent constant ripple associated with argon ion pump 3-phase regulated power supply. There was a higher frequency modulation (Figure 3.12a) at around 1 MHz which manifested itself as amplitude modulation sidebands about the 200 MHz carrier (figure 3.12b) but these could be readily removed by the limiter. The origin of this

^ --- 10 M H z---►

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Figure 3.12 a) High frequency modulating sidebands on 200 MHz Fourier component and b) associated modulation present in electrical pulse train.

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