charlie@oakhill.UUCP (Charlie Thompson) (09/29/89)
Speaking of digital mixers.... the DSP56ADC16 (and any other oversampling converter for that matter) samples an analog input at a high rate of speed. For example on the DSP56ADC16, the input sample rate can be selected to be from 500 KHz up to 6.4 MHz. Signals whose center frequencies are in this range can be 'aliased' down to d.c. ( i.e. synchronously detected). The passband of the decimation filter following the input sampling process can provide selectivity. For a 500 KHz input rate the output passband will be +- 3.9 KHz as determined by the cutoff of the FIR filter. At 6.4 MHz the passband will be +-50 KHz. If this sounds confusing think of the oversampling A/D as a type of direct conversion receiver when used in the above described fashion. To prove that this works I connected an FET preamp/tuned circuit/antenna to the input of a 56ADC16 and set the sample rate to 7.150 MHz (a litte faster than the data sheet allows for) and sucessfully received upper sideband ham radio transmissions on 14.3 MHz. I could 'tune in' the received voice transmissions by tweaking the sample rate. The one thing to point out here is that the converter will demodulate any integer multiple of the input sample rate...7.150 x 2 = 14.3 MHz. The tuned circuit/FET preamp will effectively 'select' which harmonic of the clock defines the receive frequency. This is sort of like a thing known as a 'harmonic mixer', frequently used in RF designs. Sensitivity is defined by the LSB which in the case of the 56ADC16 is about 60 microvolts. I was in Texas the station I was receiving was in California. Has anyone else tried this?? Charlie Thompson