Two channel frequency synthesizer

Posted on May 22, 2012

This instrument is a one or two channel frequency synthesizer. DC to 35 MHz frequency with 0.1 Hz resolution. +16 to -48 dBm signal level with 0.1 dB resolution. adjustable phase between the 2 channels with 11.25 degree resolution. pushbutton and knob control over user settings; knob acceleration for frequency adjustment. TTL-into-50-ohm sync outputs for each channel; may be switched off to reduce noise. selectable internal or external 10 MHz reference. easy RS-232 interface for full external control tabble-lookup calibration for frequency-independent signal level.

Two channel frequency synthesizer
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The heart of the synthesizer is a set of 2 Analog Devices AD9850 direct digital synthesis (DDS) chips. These provide 2 channels of sinewave output which may differ in frequency and relative phase. The output stage of the synthesizer also includes separate variable attenuators for the 2 channels. These attenuators have a range of 0.0 - 63.9 dB of attenuation, in 0.1 dB steps. The user therefore has control over five parameters: 2 output frequencies, 2 output attenuations, and relative phase between the 2 channels. DDS The DDS chips provide a 32-bit selectable frequency, and they use a reference frequency which is set at 130 MHz. Therefore, the DDS frequency resolution is 130E6 / 2^32 or 0.0327 Hz. In this synthesizer, the user is given 0.1 Hz frequency resolution with a frequency range from DC to 35 MHz. Therefore, there are a maximum of 9 digits of frequency which may be set by the user. The DDS DAC output spectrum includes aliased images of the output signal, the lowest frequency of which is located at Fref - Fout. A low-pass filter is needed to suppress these images; here we use a 42 MHz 5-pole elliptic filter, built from discrete components, that is specified in the AD9850 datasheet. The output of the DDS DAC is a current ranging from 0 to a fixed ratio of the DDS programming current. In order to produce an output signal with zero DC offset, a current equal to half of the maximum DDS current must be subtracted...

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