MINISTRY OF SiPPLY

Requxrements are discussed for an Iunpllfier slutable for subsonic and supersonx tdx&?nce work nxth hot xlres. Such an empllfier needs to have a constant gain at low frequencies, ad a gain rlslng In proportion to frequency at h& frequenmes, UI order to compensate for the the-1 lag of tho hot vzrc. A fairly sharp cutoff 1s required at the high frequency end, and it is desirable that this cutoff frequency should bc adJustable, so as to limit the bsnd width of the amplifier to no more than is essential, thus preserving the mexmwn ls~~l-Co-noise' ratlo. The smplifler should also be free of lntorforence arzsing from extexns.1 vibrations or from the popicr supply. An ampllfior is descnbcd whch has a frequency range from 1.4 c/s to 50 Kc/s, dealrng xith a rango of the-1 time lag from 0.1 m.s. to 5 m.s. An iron dust-cord lnduotance IS used to @VC the required compensattlon for thermal lag, the circuit being a modifxation of Dryden's circuit. The upper frequency cutoff is adjustable jn s1x steps from 1.5 Kc/s to 50 Kc/s. The output can be applied to a thermocouple meter and to an osc~lloscopo, and the sensltivlty is such that full output is obtazned for 100 PV low frequency input, at a setting of 0.5 m.s. t3me lag. The output in thus case is about 5 volts giving an over&l gain 0f 50,000 for low ~requcnmes. The upper frequency ga=n for negligible nose lntcrference 1s about'l.6 x 106, The low lxmit of turbulence v&nch can be measured varies from 0.1% with1 m.s. time lag and 10 Kc/s ;;Eezfrequency out-off, to 5$ viith 5 m.s. thermal time lag and 50 Kc/s-. These values can be decreased if necessary by the use of a transformer input.