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!

HNY

Yes, a belated Happy New Year greetings! It's hard to believe that 2013 is already well under way. I figured it was about time to give you a quick update on what's going on in the shack right now.

First up is the discrete component grabber receiver for 14.141 MHz that I prototyped to be paired with the OpenBeaconMini project. The receiver itself consists of a roughly 2 kHz wide crystal filter on the front end, feeding into a single-balanced diode ring mixer, which drives an AF amp using 2N4401 and 2N4403 transistors. Because I'm not able to put up a proper outdoor antenna for the grabber right now, I decided to put the VE7BPO cascode active antenna on it instead. It seems to work well, but I don't know for sure because there are basically no signals on this part of the band. I intended to use my Raspberry Pi with the receiver as a grabber, but I had no luck getting either LOPORA or QRSSVD to work properly and reliably. It may just be asking too much of the poor beast. So I'm going to try to appropriate another PC in order to get the grabber receiver QRV so that on-air testing of OpenBeaconMini can begin in earnest.

Discrete component monitor RX for 14.141 MHz
Discrete component monitor RX for 14.141 MHz

Next, I wanted to give you a very brief overview of my most recent purchase for the lab: a Rigol DS1022U arbitrary waveform generator. As far as I can tell, this appears to be pretty much the same as the DS1022A model that is sold in the US. But being a typical ham, I wanted to save a few dollars, so I purchased it off of eBay from seller who says he is an authorized Rigol dealer.

Rigol DG1022U Arbitrary Waveform Generator
Rigol DG1022U Arbitrary Waveform Generator

The DG1022[U|A] has two channels that can output a sine wave up to 25 MHz in 1 mHz (as in millihertz) steps. It can also provide square, ramp, pulse, noise, and arbitrary waveforms at lesser frequencies. It can modulate the waveform in a variety of ways, including AM, FM, PM, PWM, and FSK. It can, of course, also do sweeps of various parameters. The output amplitude into 50 Ω ranges from 10 Vpp on Channel 1 or 3 Vpp on Channel 2 down to 2 mVpp on both channels (or -50 dBm). The shielding on this AWG seems to be excellent. Using my HP 355C/355D attenuator combo, I can get a signal down to about -140 dBm (disclaimer: not a scientific measurement, made using my ear as a detector and listening on my IC-718). The dual outputs makes it very useful for a variety of two-tone receiver measurements, one of the big reasons driving my purchase. The Channel 2 output also doubles as a 200 MHz frequency counter input. Paired with the USB connectivity of the device (it seems to enumerate as a usbtmc device), that will be extremely handy for measuring oscillator drift. The DG1022 can also link the two channels together and give them a specific phase difference, as you can see below. This will make it very handy as a I/Q LO when I want to experiment with phasing and SDR rigs.

I/Q Output from DG1022U

So far, I've been very pleased with my purchase. I don't feel like I've had it or used it long enough to give you a full review, but I thought that this preview would at least be a bit helpful for those thinking about using it. One of my goals for the new year is to do a much better job of characterizing everything that I build. Since I intend to start selling transceivers in the near future, it's doubly-important that I can make accurate measurements of my products so that I can properly state their specifications. To this end, I've decided to sell off a bunch of my unused or replaceable test equipment (please take a look at the for sale posting) in order to finance the new, calibrated test gear. Next up on my purchase list is a Rigol DSA815TG spectrum analyzer (just reviewed favorably in the February 2013 QST), but that's going to require the sale of everything on that page!

Finally, I've got the CC1 prototype PCBs on their way from Seeed Studio right now. It looks like they just cleared customs in the US, so hopefully they will be in my hands in the next few days. With any luck, I'll have the first one built by the weekend and will be well on the way to a new beta test. I'll put up a quick post to show off the PCBs, and when the first prototype unit is completed. Stay tuned!

A Few Questions

Hello Dear Readers,

Sorry for the thin content on the blog once again. In the insufficient free time I have, I've been swamped with trying to keep OpenBeacon in stock and development of new products going. I've got a couple of questions for you, if you don't mind chiming in.

First, I tried asking a question similar to this on the KnightsQRSS mailing list, but it rapidly devolved into a flamefest and I never really got much good, constructive feedback. So I put it to you. I'm interested in putting an 80 meter version of OpenBeacon on the market. There doesn't appear to be much 80 meter activity in North America, but what there is seems to be located just above 3.500 MHz. The issue that I'm seeing is that choice of frequency excludes all American non-Extra Class hams. With 80 meters being such a large band, I don't see any reason why another frequency could not also be used. I'm proposing to put the 80 meter OpenBeacon on 3.582 MHz. If operation was kept between 3.581800 MHz and 3.582000 MHz, I don't believe it would interfere with any current informal band plans, but I'm not certain about that. I have a very large stock of 3.582 MHz crystals, which obviously also plays a factor (I would be willing to sell them individually to anyone who wanted the for their own homebrew endeavors). So my questions are: does this look like a decent frequency and is this something that would interest you?

The second query is in regard to a potential new product. I'm giving consideration to bringing to market a sort of companion receiver to the OpenBeacon. It could be used for a QRSS grabber or a dedicated monitor receiver for any of the digital modes with automatic propagation reporting such as WSPR, PSK31, or JT65A. I envision it being paired with a small SBC such as Raspberry Pi so that it could make a complete, stand-alone, efficient HF monitoring solution for around $100 total cost (Raspberry Pi currently costs $35). In my opinion, there is a lack of QRSS grabber stations in North America, and using OpenBeacon or other MEPT transmitters will be a lot more fun when there are more stations that can listen for your signal. If you use the receiver for the automatic reporting modes, you can build up a very nice set of data about propagation to your QTH. Here is a list of preliminary specs:

  • DDS or Si570/Si514 LO for wide tuning range and stability
  • Multiband
  • PC tuning and control via USB (similar to OpenBeacon)
  • Single-signal reception (probably filter method, but maybe phasing)
  • Line-level output for PC consumption

So I ask you: is this something you would be interested in? Is there anything feature-wise you would like to see included?

Thanks for letting me pick your brains. I hope you stop by in the comments and leave some feedback!