Here's a quick update for today - the ramp generator / IF amplifier / filter / ADC board. This ramp generator design is just something whipped up really quickly that is essentially the same as Greg Charvat's MIT IAP 2011 design, using an Exar XR2206 to generate the triangle wave.

However, in a future iteration I'm planning to move to a different design - probably synthesis of the modulation waveform on a small microcontroller. The XR2206 is a very old chip, obsolete now, and it's hard to get. I've still got a few in stock, but it is not really a chip to use in a new design today in terms of openness, manufacturability, low cost, reproducibility and accessibility for other people around the world to be able to easily replicate the design.

I've modified the DC bias circuit on the output of the XR2206 to bias the output at about 5V DC, compared to +2.5V on Charvat's original schematic - this is because of the different VCO I'm using. The HMC431 wants a modulation voltage of 0-10V (across its full range) compared to 0.5-5.0V for the ZX95-2536C-S+. I will be running the VCO across about 60% of the full bandwidth at most using the current modulation circuit - not quite all the bandwidth is accessible before we start to get distortion of the ramp - which we do not want. But this possible bandwidth is plenty for our purposes.

The yellow and red wires you can see tacked onto the board there are just a temporary +12V and +5V power supply, because I'm still missing a couple of power supply regulators needed to run the whole circuit off the USB port (USB connector visible at left.) This board also includes the IF amplification and processing components, but these haven't been tested yet. The IF input comes in from the RF mixer on the other board, into the SMA connector visible at right.