Assembly and Testing of the Frontend Divider!

A project log for 60MHz Bandwidth 250Msps Probe-Scope

The Probe-Scope is an open source 60MHz 250Msps oscilloscope probe with all guts built right in, that plugs directly into your PC via USB

Mark OmoMark Omo 07/09/2019 at 05:160 Comments

We just got in the parts and PCBs for all our test boards, so this will be the first of several posts on the testing and bring up of the boards. 

The first board we built and tested is the Frontend Divider test board (for a detailed explanation of why this board is so complex and needs careful attention, see our previous post).  Below is the assembled Frontend Divider board. It contains a 30x resistor divider with special layout considerations, a tuning capacitor to ensure a flat response, and the frontend buffer amplifier.

Ideally, we’ll be able to tune the divider so that the frequency response is very flat; in order to test that, I connected it up to our Bode 100 Vector Network Analyzer, a low-cost, (relatively) low bandwidth USB VNA. It can perform several measurements over a range of frequencies (from 1Hz to 50MHz), most importantly for us, it can measure Gain Magnitude, Gain Phase and Group Delay.

Here is a picture of the Frontend Divider board hooked up to the Bode 100… do ignore the mess.

After a bit of fiddling with the tuning capacitor’s value, I was greeted by this beautiful bode plot:

The gain is almost perfectly flat across the board, and while the constantly decreasing phase might instill some concern at first, that too is our desired outcome. (It’s what we want in order to get a constant group delay, which we’ll cover in detail in a future article.)
Here’s some more detailed plots of the Gain, Phase and Group Delay (shown as Gain Tg):

Now, in order to measure past 50MHz and fully understand the performance of our Frontend Divider, we need to switch to another instrument: the DG8SAQ VNA from SDR-Kits. The DG8SAQ is a low-cost VNA which can measure from 1kHz to 1.3GHz. I swept our fronted from 100kHz to 250MHz. You can see from the plots below that the Frontend Divider is not the bottleneck of the Probe-Scope; it actually has a bandwidth of more than 150MHZ!

For the curious readers, the Frontend Divider schematic is shown below, and as with everything we do, the source can be found on our Github.