Reflow soldering exploration

Soldering using hand is slow and not easy in quality control, so try reflow soldering method, a solder paste in syringe package is bought and, it contains Sn63/Pb37.

Solder paste is usually apply through solder paste stencil and we don't have a stencil today, we apply it by hand, it takes a steady hand.

next step is placing components on the pads, requires steady hand and intense focus. the pads is bit sticky because of the solder paste, i can't see clearly if the SOIC package IC AD607 is correctly placed, i hope it would goes to the right alignment during reflow.

and insert the board into reflow oven. heat it up using the default temperature profile. it takes about 10 mins, with some bad smell.

after reflow, most of the component is properly soldered, we noticed there are two pins on AD607 is falsely joined together, because of too much solder paste, and some legs of resistor array is not soldered, because the solder paste is not enough, and the lower left side clock IC is drifted away from the pad totally, because the pads are too apart and it not suit for reflow soldering.

The reflow soldering process looks good and it saved a huge amount of manual soldering time, it can be improve in the way that some pads needs to revise in shape to suit for reflow soldering.

a feedforward bpsk demodulation algorithm

SDR receiver system development board

a completed software defined radio receiver system development board, covered frequency range up to 500MHz. going to submit to PCB factory. This design utilized programmable MEMS oscillator as local oscillator.

MIXER and IF amplifier chain: AD607 IF sub-system

ADC: AD9201 dual channel 10-bits ADC

Signal processing: PIC32MX MIPS based microcontroller

USB connectivity: FT245R USB-parallel FIFO

SI501 32kHz to 100MHz Programmable oscillator

Silicon labs SI501 one-time-programmable CMEMS Oscillator

We use crystals oscillator to generate local oscillator, however crystal oscillator comes with fixed frequency and we can only pick the standard frequencies listed by manufacturer, sometimes we needs a particular frequency which is not available in the market. Recently there are some programmable oscillator IC on the market, and these IC using on-chip MEMS resonator as source and PLL circuitry to convert the base frequency to the target frequency, in the range of 32kHz to 100MHz. 

The programmer, comes with a windows GUI, through USB.

I hope I can use these oscillator to replace some of the crystal oscillator in the projects

detect a signal in a noisy enviroment

to detect a signal in a noisy enviroment, setting a received power level threshold doesnt always work, because noise power often varies, for example in a quiet enviroment, the noise power is -100dbm and we think its ok to set a threshold at -90 dbm and anything above -90dbm is a signal, it works until we moved to an enviroment that is abit more noisy, say -80dbm noise and -70 dbm signal, then we would falsely report that there's always having a signal present.

what does really matter is the signal to noise ratio.

we have to normalize the receiver gain such that the output of receiver amplifier chain is always closed to saturated, in the above example we should normalize the received signal to 0dbm so the noise become -10 dbm. And the demodulator should always try its best to perform demodulation, once the  received signal to noise ratio is above a certain level, the demodulator would giving correct output data, by verifing the checksum of demodulated data, we can report with confident that a signal is presented.

Mini-circuit ADE-1 mixer DC modulation measurement

Input DC voltage is applied to IF port, RF input:40MHz, 5dBm

Voltage mV	Current mA	Attenuation db
600	130	5.1
550	105	5.3
500	79	5.6
450	54	6
400	31	6.4
350	14	7.1
300	6	8.3
250	4	10.3
200	3	13.1
150	2	15.2
130	1	19
100	1	23
80	1	28
60	0.5	37

Mini-circuit ADE-1 mixer S11 Reflection coefficient measurement

The mini-circuit ADE-1 mixer is a diode-ring type passive mixer. As a +7dBm mixer work best at +7dBm of local oscillator input, or at least +4dBm to make it start working. It sounds like a quite huge power, so may be impedance matching is needed to optimize the efficiency of driving this mixer.  

How to do impedance matching? ok, may be we need to know the impedance of this particular component. As not very obviously showing in the picture, the IF and RF port of the mixer is connected to 50 ohm resistor for proper termination.The LO port is connected to the network analyzer for reflection coefficient measurement. Through a 1nF (-16j ohm@10MHz to -1.6j ohm@100MHz) capacitor in series.

The network analyzer is set to 10MHz to 100MHz, in -10dBm low power level, it have a quite high impedance(313ohm@10MHz) and a bit of capacitive (-22j ohm@10MHz).

In -5dBm power level, it's impedance is lowered.

In 0dBm, it's impedance is quite close to 50ohm

In +4dBm, even lower.

In +7dBm, even lower

In +10dBm, Well, it's quite hard to drive because impedance is quite low.