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Switching the second local oscillator frequency alone will not eliminate this type of spur problem. Thus, there remains a need for improvement in the elimination of receiver spurs. The present invention overcomes these and other disadvantages of the prior art by providing a combined signal level meter and leakage detector into a single handheld unit having a built-in antenna for leakage detection. Provisions are made in the preferred embodiment to maximize the common circuit portions for both modes of operation while maintaining other portions separate as appropriate in view of considerations such as circuit complexity and cost as well as RF interference and other performance criteria.
According to another aspect of the invention, a multiple conversion receiving system is capable of avoiding receiver spurs by combined switching of multiple local oscillator frequencies. The system includes a means for selecting an alternative frequency for a first IF signal and for correspondingly changing the frequencies of first and second local oscillators, as well as a means for switching the frequency of the second local oscillator between a low-side injection frequency and a high-side injection frequency.
These and other objects of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same.
It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
With reference to FIG. The switch functions are generally described as follows:. The meter is adapted for vehicle mounting and unit 10 is shown in the vehicle mount 20 in FIG. The meter is also adapted for snap-in mounting in a protective case for handheld use, as will be further described later.
The rear bezel 22 of the meter case 24 has a built-in loop antenna 26 for the leakage detector as shown in FIGS. The antenna is placed in a slot 28 of the rear bezel and held in place with potting compound.
The above-described function keys 16 and spin knob 14 provide inputs to a microcontroller 30 which, among other things as will be described, provides control signals to a three-stage attenuator ATTEN 32 and first and second local oscillator phased-lock loop ICs 34 and 36 PLL1 and PLL2.
The three-stage attenuator preferably provides 20 dB of attenuation in each stage, and each stage is designed around a Takamisawa UR1LD5WK5V latching relay arranged to selectively switch an attenuator pad into or out of the signal path. The first local oscillator includes a low-band oscillator 38 and a high-band oscillator 40 connected to the PLL IC and associated loop filter in a conventional manner.
Similarly, the second local oscillator includes an oscillator 42 operable during SLM mode and an oscillator 44 operable during leakage mode, these oscillators being connected to mixer 46 and mixer 48, respectively. Further details of the first mixer 50 are shown in FIG. For purposes of spur elimination, as will be described in detail, the first IF filter preferably has a bandwidth of approximately 30 MHz centered at MHz, with a relatively flat response from MHz to MHz, as illustrated in FIG.
A suitable mixer 48 is Avantek type IAM, an active mixer. In SLM mode, the receiver is tunable from 5 to MHz in steps as small as 50 kHz, under control of the first local oscillator which is tunable from to MHz.
The first IF frequency is nominally MHz, although that frequency can be varied as will be described, and the second IF frequency is fixed at 47 MHz. The second local oscillator is tunable either to MHz or MHz, as will be described in further detail. The input frequency range for leakage detection is MHz, and, correspondingly, the second local oscillator is tuned from MHz during leakage detection mode. The second local oscillator is capable of tuning in The leakage detector input is from either the internal antenna described above with reference to FIGS.
In either case, the leakage detector input from the antenna is supplied first to a varactor-tuned filter 58, an RF amplifier 60 and a 30 dB attenuator 62 as shown in FIG. The output of the 30 dB attenuator is connected to mixer 48, which is an active mixer the output of which is combined with that from second mixer 46 in a diode network 64, as shown in FIG. The third mixer is an active mixer, preferably Signetics type NEAD, having an oscillator input from a third local oscillator 72 which operates at The third mixer provides a third IF signal at a nominal frequency of Filter selection is controlled by microcontroller The amplifiers are preferably LM op amps, commercially available from Motorola.
The RSSI output is coupled to a peak detector UD before it is supplied to the microcontroller, for purposes of measuring peak video level. A 5 volt level has been found suitable for such purposes. The outputs of the pot are summed together and supplied to an audio amplifier 88 and speaker 90, as shown in FIG. The microcontroller provides a volume control signal to the digital pot, and also supplies tuning control signals to IF switches and to a power switch circuit, as will be described, in addition to data and control signals provided to the LCD display.
The left LCD is preferably used for display of signal level and function number, and the right LCD is preferably used for frequency and channel and related information. In SLM mode, the level of the RF signal is displayed numerically, and in leakage mode the leakage level is displayed in the form of a series of six vertical bars on the left LCD, with the level corresponding to the number of visible bars.
In addition to the visual indication, the leakage level is also indicated by an audible signal which increases in frequency as the level increases.
The RS output enables transfer of data collected during data logging to an external device such as a personal computer. The EEPROM is used for storage of data collected during data logging and has sufficient memory to store the level of all system carriers at up to 24 sites.
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