20140429_094034

Si5351A Investigations Part 3

Since the previous Si5351 Investigations post, I've made quite a bit of progress integrating the Si5351 into a grabber receiver. First, let's look at the grabber receiver architecture and how the Si5351 is used in it.

GrabberRXBlockDiagram

Above you can see the basic block diagram of the receiver. The front end has two switchable plug-in bandpass filter modules (which is switched by a pair of TI TS5A3157 analog switch ICs controlled by the AVR microcontroller). An ADE-1 diode-ring mixer is fed directly from one of the Si5351 ports set for the lowest output level (which is about +8 dBm). The pair of IF amplifiers are TriQuint AG203-63G MMICs, and the actual IF filter is a 6-crystal ladder filter with a bandwidth of 3 kHz (filter plots provided below). Another ADE-1 is used as the product detector, with the BFO signal provided by another output port on the Si5351. Finally, the recovered audio is amplified to line level by a NE5532 op amp and delivered to a 3.5 mm stereo socket by way of a transformer in order to provide good ground isolation between the receiver and sound card.

grx1 grx2

As an interesting side note, you can see my modular bandpass filter boards below. They are your standard double-tuned circuits placed on a 20 mm x 50 mm piece of copper clad, with 0.1 in SIP headers soldered to pads cut out of the material with a rotary tool. There are some extra pins which are reserved for a future module identification system.

DSCN0641

Happily, the receiver mostly worked as expected right out of the gate. A few minor tweaks need to be made, such as in the gain of the AF amplifier, but I'm pretty happy with the results so far. Although it's my intention to pair this receiver with a Raspberry Pi running LOPORA in my garage, right now the receiver is sitting on my bench capturing WSPR RF on various bands, and looks to be doing a fine business job doing that. Frequency stability looks at least as good as my IC-718 on WSPR, based on an eyeball assessment of WSPR spots.

WSPR

The Si5351 seems to do a bang-up job acting as both the VFO and BFO. What's nice is that I can easily switch sidebands merely by changing the output frequency of the CLK1 BFO output with one line of code. I haven't seen any indications of spurious products affecting the receiver yet.

My Si5351A library for avr-gcc is still pretty rough, so I'm not quite ready to publish it yet. Next up on my todo list is to get it in a state where I'm ready to put it up on GitHub, then implement the USB control protocol and client program so that the grabber receiver can be tuned via command line (especially handy when using SSH).

At this point, I'm fairly confident that the Si5351 will make a very fine oscillator system for a grabber receiver. My next line of investigation is to look at the temperature stability of the Si5351 via a thermal chamber lashup. I've got a nice Styrofoam cooler, an Arduino knockoff, a 120 V relay shield, a temperature sensor, and a frequency counter I can read via serial port, so I just need a heat source (probably in the form of an incandescent bulb) and I should be good to go. Keep watching for more experiments!