Sunday, 20 November 2011

First few prototype PCBs

It has been a long session of evenings but almost all modules are now built. The set of phase locked oscillators (PLO) is ready for testing as are the four mixers (MXR), the phase detector module (PDM), logarithmic detector (LD), the 16-bit analogue to digital converter (ADC) the control board (CB) and the master oscillator (MO).

With a bit of luck I should be able to conduct the first few basic function tests to see if I did any mistakes during the assembling.

These are the three PLOs. PLO1 and PLO3 are essentially identical while PLO2 uses a different VCO that is commanded at a fixed frequency. The function of the modules are:

  • PLO1: the variable Local Oscillator (0.9-2.2 GHz) for the first coverter/the source for the VNA phase circuit
  • PLO2: the fixed LO (1024 MHz) for the second converter/fixed oscillator for the tracking generator.
  • PLO3: the LO (1034 MHz) for the tracking generator and the source for the VNA phase circuit
The VCOs used are Mini-Circuits ROS-2150VW and ROS-1500 paired with a PLL from Analog Devices, the ADF4118.

The mixer modules all use the same double balanced mixer from Mini-Circuits: the ADE-11X.

MXR1 is actually the input of the spectrum/network analyser and MXR3 the output of the tracking generator. MXR2 is the second mixer in the signal path and MXR4 is located in the VNA phase detector section. They vary slightly with different attenuators or diplexer additions but they essentially all use the same PCB.

In this picture you can see the two DDS modules, with DDS1 to the the left, DDS3 to the right and the master oscillator module in the centre. They generate a filtered CMOS compatible output from 0-32 MHz that can drive a 50 ohm load. Another output provides an unfiltered signal with lots of harmonics present. The master oscillator is simply a 64 MHz quartz crystal with three buffered outputs to feed PLO2, DDS1 and DDS3. Later on, I will evaluate the possibility to replace the master oscillator with my rubidium standard.

Here is the 16-bit ADC, the phase detector and the log detector module. This team of modules converts the magnitude and phase of the test signal into a DC voltage that is sampled by the ADC module and sent to the software on the PC by the control board. Components used are mainly from Analog Devices, the  AD7685 16-bit ADC and AD8306 log detector. The phase detector module uses Fairchild D-type flip-flops and inverters to output a square wave whose duty cycle depends on the ratio of its two input signals.

The control board provides the power lines for modules of the system with its 20 V, 10 V and 5V voltage regulation and conditioning circuitry. It also holds the interface to the software, which consist of four 74ACT573 latches for a total of 32 control and data bits connected via a standard LPT printer port.

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