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Testing other components

A project log for Microwave Transmitter

30 - 6000 MHz synthesizer with power amplifier

PeroPero 08/09/2017 at 22:590 Comments

After verifying the main chip, MAX2871, it would be nice to see if other components work as well.

First, I tested 4PST switch SKY13322-375LF. The switch is controlled with 4 parallel inputs that switch RF input to one of four outputs. I wrote a simple code that allows user to chose (and change) desired output and measured the connection between input and corresponding output. I was hoping to hear that lovely "beep" when I send certain character on the serial terminal. Guess what, it didn't work. There was short circuit to the GND from all outputs. Bad soldering job with my reflow oven. After the rework, everything worked fine.

Second thing was the attenuation chip PE43711. It is controlled over SPI and attenuation can be set in steps of 0.25 dB with more or less precision over the frequency range. The attenuation is calculated as an 8-bit word/4. Since the chip doesn't provide any data output, I only could test it with a multimeter. I measured input and output resistance as well as resistance from input to output with various settings. My observations are in the table below:

Attenuation, dBR inR outR 1-2
042.7k42.7k11.6
138136916.7
219517521
2.2517515522
313811925
6816238
10634552
20523673

So, the resistance changes when different setting is chosen. I guess this suggests that SPI and chip work. 

Power amplifier doesn't provide any meaningful testing options - I can only hope it works when it is powered on. 

RSSI detectors are weird. I can measure detected signal using the ADC on Teensy. However, measured voltage is somewhere around 0.3 - 0.5V, even though there is signal at the input. Interestingly, the measured value drops to 0 when I put the voltmeter on the output pins. I have no explanation at the moment. My only guess is that troublesome soldering of BGA chips resulted in bad joints and ADC input measures any picked up voltage. Voltmeter probably than acts as a pull-down resistor and sets the measured value to 0. 

Overall, since synthesizer, switches, filters, attenuation and power amp work, I can hope to see generated signal with a spectrum analyzer.  When I get one. 

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