QST Forty-9er with DDS VfO

(Since this was originally posted, this transceiver has also gained a laser-cut faceplate.)

Long story short, I’ve got a new transceiver!

Back in March, QST magazine published an article about modding a cheap Forty-9er kit from eBay to incorporate a digital VFO. The original Forty-9er was a kit from the NorCal QRP club, a 40m transceiver designed to run on a 9V battery, hence the name. It, like many other kits, is based around an NE602 and an LM386. In the last couple years, cheap kits bearing the same name have been appearing on eBay, which bear only a scant resemblance to the original. The biggest difference is that where the original kit had a VXO attached to the NE602 mixer, the eBay kits are designed to be rock-bound to a single frequency. Perhaps this was the motivator for the QST article, to restore some frequency coverage to these fixed-frequency kits.

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The Forty-9er kits from eBay – not a huge number of parts, but not a tiny kit either.

The process of using a digital VFO with an NE602 architecture is simple enough. Specifically, an AD9850 breakout board is used to provide the signal, and a small BJT amplifier increases the power output from the DDS chip. The oscillator power is adjusted to show about 300mV P-P in-situ. After the article was published, one of the authors, K2ZIA produced a limited run of kit boards, which include the amplifier and sockets for both the AD9850 board and an Arduino Nano to control the DDS over i2c.

K2ZIAboard
The K2ZIA kit board, with attached Arduino Nano and AD9850 board. Image credit: K2ZIA

I purchased the breakout board, AD9850, Forty-9er kit, Arduino Nano, and a rotary encoder from a fellow ham, Justin AJ2Q, who had gathered the pieces but was focusing on other pursuits and wanted to pass the project on. Final assembly was pretty straightforward; there are a couple of mods that need to be made to the Forty-9er (specifically, replacing the oscillator crystal with an input for the VFO, and swapping the crystal input bandpass filter for a much-wider two-element bandpass filter), and connection made so that the Arduino can detect when the key is down and shift frequency. AJ2Q had already done most of this, so only some final tweaks and cleaning up some soldering were necessary.

The original Arduino code was designed to make use of a 16×2 LCD display to display the current frequency, as well as licensing information according to the 40m band plan. Since I’ve already been playing with an LCD display on another ongoing project, and since I wanted this to be a simple and durable bit of kit, I wrote a bit of code that instead flashes the current frequency in morse code on a panel-mounted LED. The display is triggered by the press of a button. The number of digits to display is configurable. I’ve found that just displaying the three kilohertz digits is plenty (I don’t need to be reminded I’m on the 7 MHz band every time, and I don’t need precision better than KHz for simple operating). As always, you can see the code on Github.

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The internals of the transceiver. The two green perf-boards are an experimental audio filter to be documented later. The 40-9er board is bottom-left, the K2ZIA board is bottom right, and the small black board on the upper right is the rotary encoder mounted to the front panel.

 

The full schematic of the original Forty-9er as well as the necessary mods can be found on Farruk K2ZIA’s website. The only additional hardware changes I made were to wire an LED and a 1k resistor between Arduino pin 14 and ground for the LED, and an SPST button between Arduino pin 4 and ground to trigger the frequency display. Like the original code, depressing the encoder changes the tuning rate, though I modified the possible step values to be only 1000Hz, 100Hz, and 10Hz, in that order. 1KHz is useful for zooming around the band, 100 Hz is useful for tuning a specific signal, and the 10Hz step is mostly for resolving SSB/DSB/AM signals cleanly.

On the rear of the radio are the BNC antenna jack and the power pole power input. (Useful tidbit – a pair of connected 30A powerpoles fit neatly in the cut-out for a VGA connector!) I’ve yet to fashion a front-panel, so the connections on the Forty-9er board for a key and headphones are directly accessible.

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I’ve had the rig out to the park a handful of times now, and it sounds good! Like my other direct-conversion NE602-LM386 experiments, the audio quality is great, but broad as barge, so selectivity suffers. The sidetone is clean, at around 700Hz, from the Forty-9er’s little BJT oscillator. I enjoy being able to tune up into the phone portion of the band and listen to SSB QSOs and nets and such, which provide a good sketch of current propagation conditions. Being able to quickly switch between tuning steps is helpful, as I tune around and try to find someone sending CW slow enough for me to keep up.

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The top of the rig. Not fancy, but functional.

