Multichannel RF signal generator
BACKGROUND OF THE INVENTION 
     This invention relates generally to test equipment for radio apparatus and more particularly to means for testing radio receivers without the requirement of knowing a priori the frequency to which the receiver is tuned. Present methods of generating modulated RF signals for utilization in receiver testing rely on discrete frequency generation techniques. This requires knowledge of the frequency of the receiver under test and tuning of the RF signal generator is required. Signal generators for testing receivers for various modulation formats, e.g. amplitude modulation, frequency modulation, phase shift key modulation, frequency shift key modulation, and double side band modulation, tend to be rather large and exhibit moderately high power consumption. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a simple reliable means of generating a plurality of RF signals of a given modulation format, e.g. AM, FM, PSK, FSK and DSB, all RF signals exhibiting identical modulation characteristics. That is, for example, in the case of amplitude modulation, the modulation percentage would be the same for each of the RF signals generated. The present invention also provides for a simple means of generating test signals suitable for use in receiver testing without the requirement of knowing a priori the frequency to which the receiver is tuned. This technique is applicable to any receiver which can be tuned to any one of a plurality of equally spaced channels. A narrow pulse train is generated with spacing equal to the receiver channel spacing and is filtered to remove undesired high frequency components. The pulse train is fed to a first double balanced mixer where the signal is mixed with the output from a signal provided by a variable frequency oscillator which is controlled by FM or FSK modulation inputs. This modulated signal is further mixed in a second double balanced mixer with AM, PSK and DSB modulation signal inputs and provides the desired plurality of RF signals for performing the desired testing. 
     Accordingly, an object of the invention is the provision of a simple and reliable means of generating a plurality of RF signals of a given modulation format. 
     Another object of the invention is the provision of a simple means of generating test signals suitable for use in the receiver testing without the requirement of knowing a priori the frequency to which the receiver is tuned. 
     Another object of the invention is the provision of a simple and reliable means of generating a plurality of signals of a given modulation format, all signals exhibiting identical modulation characteristics. 
     Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein there is shown in the single figure a block diagram of the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein there is shown in the single figure a fixed frequency oscillator 10 for triggering pulse generator 12. Oscillator 10 should operate at a frequency equal to the channel spacing of receiver under test. The output of mixer 16 is fed to a second doubled balanced mixer 20 and is of the type that has the capability to vary the amplitude and phase (zero degrees or 180 degrees) of the output of mixer 16. AM, DSB and PSK modulation inputs are coupled to mixer 20 to provide the multiple modulated RF signal to attenuator 22 which is used to set the level of the RF output signal at output terminal 24. 
     In operation and by way of example, the signal generator is designed to operate with a receiver, operating in the 225 to 400 MHz frequency range, and which has a 25 kHz channel spacing. Fixed frequency oscillator 10 then is operating at 25 kHz. Oscillator 10 causes pulse generator 12 to output a pulsed train with a PRF of 25 kHz. The Fourier transform of this pulse train is a comb of frequency components equally spaced at 25 kHz. 
     The output from pulse generator 12 is fed to low pass filter 14 where undesired high frequency components are removed. If these undesired components were allowed to pass the double balanced mixer 16, unwanted frequency components would be subsequently generated. 
     The output of low pass filter 14 is fed to double balanced mixer 16 where it is mixed with a signal provided by variable frequency oscillator 18. If the output frequency of oscillator 18 is caused to change by applying a suitable voltage to the VCO input, the mixing process of double balanced mixer 16 causes all output frequency components to be simultaneously shifted by an amount identical to the oscillator shift. It is this control voltage which allows for variable frequency modulation schemes such as FM and FSK. 
     The output of mixer 16 is fed to a second double balanced mixer 20 where it is mixed with a suitable control signal. By virtue of the capability of mixer 20, this control signal allows the implementation of amplitude and phase (zero degrees or 180 degrees) modulation schemes such as AM, DSB and PSK. Balanced mixers such as model ZAD-2, manufactured by Mini Circuits Laboratory may be used as double balanced mixers 16 and 20. The output of double balanced mixer 20 which is the desired comb of modulated RF frequency components, is fed to RF attenuator 22 so that the output level to the receiver under test may be accurately controlled. Therefore, regardless of which channel the receiver under test is operating, there will be a test signal present. Also the modulation type, and amount will be the same for each of the individual comb components. 
     Obviously, many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.