A simple direct conversion receiver

Back to my roots - simplicity
I tried a little experiment. I hooked-up my Poundshop receiver (Google it...) to my netbook, and fired-up the wonderful SAQrx Panoramic Receiver software. I found I could use SAQrx as a digital signal processing AF subsystem, and tune-around inside the Poundshop's gigantic audio bandwidth. I was satisfied with the results, but the instability of the Poundshops's 7088 chip meant it drifted too quickly for comfort when the passband was narrowed. I needed crystal control, and a fixed reference.

Why use batteries?
Most of my projects run from 12V lead-acid gel-cells, or 9V alkalines if low-powered. The current project has two features; a single active device with a 4.5V - 8V supply range, and a close association with a small computer. Why not use the power from a USB port? It's 5V, and capable of supplying 100mA (more if care is taken). So, I found a redundant USB cable, an old earbud cable (the kids tread on the earpieces!) and scribbled out a plan on the Schematic Editor.

My favourite chip
It had to be a SA602 mixer-oscillator. These have poor big-signal handling, but are very easy to use. I attached a 7030kHz crystal, a smattering of capacitors and a loop antenna (one metre length of four-core cable), and hooked-up the computer power and signal cables. It worked as soon as I'd turned the SA602 over; a blunder which deserves it's own paragraph...

A creature of habit
Nearly all my projects are produced in 'ugly' style, where chips are turned-over, and used legs-up. So, when I came to use a 'familiar' chip in a piece of stripboard, I reversed all the connections. Happily, the SA602 survived this abuse; I simply desoldered it and mounted it in the solder-side of the board. Success!

A little deaf, but it works
It's a starting point. I may try to improve the antenna, and / or add another SA602 as a RF amp stage. Because the antenna and mixer inputs are balanced, this is an ideal choice. I get an extra 15 - 18dB of gain, controllable by applying a bias voltage to the oscillator emitter pin (6). This works well. I have a home-brew guitar amplifier which uses this scheme as a tremolo effect, with pin 6 fed from a sine-wave generator via a 'level' pot.

I'm getting restless.
The station is essentially dormant; stored in the garage in boxes. However, I'm missing it already...

Today, I hooked-up my Poundshop radio (a 40m DC receiver made from a £1 scanning fm toy radio) to a simple and magical piece of software; SAQrx. This tiny Windows application (it runs under Wine on my Acer Aspire One) turns the Poundshop into a very useful CW receiver. I set the radio up on 7MHz (the band edge), and use the DSP filter in SAQrx to probe around the band. I can watch the whole 22KHz going into the PC on a real-time FFT display, and slide the filter onto any juicy-looking carrier. I would now like to make a dedicated receiver, using a 7MHz crystal for stability and USB power for convenience.

Downsizing and other nonsense

Beyond amateur radio, I have other things to do. Oh, well...

Since the last post, I've discarded a lot of radio stuff. Old, unfinished and abandoned projects, items which 'might be useful one day', most of my surface-mount stock, and a pile of magazine cuttings. It had got thoroughly out-of-hand, and the carnage may yet continue. I have several A4 ring binders full of datasheets, all of which exist as PDFs or other electronic documents. Why keep all that paper?

An adventure at a local carnival convinced me that a lean and portable radio station was all I require, and anything I couldn't carry around in a shoulder-bag was a liability.

I'm a Petty Officer in the Sea Cadet Corps, and I instruct in communications at our local Unit (Worthing). As part of a recruitment exercise, I took an FT101-B, a field antenna, a tuner and a large (24Ah) gel-cell battery to Broadwater Carnival. I cannot use our Sea Cadet frequencies away from the Unit, as the transmitter permit won't allow that, so I operated as G1INF/P. All this should have been easy, and provide an interesting display.

The antenna, a 33-foot copper screw-together pole, was a success (it supported our Ensign). The tuner wouldn't load the thing on 20m, and there was no activity on 40m. The Yaesu's tuning knob seized (thirty years of zero-maintenance finally too its toll), so the Cadets were reduced to running 'errands' with PMR446 hand-helds, and helping to locate lost children in the crowds.

The whole affair was grossly overweight, and unmaintainable. The tuning gear on the radio needed a strip-down, clean and lube (gun-oil is excellent), something I couldn't hope to do with my 'field tool-kit'.

