FI63645C - BLANDARFOERSPAENNINGSKRETS FOER VARAKTORTELEVISIONSMOTTAGARE - Google Patents

Description

EäPTl M ^ ANNOUNCEMENT - JRSff l J UTLÄCCNINOSSKIIIFT 63645 c Patent granted 11 07 1983

Patent meddelat ^ T ^ (51) Kv.ik.3 / i «» ta3 H 03 J 3/06 SUOMI — FINLAND pi) F »eMeiw ^ -hM« i «iew» i 762357 (22) Htkemtopllvt — Amttnlnpdtg l8 .O8.76 '' (23) Alkupllv · —GlMglMCadag 18.08.76 (41) Tullut JulklMkal - illvtt offuntllg 26.02.77 ^ Mäntti · ja rakiatarihallitin .... ....._________ t, .. „, _ ' (44) Date of entry into force of the contract. - ratant · och raglstaratyralMn '' Ameku uttagd och uti. * KrtfMo puMkand 31.03.83 (32) (33) (31) Pjryd ** ty «tuelkuui — Bugtrd prtorlut 25.08.75 USA (US) 607698 (71) RCA Corporation, 30 Rockefeller Plaza, New York, NY 10022, USA (72) George William Carter, Indianapolis, Indiana, USA (7 ^) Oy Kolster Ab (5 * 0 Agitator bias voltage circuit for varactor television receivers - Blandarförspänningskrets för varaktortelevisionsmottagare This invention relates to a radio frequency tuner, is a varactor-controlled local oscillator, said oscillator providing output signals of different amplitudes at different frequencies, the tuner comprising parts for switching the selected excitation level voltages to the oscillator for each channel to be tuned, a mixer connected to this oscillator

When using superheterodyne television receivers, the local oscillator is tuned to a relatively wide frequency band to select television channels in the VHF range. For example, in the USA, the frequency of the VHF local oscillator may vary from 101 MHz on channel 2,129 MHz on channel 6 and then in 6 MHz steps from 221 MHz on channel 7 to 257 MHz on channel 13. When such tuning is performed, the range varies from 2,63645 the amplitude of the signals produced by the local oscillator typically from a relatively small amplitude on channel 2 to a relatively high amplitude, e.g. on channel 6. Large variations in the amplitude of the local oscillator signal cause the mixer gain to change substantially and thus adversely affect signal-to-noise ratio and / or cross-modulation. As a result, it is desirable to provide a mixer circuit that provides improved gain for relatively low amplitude oscillator signals and still also high gain for higher amplitude local oscillator signals. In particular, it is desirable to select the mixer bias voltage and oscillator amplitude relatively accurately for channel 6 operation due to channel 6 color jitter problems.

The tuner according to the invention is characterized in that it has parts for connecting at least a part of the above-mentioned selected tuning DC voltage to the mixer to bias the mixer so as to provide a higher conversion gain for low-amplitude oscillator signals than for high-amplitude oscillator signals.

In the accompanying drawings, the figures show the following:

Figure 1 is a partial block diagram and a partial schematic view of a superheterodyne receiver incorporating the present invention.

Figure 2 illustrates typical operating values for the tuner of Figure 1 for both low and high level oscillator supply voltages.

Referring now to dry 1, RF amplifier 12 is tuned to receive signals representing modulated images. The signals processed by amplifier 12 are coupled through capacitor 14 to mixer amplifier 44. The varactor-controlled local oscillator 16 further provides selected fixed frequency oscillator signals to mixer 44 via capacitor 18. The channel selector 20 provides a channel selection tuning DC voltage and connects this voltage via a switching terminal 19 to an amplifier 12, an oscillator 16 and a mixer 44. The tuning voltage applied to the oscillator 16 is connected to a certain voltage-varying capacitance or varactor diode 102.

II

63645 a frequency of the oscillator 16, wherein the oscillator has a bipolar transistor 100 arranged in a ground base connection. The excitation voltage connected to the RF amplifier 12 is also connected to varactor diodes (not shown) to control the frequency response of the amplifier 12. The channel tuning voltage is provided by an apparatus that includes a plurality of channel selection switches 200 connected to a voltage divider circuit 202 and a reference voltage supply source 204.

