Some thoughts on the K5BCQ Si570 signal source

For the last few months, I’ve been working on building a radio from a pair of kits old enough to vote: KK7B’s R2 receiver and T2 transmitter. A complete transceiver built from these kits needs some other pieces, including a VFO (variable frequency oscillator). After looking around a bit, I picked an Si570 signal source kit from Kees Talen K5BCQ and John Fisher K5JHF.

Si570 signal source and frequency counter, showing 17 Hz difference at 10 MHz

I’m not going to try to review the kit, because Jack Smith wrote the canonical review of it already. Instead, I will share some impressions of the kit.

First off, the PCB layout has not gotten better. There are some crazy things about it, including through-hole parts mounted on opposite sides of the board, such that one component prevents access to the terminals for another. The layout of the output circuit is also odd. In an effort to provide for many different output options, apparently while keeping trace lengths to a minimum, the output section is crowded and hard to navigate.

I had to do only a little debugging after assembly. The first problem was that the two pads at the ends of the Si570 were not well-soldered. Jack Smith ran into the same problem. My second problem was a little more subtle. I fired up the circuit and saw output at the right frequency, but 0.5 V in amplitude. I purchased the CMOS output option, so it should have been a 3.3 V square wave. Tracing out the circuit, I found that the signal was getting knocked down when it passed through a DC blocking capacitor. Oddly enough, I noticed that when my ‘scope probe pushed down on the right spot on the capacitor, the output jumped up to 3.3 V. I probably fractured the cap with the heat of soldering. I had changed my mind about capacitive coupling anyway, so I replaced the cap with a 0 ohm resistor and now I get the output I expect.

Waveform from the Si570 VFO

The ‘scope shot shows the output, which is a reasonably clean 3.3 V CMOS signal. There is almost a volt of overshoot on the transitions, which is a little concerning, but I am not too worried about it at this point. The CMOS edges are fast, making overshoot understandable, and the size of the overshoot is likely to change as the VFO is integrated into the radio. I will worry about it later.

All in all, I have to say that the board works a treat. It is awesome to see 1 Hz tuning resolution and crystal oscillator stability coming out of a tiny 8-pin surface-mount part. I measured the frequency as being off by about 14 to 17 Hz at 10 MHz, which works out to about 1.7 ppm. Drift is confined to that 3 Hz range, at least when sitting on my basement workbench. The board does have a provision to calibrate out the frequency error, which I have not used yet. This much error really doesn’t matter.

The microcontroller firmware with the kit works well. The user interface is slightly unusual, with a decimal point used as an input cursor, but it works fine in practice. I do wish there was a way to configure band limits. The board has provisions for band selection, with up to eight bands possible. However, regardless of the band selection, the device will tune over its entire range. In a multi-band radio that is a poor idea, because it would be too easy to transmit at the wrong frequency and damage the final amplifier. With hundreds of on-board memories (100 per band, minus 20 reserved for setup), it would be nice if a few configuration memories were used to choose tuning limits for each band.

Over all I am happy with the board. It will serve just fine as my VFO.

Working slowly on the R2/T2 radio

I was at my workbench when my son came downstairs to talk and hang out. A budding reader, he soon started singing the A-B-C song with the letters mixed up. It crossed my mind that this was odd, but I didn’t think much about it until I heard

That rang a bell, and suddenly I realized he was singing my oscilloscope’s front panel. Music by Mother Goose, lyrics by Tektronix.

When he came downstairs, I was working on assembling the K5JHF/K5BCQ Si570 synthesizer kit. It turned out to be a bit fiddly, but it is a decent kit. That said, I would not lay out a PCB like this one. It has a number of vias in the middle of pads, and while that’s not a big problem for hand-soldering, I would avoid it in a PCB as a matter of good practice. You never know who might decide to use solder paste and reflow to assemble the board, and then those vias will be trouble.

Worse, though, is the positioning of the connector for the LCD. It is laid out smack in the middle of the footprint for the microcontroller, but on the opposite side of the board. If you look at the big black socket in the picture above, you will see the back side of the microcontroller’s pins poking through the board on either side of it. This positioning means one has to solder the header on one side of the board, then insert the micro on top of the pins one just soldered, flip over the board and solder the micro’s pins next to the header. The header pins are non-serviceable once the microcontroller is installed, and the microcontroller won’t be easy to get out, either. DIP desoldering tools won’t fit with the header in the way. I’m not sure why the board is laid out this way, except that it makes the board a little smaller. Maybe that was the only concern.

At this point, it’s fully assembled except for the encoder/switch, which is about half wired. What am I doing blogging? I could be soldering!

An Si570 VFO for the R2/T2 transceiver project

I’m continuing to work on my R2/T2 transceiver project as time allows. My goal is to get on the air before the sunspot cycle peak passes. That gives me a little time yet, but at the rate I get things built around here, it’s going to be a close race.

Even when building a radio from kits, as I am here, there are many decisions to be made. When I bought the KK7B R2 and T2 kits, I had no thoughts about what to use for a local oscillator. Technology has advanced mightily since then, and now I have the option of an Si570 frequency synthesizer. This little chip provides a precise, low-noise  digital clock at programmable frequencies between 3.5 MHz and 1.4 GHz, depending on the variant one buys.

After looking around a bit, I picked John Fisher K5JHF and Kees Talen K5BCQ’s SI570 controller/frequency generator kit. Once it arrived, I had trouble figuring out how to fit it into my case. This case has a 0.125″ thick aluminum front panel. The threaded bushing on the kit’s encoder/switch was not long enough for this thick panel and a mounting nut, let alone a washer. There were also some mechanical things I didn’t like about the circuit board. I thought a bit about designing a new board for the parts from the kit, but I decided I could fix the worst of the problems with a new encoder. A little browsing at Mouser turned up an extremely similar model that had the longer bushing I needed. It even has the same footprint.

I’m a little stumped by how similar they are. The Mouser one (on the right) is from Bourns, but looking over the data sheet, I couldn’t find a model with a bushing and shaft length matching the one from the kit. The body of both units is essentially identical. Hopefully they are electrically close enough, too. I had to guess at how many pulses per rotation it should have.

I’m still chewing on another mechanical question. The kit is designed to have the PCB soldered to one end of the LCD, with the encoder mounted off the PCB, on the right of the LCD. I want to have the tuning knob centered below the LCD, so the PCB is going to have to stay with either the LCB or the encoder, and the other will have to be connected with wires. My initial thought was to mount the encoder on the PCB and wire the LCD remotely, but I’m beginning to favor mounting the PCB on the LCD and running wires to the encoder. The connection between the PCB and LCD will involve high-frequency digital signals, while the connection to the encoder is analog switch closures that have less potential for RF interference. It would be better to have the LCD signals cover a shorter distance so they radiate less.

On top of that, putting the PCB and the LCD together will make it easier to surround them with a shield.

All this rambling aside, yes, I’m making slow progress on the R2/T2 rig. When I’m working on a project, sometimes I spend a lot of time doing and other times I spend my time thinking. I’m a little out of my element with the mechanical design of the radio, so lately I’ve been planning the design carefully.