Wireless ADC Measuring and CAN Controller PCB

Features

• 57K6 RF 70cm Data transceiver.
• 8 Channel 14(or 16) bit ADC convertor.
• Frequency measuring input.
• CAN-bus interface.

Data from the ADC and info of the measured frequency input is transmitted to a (RF remote) host.
Data from CAN devices connected is sent to the host.
The remote host can sent data to the CAN bus.
The remote host can sent other RF commands that will be interpreted by the onboard CPU.
This allows the local CPU to control devices on the CAN bus without intervention of the (remote) host.

Measuring the transmit frequency.

The frequncy measuring is done by using a 12bit binary counter (74LV4040).
The output is then fed to the microcontroller section for further processing.
The input of the prescaler is driven by an LVDS line receiver.These driver provide 3V output signals and have a differential sensitivity level of around 100mV and have a 400Mhz bandwidth.
The datasheet can be found here.

Measurement Bridge : Modifications

In order to adjust the frequency response of the voltage divider the capacitive leg was splitted into 2 dividers.
The first is formed by C2 and C1 + C32. The secondary divider is formed by C30 and C3.
This construction makes it easier to use a low capacitance trimmer in adjusting for a flat response.

To compensate for the error caused by the inductive part of R4 a small capacitor (C33) was added in parrallel with R6. 

Measuring Bridge : First Measurements

Here are some results of the initial phase measurements made.
Only the voltage divider has been adjusted by changing the capacitive dividing part.
I used 1.8Mhz as reference and the load was a 50 Ohm Dummy load.

Freq       Phase Error
1.8Mhz        0°
18.1Mhz      1°
24.9Mhz      1.65°
29.7Mhz      2.65°

These errors seem te be caused by the self inductance of the shunt resistors, and i was able to compensate this by adding some shunt capacitance to these shunt resistors.
At 30Mhz and using an estimated resistor inductance of 1nH yields an inductive impedance of 180mOhm. Since our shunt resitance is only 5 Ohm, this presents a large unwanted inductive part.

I added a variable capacitor on the current shunt side and adjusted it so that i had zero error at 29.7 Mhz and then made the next measurement.
The Magnitude adjustment was done likewise, i adjusted the trimmer on the voltage divider for the so that i had the same magnitude on 1.8 and 29.7 Mhz.

Then i did a measurement on all bands.

Measurement

	1.8Mhz	3.5Mhz	7Mhz	10Mhz	14Mhz	21Mhz	29.7Mhz
Phase Error	0	-0.48°	-0.44°	-0.29°	-0.04°	-0.22°	-0.18°
Magnitude Error	0.02dB	0.04dB	-0.16dB	-0.2dB	-0.17dB	-0.11dB	0.03dB

As you can see from the above table, the remaining error is very acceptable and beyond expectation.

Here is an image from another angle showing the capacitive divider adjustment.

You can see the 3 small 1.5pF capacitors in series to create a total series capacitance of 0.5pF.
All used smd capactors are NP0//COG (Chip on glass)  These 1.5pF C's are rated for 1000V so they can be used even for high power levels.