Abstract:
The gain characteristics of certain stages of a tuner circuit are selected to be proportional, but opposite, to selected other stages such that the overall gain of the tuner remains constant over a wide temperature range. In one embodiment, the gain of the IF stage has a temperature slope opposite to the temperature slope of the input RF stage thereby avoiding the use of AGC circuitry.

Description:
TECHNICAL FIELD 
     This invention relates to single RF tuners and more particularly to a system and method for temperature compensation within single conversion tuners. 
     BACKGROUND 
     Single conversion tuner circuits are very prevalent. The first element in the frond end of the typical RF tuner is a dual gate FET amplifier. The inherent problem with these devices is that they are sensitive to temperature causing the signal level to change. Over an industrial temperature range of +85C to −40C, there is typically between 6 and 10 db of gain variation. One method of compensating for gain variation is to use automatic gain control (AGC) in a feed back manner. However, when gain is retarded, the noise level rises for this application by approximately between 6 and 8 db. For this application RF AGC is the only available method of retarding the system gain. As the temp goes down the RF gain goes up to counteract this action and the RF gain then needs to be reduced. Reducing the RF gain unfortunately increases the system noise figure, thereby decreasing the SNR out of the tuner. Thus, it is desired not to have to retard the RF gain setting. Also, it is desired to save the power of an AGC circuit which can be in the order of 40 mA, which does not seem like much, but when total available power is limited, and where the heat from excess power usage is important, 40 mA can be critical. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method which takes advantage of the fact that the first stage dual gate in the RF input circuit of a tuner is actually quite well behaved in terms of its gain variation versus temperature. In fact, there is almost a linear degradation in gain as temperature increases. For example, assume that at −40C, the system gain is 40 dB. In this example, we found that gain would degrade to 32 dB with an almost linear slope at 85° C. Using this almost linear property, I have designed an IF stage which has an opposite slope as a function of temperature and which maintains the proper linearity over the gain variation. 
     The circuit and method of this invention implements a thermistor in the common emitter bias of an IF amplifier tailored to precisely pinch off the bias current to reduce the gain of the IF amplifier as a function of temperature. The characteristic of the IF amplifier is selected to be opposite of the gain of the input FET so that the two gain curves exactly cancel. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 shows the slope of the gain of the tuner input as a function of temperature. 
     FIG. 2 shows one embodiment of a system and method for compensating for the input gain change as the temperature changes; 
     FIGS. 3A-3D show how to derive the proper compensation parameters; and 
     FIG. 4 shows a typical prior art circuit which requires some form of automatic gain control (not shown). 
    
    
     DETAILED DESCRIPTION 
     Prior to beginning a description of the invention, it might be helpful to understand a typical prior art circuit as shown in FIG. 4, where there is shown a standard configuration of a single conversion tuner  40 . The RF input  401  goes to band pass filter  41 . The RF which passes through filter  41  goes to the dual gate FET which is biased by element  43 . The FET gate is normally used to control the gain of the entire system for a single gain type of design. 
     The RF then goes to band pass filter  44  which actually is a channel select filter. The signal then goes to PLL mixer oscillator  45  where a frequency translation to an IF frequency occurs. For example, an RF frequency of 500 MHZ could be input to the tuner. The local oscillator would be programmed to 544 MHZ and it would mix the input signal down to IF to frequency of 44 MHZ and then to saw filter  46  which selectively selects out the specific signal. The IF signal then goes to the IF amplifier stage,  46 ,  47  which amplifies the signal to output  402 . 
     Turning now to FIG. 1, as discussed above, the gain of the input of the tuner is almost inversely proportional to temperature as shown by line slope  101 . The problem with this is if you have a fixed amount of input power on the front end, and the temperature varies, for example, if the temperature went very cold, you would have to reduce the gain by 4 db because the gain of the FET increases by that amount. When the 4 db of gain reduction is implemented (by an automatic gain control (AGC) circuit—not shown), the noise in the RF signal would be increased by 4 db, thereby changing the signal to noise ratio. For example, assume gain was 21 db at room temperature, it would become 25 db at the coldest temperature. This is a severe penalty considering the system requires a minimum 22 db of gain to operate. This means that the system would fail unless some type of compensation were to be used. 
