Method and apparatus for testing a frequency synthesizer in an RF modem

A method of testing a frequency synthesizer in a radio frequency modem having a transmitter section in which incoming data modulates a carrier provided by an IF oscillator before the modulated IF signal is translated to an RF signal as a function of a local oscillator signal generated by the frequency synthesizer and a receiver section having a double balanced mixer circuit for mixing a received RF signal with a local oscillator signal normally provided by the frequency synthesizer, said method including the steps of:

• causing said frequency synthesizer to produce a local oscillator signal having a nominal 3 IF frequency;
• driving said transmitter section to generate a modulated signal having nominal frequencies of 3 IF.+-.IF;
• transmitting the modulated signal having the nominal frequency of 4 IF to the input of the receiver section;
• selectively isolating the double balanced mixer circuit from the local oscillator output of the frequency synthesizer and supplying directly the output of the transmitter section intermediate frequency oscillator as a local oscillator input to the receiver double balanced mixer circuit for mixing an IF input with the 4IF signal received from the transmitted signal;
• passing the mixed signal through the receiver section IF filter;
• determining whether a significant signal has been passed through the IF filter, such signal indicating an RF signal having an actual frequency of 4 IF has been mixed with a third harmonic of the IF input to the double balanced mixer circuit.