The only major limitation of the rig is that, with only a single-resonator input bandpass filter, out-of-band signals can get into the radio and cause interference. Specifically, World Harvest Radio WHRI, who maintain and 500KW (no that’s not a typo) transmitter in South Carolina, is often audible everywhere on the dial, which is distracting at best. A stiffer bandpass filter will be necessary soon. I’ve also been experimenting with a peaked audio filter to help with reception of CW, but that’s still experimental.

The unit puts out about 3 Watts, which proved to be enough to make my first CW contact and, over the weekend, my second. The gentleman on the other end this time was Gary N4PIR, who was running 5W on his FT-817 into a trapped vertical. I think a little afterburner would be helpful on my end, as we were definitely fighting QSB. But that’s a later project.

Hear you on the air!

73

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“The Virgin” – A DC 40M Receiver

My first HF receiver project is complete.

It’s not fancy. It’s rock-bound with no ability to QSY. It only has a single RF gain control up front. It’s direct conversion, so it hears all signals on both sides of its frequency. It’s bodged together and probably not super durable.But it’s my own, made from scratch, and I couldn’t be more happy. So without further ado, here’s the finished product:

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The Virgin Receiver – safe for use, if not safe for work.
I received this (scandalous!) container in a White Elephant gift exchange at a holiday party this Christmas, and I immediately thought it would make a good home for a radio. It seems a fitting case for the receiver that’s taking my homebrewing virginity.

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The Virgin DC Receiver. Two IC’s and a power regulator. One knob. No fuss.
The circuit is nothing particularly new: it’s based, as I’ve said before, on the work of Dave Richards AA7EE, some fine projects from GQRP, and a very useful document from Bill KV2AWC. The unit is a direct-conversion receiver based around the ubiquitous NE602/NE612  mixer/oscillator IC and an LM386 audio amplifier circuit. Gain is controlled solely by the RF gain pot in the front-end. I find this provides more than enough gain control. There’s also a position for a jumper to boost the audio output from the LM386, but with the amount of RF noise in my apartment, this proves more detrimental than helpful. I may turn that jumper into a switch if experimentation shows its use.

The only filtering present is the handful of passives that sit between the NE602 and the LM386, filtering the audio a bit between converting to baseband and the audio amplification stage. A little experimentation showed that the present passives only seem to round off higher audio frequencies, say about 6 khz. They’re not really meant to enhance selectivity, but just to reduce noise introduced by signals further away from zero beat.

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A naked view of the inside of the receiver. Avert your eyes, children!
I found some nice panel-mount BNC connectors at our local Fry’s electronics, and fitted some Anderson Powerpole pigtails as a power input. (Pro-tip: if you use Powerpoles regularly, as I do in my theatre job, get yourself a ratcheting crimper. Less than $40, will change your life.) There’s a 1/8″ jack attached to one side and the RF gain control is mounted up front. The whole thing weighs maybe half a pound.

Signals come in clear and hot on this thing! With minimal audio filtering and just a basic front end filter, it’s pretty wide open, but that’s kind of the point – at this point, I’ll take sensitivity over selectivity. Particularly up in the QRS part of the spectrum, I’d rather be able to hear the one guy who’s within 3kHz of my crystal than lose him. Remember, this thing is rockbound, so changing frequencies means swapping in new crystals.

The receiver has gone through several revisions and adjustments in the past couple months. See the original post, changes to the front end, and some power problems for those adventures.

While I’ll probably never really be ‘done’ with this receiver – with tinkering projects, are we ever really done? – it feels great to have it packaged up, in a case, and usable. It’s freeing to have something complete enough to show off.

Hear you on the air!

73

 

 

DDS Output Screen in Polyakov mode.

40m Direction Conversion Receiver “In the Polyakov Style”

tl;dr: My build of KE3IJ’s DC-80 Receiver for 40m works! Building radios is fun!

I stumbled across KE3IJ’s version of an RA3AAE-style (V. Polyakov) receiver some months ago as I started looking into homebrewing my own receiver/transceiver. I figured a direct conversion receiver would be a good place to start, as it would give me immediate reception with a simple pre-selector filter, mixer, and audio amplifier. RF filter designer, and IF filter design in particular, scares me a bit as a new radio builder, so I figured a superhet was probably a little too advanced to start with. So when I stumbled across KE3IJ’s version of a direct conversion receiver that seems to have reduced some of the issues with direct conversion receivers (microphones, AC or ‘common mode’ hum), it seemed like a reasonable place to start. You should absolutely read his full write-up on his build.