     As will be seen, the inventive circuit adjusts the IF amplifier gain to compensate for the input gain change therefore does not require adjustment of the RF input gain to maintain proper output power. Essentially the inventive circuit is self-leveling over a particular temperature range so it maintains optimum noise performance of the system. This is accomplished by the circuit shown in FIG.  2 . 
     FIG. 2 is one illustrative implementation of IF amplifier  20  which implements the gain correction. This is accomplished by using thermistor  27  having a specified temperature sensitivity to it in the emitter resistor circuit. The selection of the proper transfer function of thermistor  27  is accomplished by measuring the gain of amplifier pair  21 ,  22  as a function of elements  25 ,  26 ,  27 . Gain is measured between the input terminals and the output terminals. Then a thermistor is selected which tracks as close as possible the measured curve. The effect is to design a circuit which has a gain increase as a function of temperature which is the opposite slope of what the FET (in the input) is doing. 
     Circuit  20  is basically a differential common emitter. Transistors  21  and  22  form the amplifier and are driven 180 degrees out of phase and biased with a T-structure. This is done to stabilize the design over temperature between components and from unit to unit. The emitters of transistors  21 ,  22  are tied together to produce emitter degeneration in terms of the DC bias points. Thus, if the temperature goes up and the bias current were to increase, the emitter voltage would rise, but the base voltage would remain the same. This effectively turns the transistor off regulating the gain in that regard at a given temperature. A problem is to find the right impedance for element  27  which will give the desired output to match the opposite of the gain slope of element  42  of FIG.  4 . 
     This calculation is shown in FIGS. 3A and 3B such that the gain of the IF amplifier  15  is measured as temperature is varied. In FIGS. 3A and 3B this is called the R bias and as that resistor is decreased, the gain is increased. Thus, as resistor  27 &#39;s value is increased, the bias current is pinched off thereby reducing the gain so the slope of the IF amplifier of FIG. 2 is negative in FIG.  3 A. FIG. 3B shows the selection of a thermistor (or any other element having a similar resistive change as a function of temperature) which will provide the desired bias resistor change as a function in temperature. When those two slopes are multiplied together, the result should be substantially equal and opposite to what the desired slope of FET  42  is. 
     FIG. 3C is a mathematical representation of the process used to calculate and measure these gains as a temperature function of the bias resistor. 
     FIG. 3D shows a circuit which results from actual measured data and the actual solution that was implemented. We know we want the slope to be equal and opposite to what we measured in FIG. 1 which was 0.059 db per degree C. Then the equation in FIG. 3C is solved for the resistence as a function of temperature. This calculation yields the resistor characteristics needed to produce a given gain change. In our example, it is 1,180.5 ohms per db. After multiplying these two together, we get 69.5 ohms per degree C which is the slope of the thermistor. We now select a resistor (or other element) with that performance. These calculations are based upon a particular circuit and, of course, different circuit parameters. For example, different differential IF amplifiers may have different characteristics and will have to be calculated in the same way. 
     While the embodiment shown has a single RF stage matched by a single IF stage, in fact, the same principles can apply for any set of stages. Thus, compensation for a single stage might be made by several stages or several stages might be compensated by a single stage or compensation for multiple stages can be made by multiple stages. Also note, that the compensation adjustment need not be perfect for applications, but should be within a certain range. For example, in the tuner discussed herein, the range would typically be about 8 db. 
     Also note that while temperature has been discussed herein as the variable against which gain has been measured, any variable could be substituted with a proper substitution of element(s) in the compensation circuit. Thus, the word temperature in the claims can also include such other variable. These other variables could be, by way of example only, light, moisture, pressure, velocity, acceleration, or a combination thereof. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.