Jason has been working on a portable software defined transceiver design for the past year. Every day this week he’ll discuss a different part of the hardware in a series of guest posts. You can chat with the designer in the forum. Today’s post is about the quadrature sampling detector.
Here’s the design for the quadrature sampling detector (QSD), which is a type of mixer. This mixes the output from the pre-amplifier with the output from the VFO, resulting in the sum and difference frequencies appearing at the output.
Lots more below.
In a conventional superhet design, the intermediate frequency (IF) would be at some frequency like 10.7MHz or 455KHz. However, this design is for a direct conversion (DC) receiver which means the IF starts at DC and extends upwards in both the positive and negative directions (upper sideband and lower sideband). There are advantages and disadvantages to both approaches. In the past, the superhet design was considered the best approach, but modern methods have allowed the DC design to challenge its status as the top performer, especially in the SDR world.
Since the IF starts at 0 Hz and extends upwards, the bandwidth is limited with a lowpass filter (as contrasted with a bandpass filter — usually a crystal filter — used in a superhet design). A later stage of the IF contains an 8th order Bessel lowpass filter to prevent images of strong signals from appearing as aliases after the ADCs, but here we also use a simple RC filter to reduce the response of the succeeding amplifier stage to out-of-band signals, thus improving resistance to strong nearby signals overloading the amplifier.
The amplifier itself is an instrumentation amplifier which allows symmetric differential inputs to be applied from the output of the mixer. A conventional op-amp used here would present two different input impedances for the inverting and non-inverting inputs, which may negatively impact performance by introducing distortion. The gain of the instrumentation amplifier is rolled off to 1 at DC to minimize the effect of amplifier offsets further down the line, as the entire IF chain is DC coupled.
Here is the schematic, and once again a PDF version is attached for better clarity
Via the forum.