The mixer amplifier 44 includes a dual input MOS transistor 10, the first input electrode 1 of which is connected via a capacitor 18 to the output of the oscillator 16. The excitation-DC voltage applied to the switch terminal 19 of the channel selector 20 is also connected to the taste of the resistor 22 on the first input electrode 1 to provide a gain-adjusting bias voltage, as will be explained below. A bias circuit consisting of resistors 24, 26 and 28 connected to a bias supply voltage source (+ 16V) provides relatively fixed biases to both the first input 1 and the second input 2 of the transistor 10. The source electrode of the transistor 10 is connected to the bias resistor 30 and the RF bypass capacitor 10. connected to a low pass filter consisting of an inductor 34 and a capacitor 36. The signals provided by the low pass filter 34 and 36 are connected via a resistor 38 to the television processing circuitry 40. The processing circuitry 40 processes intermediate frequency signals generated by mixer 44 and generates suitably demodulated video signals to

In the operation of the circuit described above, the television channel is selected by developing a suitable tuning voltage at the tuning control terminal 19. This voltage may be in the range of 1 ... 20 volts on channels 2 ... 6 (lower UHF range) and 9 ... 26 volts on channels 7 ... 13 (upper UHF range), and the voltage in question is determined by the viewer (or service technician) by adjusting the potentiometers 202 so that the desired channels can be received. The desired channel is selected by closing one of a plurality of channel switches 200 connecting the corresponding channel excitation voltage to RF amplifier 12, local oscillator 16, and mixer 44. The excitation voltage applied to RF amplifier 12 is used to bias variable capacitance varactor diodes (not shown) in the RF circuit. The selected RF signals received by amplifier 4 63645 are amplified and applied through capacitor 14 to the first input electrode 1 of MOS transistor 10 in mixer 44. The excitation voltage applied to oscillator 16 via switch terminal 19 is similarly used to bias the associated varactor diode 102 to match the frequency of this oscillator. .

The amplitude of the signals provided by the oscillator 16 typically varies as a function of frequency. For example, in the lower UHF frequency range, the amplitude of the oscillator signal in the illustrated arrangement may be as low as 0.2 volts when selecting channel 2 and as high as 0.8 volts when selecting channel 6. This large variation in the amplitude of the oscillator signal adversely affects the conversion gain at which mixer 44 operates. In order to provide optimal conversion gain from the mixer 44, it is desirable that the oscillator signals from the oscillator 16 be of sufficient amplitude to change the conductivity of the transistor 10 from its extremes, e.g., in the saturation state to the clear off state. This operating situation would require a bias voltage in the mixer that is relatively low in the transform gain characteristic, as will be shown below. For oscillator signals with a smaller amplitude, the maximum possible mixer conversion gain is realized when the oscillator signal varies in the range of the maximum slope in the characteristic curve of the transistor 10. In the case of MOS transistors (and other mixer devices), the range of maximum slope generally does not occur midway between the saturation state and the quench state. As a result, it can be shown that operation for low-amplitude signals requires the mixer to be biased differently than when operating with higher-amplitude signals.

An additional factor used to determine the mixer bias is the effect of the color silicon of channel 6. The color silicon of channel 6 generally occurs in locations where a strong signal is received at channel 6 and is due to the fact that due to the frequencies associated with channel 6, mixer and intermediate frequency amplifier nonlinearities may cause harmful silicon frequency components located in the color receiver color channel. To minimize these peaks, a 5 63645 mixer sleep bias is typically selected to provide a relatively silicon-free image on channel 6.

However, as stated above, in order to obtain the maximum possible mixer gain with low-amplitude oscillator signals, it is desirable to change the mixer bias voltage according to the amplitude of the oscillator signals. When this bias voltage is appropriately changed, the mixer can be made to provide a higher conversion gain despite large variations in the amplitude of the oscillator signal without unnecessarily distorting the resulting image.

