I’m beginning to try getting into some RF stuff.
I decided to start with a SDR receiver, based on the LT5517 chip.
You can find it’s data sheet here: LT5517
The chip is a quadrature demodulator that it’s specified from 40MHz to 900MHz. It can be pushed to lower frequencies by increasing the LO drive.
It works pretty good down to 3.5MHz ( 7MHz LO ) with ~ 0dBm drive.
Once LO gets over 40MHz ( 20MHz RX ) the LO can be as low as -15dBm with no problems. Usually I keep it at -10dBm for convenience – the signal generator I’m using now to generate the LO has a dedicated key to modify the output signal in 10dBm steps.
Lately I’ve got my hands on this little toy:
It’s a 1981 vintage CRT camera viewfinder that has a huge resolution even if it’s very small. I had to modify it to make it work without the camera: I’ve added a voltage regulator for the 6V rail that was provided by the camera, and created a constant current sync for the CRT heater – oddly enough it seem to be also in the camera body.
the current limit is simple : a NMOS pass transistor , a 10K pull-up resistor from gate to VCC, a sense resistor from source to gnd and a BJT with collector to mosfet gate, emitter to ground and base to the sense resistor -> the entire think tries to keep 0.6V across the sense resistor. The filament current is at ~ 80mA – enough to make the CRT tube work with a very faint glow from the heater. The entire thing work from 9V and uses ~ 300mA
Lately I’ve been interested in some RF stuff.
The “goal” is to build an entire RF spectrum analyzer with a range of about 50-100MHz.
For this I have to learn basic stuff like filters, oscillators, amplifiers, etc.
This implies having some basic tools like an oscilloscope, RF power meter, signal generator, etc.
The oscilloscope was already present so I’ve build a simple RF power meter using an Analog Devices log amplifier: AD8307
The device uses the basic connection – figure 32 from the data sheet. The INP signal is connected to a 50 ohm terminated BNC input via 100nF capacitor.
The INM pin is connected to ground via another 100nF capacitor.
The output signal is fed to a LM358 op-amp connected as a buffer. The op-amp output goes via a RC filter to the output.
The entire RF section is build dead-bug style in a small enclosure made from FR4 pieces. I’ve run out of space on the base plate, so I had to build it in a “3d” fashion.
Here are some pictures:
The read-out is based on an 4×20 lines LCD display driven by a STM32F0 board. The detector’s output is connected to one of the ADC’s input.
The microcontroller code samples the ADC, creates a buffer containing 256 values from witch it extracts the Min / Max and medium values. The displayed values are updated around 2 times / second.
I’m still working on the code. For now i’ve created a simple calibration table in 50Mhz steps, from 50 to 500MHz.
New toy – a Philips PCB80C552-5-16WP based single board computer.
The board is designed to have 64KBytes of EEPROM from 0x0000 to 0xFFFF, 32KB of SRAM from 0x0000 to 0x7FFF and 32KB of peripherals space from 0x8000 to 0xFFFF
On the same board I’ve placed a SPI RTC, EEPROM and 4.096 voltage reference for the on-chip ADC.
Just got a new toy, to start working on some interesting projects: an HP3466A almost my age, but still kicking
Here are some pictures:
This is a quick teardown and fix of a Chinese variable power supply.
While trying to measure a negative power supply I’ve discovered that the negative terminal was shorted to main’s earth and case
In this post i’ll show a quick walk-trough on how I managed to re-use an old Asus A6RP that had a sad ending.
In a few words, the entire case got smashed to plastic splinters. Luckily the internal components survived.
The case was rebuilt around two 4mm transparent acrylic sheets and 2.5mm pcb stand-offs, 10 and 15mm tall.
This is an update for the led lighting project.
The design in complete and the PCB’s are almost ready.
This is the schematic:
I’ve started a new project: a RGB led lighting module, that can be controlled from a central communication node.
I’ll be using some SMD rgb leds, in 5050 case, that i got from e-bay.
The Imax is specified to 40mA, but i’ll be running them at 30mA max, to keep the heat down.
The design features are :
- 50x50mm per module
- cheap controller, JTAG interface, enough cpu power for high resolution/ speed PWM
- the ability to run stand-alone, following a pre-programmed sequence
- RS485 line drivers
- wide input voltage range
Ca o prima postare am ales un proiect mai “aratos”: un ceas cu afisaj VFD
Pentru afisaj am folosit un tub IV-18:
Caracteristicile tubului sunt:
- 9 digiti, 8 cu 7 segmente si virgula, 1 cu simboluri speciale.
- tensiune de filament : 5V la 85mA
- tensiune anodica: 20-30V
- curent de grila: 11mA per digit
- curent anodic: 8mA per digit, cu toti anozii aprinsi
Mai multe informatii puteti gasi aici