Here’s KE3IJ’s original design:

KE3IJ's DC-80 Receiver
Original Design of KE3IJ’s DC-80. Some revisions crossed out at a later date.

The central, exciting part of the DC-80 is that the VFO is tuned to half the operating frequency. That is, to tune in a signal at 7.040 MHz, you would set up the VFO to oscillate at 3.520 Mhz. Which explains why the original design lists the VFO range as 1.79-2.0 MHz for reception on 80 meters.

Rick has some interesting theories (and has done some experimentation) as to why the half-frequency VFO works that are well worth reading. After trying some different mixer options and settling on the two pairs of anti-parallel diodes, he concludes that what’s actually happening has little to do with “frequency doubling” or somesuch. Rather, an incoming signal (at, say, 7.0 MHz) mixes with a half-frequency VFO (in this case, at 3.5 MHz) to produce two signals at 7+3.5 = 10 MHz and 7-3.5 = 3.5 MHz. This copy of the original signal at half the frequency (3.5 Mhz) then mixes again with the half-frequency VFO (3.5 Mhz) to produce a baseband signal (“0 Hz”).

Seems like a plausible mechanism to me… and regardless (spoilers), it works!

One change I made was to replace Rick’s “quick and dirty” VFO (since I don’t have an AM radio to tear apart at the moment) with the Si5351 derived VFO I previously built. I’ve upgraded the Arduino code (available on Github) with a band-select button to jump between the various HF bands, and a mode button to change between some operating modes. For this project, I mostly left it in “Polyakov” mode, which just means the DDS VFO is outputing half of the frequency displayed on the screen.

DDS Output Screen in Polyakov mode.
Arduino-controlled DDS in Polyakov mode, outputting half the displayed frequency

Sidebar: I added a few additional features which I was updating the Arduino code. When you press the Band Select button, if you’re within one of the HF bands, the Arduino will remember your current tuning and return to that frequency when you cycle back to that band. Also, the “E” in the upper corner of the above picture indicates that only Extra-class licensees may transmit at the displayed frequency: I baked license-permissions into the code as well. Being only a General, I wouldn’t be allowed to transmit here!

The other other change I made was to implement a 40m band pass filter instead of the original 80m filter. I just ripped the values straight out of Rick’s original suggested for a 40m bandpass. I really don’t quite understand it, but it does seem to isolate the 40m band nicely.

So, with those changes in place, the actual schematic became:

Schematic for 40m DC Receiver

Which turned into the following pile of spaghetti on copper-clad and a breadboard:

Assembled 40m Receiver

With the solder cooling, I plugged a long bit of wire into the antenna input and set up my little 40m Pixie on the YL’s desk about 6′ away with a 7.114 MHz XTAL. I tuned the VFO to 7.114.500 and clicked the key on the Pixie, and…. Sound! A glorious 500 Hz tone! What a beautiful feeling, when something scratch built works. Flush with excitement, I hauled the receiver out to the backyard and plugged my ~1/4 wavelength bit of wire (10m) antenna, and boom, strong signals coming in from about 7.0 MHz to 7.060 MHz. The fact that this was during the ARRL November CW Sweepstakes meant that there were many signals on the air to listen to, many of them running 100s of Watts for contest’s sake.

Not bad for a first outing! Several issues exist, of course, in no particular order:

  • Having the 9V battery biasing the audio pre-amp transistors does increase the signal volume in my earbuds by maybe 2-3 dB, but it also increases the background noise volume by maybe 10 dB! With that 9V no longer in the circuit, signals appear faint but audible, and noise drops almost to nothing. I haven’t gone back and confirmed that it’s not an assembly error, but this does not seem like the desired behavior.
  • Sadly, the work lights in my office are very RF noisy. I have three 24″ fluorescent fixtures mounted above my desk, and when they’re on, the antenna and receiver both pick up ungodly noise.
  • Something in either the VFO, the rotary encoder, or the backlit LCD display makes an awful THUMP in the audio whenever I change frequencies. More experimentation will be necessary to figure out where that’s coming from. Initial theories are the I2C lines controlling the display, or maybe they’re an artifact of the frequency change on the Si5351 itself.
  • I suspect that driving the diode mixer directly from the output of the Si5351 breakout board may not be pushing the diodes into conducting for much of each cycle… Some kind of buffer may be necessary.

Hear you on the air!

73