Referring to Figure 2, there are shown characteristic curves of transistor 10 for a voltage (Vq2s) between input 2 and source electrode of about 4 volts. This characteristic is non-linear in nature as required to provide signal mixing. The radio frequency and local oscillator signals are mixed in the transistor 10 by feeding these signals through relatively small capacitors (14 and 18) to the input electrode 1. While maintaining an amplitude ratio of about 100: 1 between the oscillator and radio frequency signals, the intermodulation effects and the resulting signal can be minimized. As shown above, the gain of the mixer is maximized by providing the maximum possible variation in transconductance (G „) for the input oscillator signals. In the case of illustration, a relatively large local oscillator signal 202, such as for channel 6, is shown compared to characteristic 200. A large variation in conductivity in transistor 10 can be obtained by adjusting the bias voltage at input 1 of transistor 10 to a point such that signal 202 peaks from peak to peak. conductivity variation in transistor 10 and in fact force transistor 10 over the off point. With respect to the characteristic curve 200 and the signal 202, such a variation is obtained when the voltage between the input 1 and the source electrode is selected to be about -0.9 volts (the voltage of the input 1 with respect to ground is about 4.2 volts).

For low amplitude local oscillator signals such as those illustrated by curve 204, transistor 10 may also be biased to provide maximum conversion gain. In the case of low-amplitude oscillator signals / 63645, e.g. those provided by the oscillator 16 for channel 2, maximum mixer gain can be achieved when the input 1 of the transistor 10 is biased to form an operating point within the maximum slope of the characteristic 200. The magnitude of the bias voltage between input 1 and the source electrode, which is suitable for providing a maximum mixer gain for the signal illustrated as 204, is about -0.5 volts (the voltage of input 1 with respect to ground is about 2.3 volts). In order to obtain a maximum amplifier gain, it is therefore desirable to supply a bias voltage to the input 1 of the transistor 10 which varies according to the amplitude of the local oscillator signal. Since the amplitude of the signals of the oscillator 16 increases as the frequency increases, it is desirable to provide a bias voltage to the input 1 of the transistor 10, which also increases as the frequency increases. In the case of a tuner using varactor diodes, the excitation voltage provided by the local oscillator excitation voltage supply source 20 is higher at higher frequencies. When this excitation voltage is suitably applied to the input 1 of the transistor 10, the bias voltage of the mixer can be suitably varied according to the frequency of the local oscillator 16. This desired purpose is accomplished by combining the excitation voltage with the bias applied to the input electrode 11 from an 18 volt supply source through a voltage divider consisting of resistors 24, 26, and 28. This additional bias sets the idle bias level of mixer 44 independently of the selected channel.

The second input electrode 2 also receives bias voltage from the above bias resistors 24, 26 and 28. This second input electrode is effectively signal grounded by a radio frequency bypass capacitor 29 and enhances the signal gain of the transistor 10 by causing the transistor 10 to act as a cascade amplifier.

In addition to the resistor 24 in the voltage divider circuit with the resistors 26 and 28 providing a fixed bias voltage to the input 1 from the fixed voltage supply source 18, it is also part of a second voltage divider including the resistor 22. This second voltage divider divides its variable excitation voltage at the excitation control terminal 19. the voltage across the resistor 24 has a fixed component that is independent of the excitation voltage of the varactor and a component that varies according to the excitation voltage.

Claims (7)

1. A tuning device having a varactor controlled local oscillator which provides output signals of different amplitude at different frequencies, which tuning device forms a combination which includes means for coupling selected tuning DCs to said tuning oscillator for each tuning channel and a pair of said oscillators which is operable in response to said oscillator output signals and to received radio frequency signals and which is adapted to provide such frequency output signals, characterized by means (22, 24), for coupling at least a portion of said selected tuning equalizer ( to provide the mixer a bias voltage and thereby provide greater conversion gain for low amplitude oscillator signals than for large amplitude oscillator signals. Device according to claim 1, characterized in that said mixer (44) includes a transistor (10) with an input electrode (1) arranged to receive said radio frequency signals, said local oscillator signals and said tuning DC voltage, and an output electrode for providing signals representative of the difference in frequency between said local oscillator signals and said radio frequency signals.