A 1.6W, 40MHz power amplifier

Considering class AB and class B bias configuration in power amplifier, for class AB bias design, it provides a better power gain, so it require less driving power, but the drawback is the power transistor pass a quit heavy idle current, for this reason, the overall power efficiency is low. When the output power required is approaching the thermal limit of transistor, the low efficiency cause a problem , pushing the transistor got overheat and burn up before it can generate a good output power.

In this attempt of 40MHz power amplifier, the objective is to squeeze as much power as possible from a BLT50 transistor, the class B bias configuration is selected because of the better efficiency.

Using power supply of 10V, 647mA of current, an output power of 1.6W is achieved.

Two stages is required because the second stage class B amplifier required a pretty large drive power ~20dBm (100mW). L1 is 0.8mm wire with 7mm diameter, 7T. L2 is 0.8mm wire with 7mm diameter, 3T.

Spectrum analyzer shows 26dBm,incorporate with two -3db attenuator, the output power is 32dBm(1.6W). the second harmonic having a level of ~15 db below the fundamental.

The two-stage power amplifier consumes 0.647A of current, at 10V.

Input drive power required 5dBm.

Having adequate heat sink is important because the heavy power dissipation can burn the transistor up.

The waveform for power stage collector. Actual voltage is 4 times larger than the probed voltage, due to the resistor voltage divider.

The waveform for output matching circuit. The circuit consist of L2 and C1 L-section. Actual voltage is 4 times larger than the probed voltage, due to the resistor voltage divider.

The output voltage is measured 28V p-p which is larger than twice of the DC supply voltage 10V, because of the output impedance matching circuit.

RF Probing issue

When probing around a 500mW 40MHz power amplifier. There is an observation of leaked RF signal on oscilloscope probe, even it is grounded like the picture above.

the leak pickup may caused by the probe ground tail inductance.

And the leaked signal is quite strong, it is suspected that the antenna radiate the carrier around the board. 

This leaked RF pickup may contribute to the unexpected waveform during measurement. This probe is on the collector of transistor.

which is not expected to having voltage dip below 0V

The measurement method needed to be revised.

RF active probe, or differential probe would they help?

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.