This is an experiment to push the limits of the A/D board. I've been using a
wireless transceiver that operates in the 915 MHz ISM band. So I decided to try
analyzing the signal with my MHZ100Q A/D board. Normally, each A/D is preceded
by a 30 MHz antialiasing filter, as shown in the figure at left. For this experiment
I removed the antialiasing filter and associated buffer amplifier, and feed
the signal directly into the A/D converter. The data sheet for the AD9283 A/D
converter indicates that the frequency response is down about 5.5dB at 600 MHz,
and the response is not specified above that frequency. So it seems likely that
the A/D will still have some response at 915.
One of the 4 input channels was modified as shown: the signal was AC-coupled directly to the input of the A/D.
The Radiotronix transceiver puts out 0 dBm to 15 dBm, depending on the
parameter settings. I used short-range wireless coupling for this test:
a 2-inch wire connected to the MHZ100Q input serves as the receiving
antenna, and the normal Radiotronix antenna is connected to the transmitter.
The transmit and receive antennas are placed close together to provide a
strong signal for the A/D.
The first waveform was captured with the transceiver set at maximum output
power level. As shown, the captured amplitude is about +- 20 counts, of a +- 128
range, so the A/D is only a small part of the A/D range is in use.
The associated spectrum clearly shows the peak at 2.539MHz. The
input signal was set to 902.712MHz, so the expected peak location is 2.712MHz, for
an offset of 173kHz. Both the transmitter and the A/D clock are based on
crystals with 100ppm accuracy, so the measured frequency is within the
Another sample shows the signal when set to minimum output power, and 906.808MHz
transmit frequency. As expected, the peak location moves and the amplitude drops.
At 915 MHz, the A/D still have enough response to verify basic operation of a strong input signal. It's possible to verify that the input signal is changing in frequency and amplitude based on control inputs to transceiver.
The A/D is being operated well above it's specified operating frequency range, so detailed measurements of parameters like absolute signal level and spur amplitudes are not reliable.
You can post comments on my blog site at blog.sensicomm.com/2009/08/915mhz-signal-with-100mhz-ad.html.
More info about the project is on my company website at
$Date: 2009/08/12 20:40:57 $