Patent GB-714684-A (GB)
Title: Improvements in devices for frequency selections
IPC: H03B21/02
Filed: 1951-03-13
Abstract: 714,684. Valve generating circuits; radio receiving systems; generating oscillations. SOUTH AFRICAN COUNCIL FOR SCIENTIFIC & INDUSTRIAL RESEARCH. March 13, 1951 [March 21, 1950], No. 6043/51. Classes 40 (5) and 40 (6). [Also in Group XXXVI] The invention relates to systems for generating or selecting a signal of desired frequency using as a reference one of a range of harmonic frequency oscillations, and in which the harmonic used is determined by means of including a variable frequency oscillation applied twice to the system such that its frequency is cancelled from the output frequency expression. According to the invention an interpolating device is also provided and said harmonic determining means comprises a filter having a pass-band higher than the highest harmonic and which is fed by the output of a mixer to which is fed the variable frequency oscillation and the whole range of harmonics. In Fig. 4 the output frequency fc of a crystal oscillator C.O. is fed to a harmonic generator H.G. which feeds a mixer through a low pass filter. A variable frequency fr is also fed to the mixer to produce an output frequency which is above the highest harmonic passed by the lowpass filter. The filter has a pass-band width less than the frequency difference between two harmonics and its output of frequency nfc + fr is fed to an interpolation device comprising a variable shift-frequency generator of frequency fi, a mixer and a band pass filter, which enables the output frequency to be varied over a range equal in extent to the spacing between the harmonics and the resulting frequency fr Œ nfc + fi is mixed with the variable frequency oscillation to produce an output frequency nfc Œ fi. The variable shift-frequency generator may be an adjustable frequency oscillation or a further circuit of the type shown in the Figure but operating on a tenth of the frequency and a still further similar circuit may provide an even smaller subdivision. In Fig. 6 the invention is used in a receiver or frequency meter. The incoming signal fs is applied through a low pass filter to a first mixer the output of which is passed through a band pass filter to a second mixer and a detector. An output fr of a variable frequency oscillator is applied to a first mixer and also to a mixer to which the harmonics are applied. The output from the latter mixer is fed through a band-pass filter and given a variable frequency shift fi by a further mixer and shift-frequency generator, the resulting output frequency fr - nfc Œ fi being applied to the final mixer to produce an output frequency Œ fs - nfc Œ fi. The latter band pass filter has a pass-band above the highest of the harmonic frequencies. The detector may be of a zero beat type, or of fixed or variable frequency type and the variable shift-frequency generator may be a continuously adjustable oscillator or may be variable step wise and take the form of the circuit shown in Fig. 5. The mixer circuit producing the frequency shift fi may alternatively be connected in the signal path after the signal band pass filter Fig. 7 (not shown). It is stated that the frequency appearing in any two of the band pass filters may be transposed to a different frequency by the use of further mixers and filters and a further common oscillator, Fig. 10 (not shown).