Browse Prior Art Database

Acoustic Surface Wave Filter for TV Tuning Circuits

IP.com Disclosure Number: IPCOM000086297D
Original Publication Date: 1976-Aug-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 66K

Publishing Venue

IBM

Related People

Grice, DC: AUTHOR [+3]

Abstract

As described in Electronic Design, September 13, 1974, pages 152-153, mechanical tuners are being replaced with electronic designs but cost and DC stability problems have not been fully solved. Acoustic surface wave (ASW) filters and digital-to-analog converters (DAC's) are used in the circuits of Figs. 1 and 2 to solve these problems.

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Acoustic Surface Wave Filter for TV Tuning Circuits

As described in Electronic Design, September 13, 1974, pages 152-153, mechanical tuners are being replaced with electronic designs but cost and DC stability problems have not been fully solved. Acoustic surface wave (ASW) filters and digital-to-analog converters (DAC's) are used in the circuits of Figs. 1 and 2 to solve these problems.

Practical design considerations of ASW filters permit implementation of filters having a frequency range of 10-300 MHz. Because the 82 television channels cover a range of over 850 MHz, a multiplexing scheme using mixer 31 is necessary The multiplexing scheme requires a bifrequency oscillator 29 which is used to heterodyne the tuner local oscillator 19 down to a band of frequencies corresponding to the ASW filter frequencies.

The channel selector 11 controls the bifrequency oscillator 29 to generate zero frequency, a first frequency, or a second frequency to divide the television frequency range into three different hands. These bands are illustrated in the table below, with each of the bifrequency oscillator and low-frequency oscillator frequencies necessary to select each television channel.

Fig. 1 shows a circuit using a single ASW filter 21 in the harmonic mode. Each time manual input channel selector 11 is changed, a reset signal is generated to reset ramp counter 15 and channel counter 25. The setting of channel selector 11 provides one of three band selections to bifrequency oscillator 29, local voltage-controlled oscillator 19 and tuner 20 to generate the oscillator frequencies for the selected band as shown in the table above. Additionally, the channel selector 11 generates a digitally encoded channel number as an input to comparator 13, which is compared with the output of channel counter 25 to inhibit ramp counter 15 when channel counter 25 reaches the proper count to tune the selected channel.

Ramp counter 15 is driven by a 100 Hz clock and the output of ramp counter 15 is converted to analog voltage by digital-to-analog ramp voltage generator 17. The voltage ramp output from generator 17 causes voltage-controlled local oscillator 19 to sweep from its lowest frequency settings such as 98 MHz to its highest frequency setting such as 931 MHz. Mixer 31 heterodynes the wide sweep of frequencies generated by local oscillator 19 down to the range of 10- 300 MHz.

As local oscillator 19 approaches a 6 MHz harmonic, the output of ASW filter 21 increases to increment channel counter 25 through low-pass filter 23. When the count in counter 25 equals the channel selection from channel selector 11, compare 13 inhibits ramp counter 15...