Control board

The 3’rd in the series is the control board.
It’s purpose is to allow easy interface from a single-board computer that has one SPI and one I2C port.
Also, it provides a common reference clock and a synchronous update signal to the two DDS boards, as required by AN-587
The SPI port is level-shifted to a MCP23S17-E/SP SPI to parallel expander.
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Category: VNA

AD8302 detector

The second board in the VNA is the phase / frequency detector.
For this one I chose an AD8302 chip.
It will measure in the range I’m interested in.
It might not be the best part for this, but for the initial design it’s enough, as it works out-of-the box.

The board contains:

  1. two low-pass filters, identical to the ones used on the DDS boards
  2. a fixed 30dBm atenuators for the reference input
  3. two switched 30dBm atenuators for the DUT input
  4. a AD8302 detector
  5. a MAX11612 12bit ADC to read the detector
  6. LDO, passives and decupling for the active components
  7. transistors and diodes to drive the two atenuators switching relays

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Category: VNA

AD9851 signal generator

The signal generator is based on the AD9851 DDS chip
The board contains:

  • AD9851 chip and passives
  • 2 5V LDO’s for the digital and analog DDS power supplies
  • footprint and passives for a local 30MHz oscillator, for stand-alone use
  • 50MHz low-pass filter copied from the N2PK VNA project:
  • several footprints for atenuators
  • a 1:2 50ohm power spliter
  • 2 gain blocks based on the MSA-0286 MMIC amplifiers
  • different connectors for RF / CLOCK / power / DDS data

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Category: VNA

General description

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While working on RF stuff I got to the point where I had to tune filters and measure frequency response of amplifiers and other tuned circuits.
This implies having access to a VNA.
As something like that is out of reach, and buying an existing one is out of my budget I started out designing one from scratch.
For that I’ve used the N2PK VNA as a starting point with some small changes that I though will be useful.

The entire system will have:

  1. Two AD9851 based generators, each with 2 buffered outputs
  2. One AD8302 based detector that measures phase and amplitude differences
  3. One system control board, that offers to the driving computer a SPI and an I2C interface

Each board has a 12V input and local LDO’s for all needed voltages.
12V was chosen because the value of the MMIC’s bias network was high enough to require just a generic inductor.
I’ve used 5V for all internal supplies as all chips used that voltage.
Also, I’ve used a common PCB template – so the boards can be stacked with M3 spacers.
That will also simplify making some shielded cases, if needed – just one case template.

The overall system architecture looks like this:

Category: VNA

WS2812B Wake-up lamp


This is a new project I’ve been working on: a small “smart lamp” that will create some programmable lighting patterns.
The main usage will be to help me wake up in the morning by beginning a light pattern before the alarm clock. It should be able to simulate some sort of sunrise, etc.
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Rohde & Schwarz SMS Teardown


This is a quick tear-down of a Rhode & Schwarz SMS frequency generator, with the 1040MHz option installed.
I had to replace all the electrolytic capacitors, as they are failing all over the unit. The PSU ones blew up and filled the unit with electrolyte vapors.

The photos are quite poor and the colors are a little off – i have some warm LED lights that look good but mess up photo cameras.
I had to take the photos at F2 -> the DOF is low and there’s a lot of fringing. I’ll try to improve them as much as i can ( sharpen, etc )


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Critikon Dinamap Plus Vital Signs Monitor tear-down

Hello, it’s been a while since my last post.

I’ve decided to add here some things I initially posted on Eevblog
The original link is here and it includes some useful comments from EEVBlog users:
EEVBlog original post

A little history:
I’ve got this device as an impulse buy from e-bay, when I was playing with fpgas and displays and for some reason i really wanted a plasma / electro-luminescent display. The device was listed as “not working” but from the photos, the display looked undamaged ( no cracks, etc ).
The whole thing cost me ~15$ + more than 30$ for shipping …
It arrived properly packaged, pretty clean and exactly as described – a big chunk of the case was cracked – just the outer case as I saw after tearing it down.

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LT5517 radio receiver


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.

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Category: Electronica

FPGA NTSC Generator


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

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Category: Electronica | Tags: , , ,

RF power meter (part 1)

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
AD8307 datasheet

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.