Regen receivers, again - I find the idea of regenerative so seductive, I had to try again. The thing works, but I have reservations. It's unstable below 8MHz, squarking loudly if oscillation is approached. It needs more gain, and a lower-Z output, as it currently uses a crystal earpiece. But it works; I've recovered SSB and cw from both 20m and 40m, and a host of commercial stations in between.

I'm going to try another, based closely on G0KJK's HF9 design, as published in Sprat 82. I'll take my time over it, and follow the design closely, using lessons learned in previous regens. Microphony, feedback and hand-capacitance all need careful attention. Stiffness, layout, and screening!

The 'simple' regen is pictured here; it's a variant on the G4KJJ 'Smid-Gen', itself a subset of the HF9. The top-left is the main tuning, under this is the bandspread. Top right is AF gain, with reaction below. Left switch adds 100pF to the tuning, an the right switch adds 9V to the radio. A random wire is seen attached as an antenna.

Onward, I hope to make a better regen receiver. I also want to make the add-on box for my Racal Minical (see previous posts), and that radio was designed for a shoulder-bag!

Next.
I don't want to fiddle with the Topband SSB transmitter again for a while; it's great for local work, and I have a bigger fish to fry...

SSB / cw / PSK31 from 1.8 - 7.2MHz
Quite a big project, but I'm really only adding sub-systems to an existing radio. I have a Racal Minical, a US-built 'green' USB packset, which I intend to extend to provide LSB, cw and PSK31/63 capability. I've already had it working on LSB, by shifting the BFO down 3kHz. PSK31 works, and cw experiments using mcw (modulated carrier wave) will be the first practical trials.

Doing morse code on a single-sideband
It's a common practice on VHF and UHF to modulate an FM radio with a sidetone to provide morse code transmission. I see no objection to doing the same with a single-sideband radio, provided the sidetone oscillator is as pure a sinewave as practicable, and the radio is not overdriven. I intend to 'key' the Minical by a phase-shift oscillator at 700Hz, and examine the output with DigiPan PSK31 software via a second radio.

Adding a VFO
The little Racal is crystal-controlled, offering six channels. I already provide an 80m VFO for it, using a variable crystal oscillator, and with the Racal's LO as a buffer. It works well, and all I need do now is extend this with a full mixer / VFO system to give coverage of all amateur bands from 1.8 to 7.2 MHz. Using a selection of crystals from my junk-box, and mixing them with a low-frequency VFO, the Racal will be transformed into a useful radio.

A smaller, tidier station
I had to compact the station into a corner, pending a refurbishment of the extension in which it is housed. I have less space, but I'm making better use of the volume. I also have a better place for the 12V charging system, and more shade for the PC and radios.

It doesn't look like much,
and it isn't. It's a 14.318MHz Colpitts crystal oscillator, pulled by an inductor and a large variable capacitor down over a 50kHz range to form an external local oscillator for one of my radios, a Racal Minical.

The Racal is normally rockbound (crystal-controlled), with a choice of six channels, but I wanted more freedom. As you can see by the dial marking, the commonly available 14.31818MHz crystal covers a good portion of the cw segment of the 80m amateur band, once the 10.7MHz IF has been accounted for. The numerous other holes in the front panel (and the Belling-Lee connector) are artifacts of previous uses of the box. It has been a regenerative receiver (not a good one), a 14MHz SSB rig, and a 7MHz superhet receiver. The current manifestation uses the mouse connector (violet, six pins) as power from the Racal and LO signal to the Racal. The mouse cable and connector are small, neat and shielded, and so ideal for this application.

It's not enough
I want even more. I want coverage of Topband, 80m, 60m, 40m and a few spot frequencies in between (for use when I'm in uniform as a Sea Cadet communications instructor). I want LSB and USB phone, PSK31/63 and cw transceive capability at 10W output, VOX, audio filters and it must look good. The Racal on its own looks funky and green, but that little plastic box with the grey-painted copperclad panel will always look nasty. So, I have to look to my junk-box for inspiration. I have an old R1132 dial; this is a wonderful object; Bakelite and celluloid, with a slow-motion drive and a big knob. I have crystals which will mix with an MF VFO to cover the bands required, I have fitting skills sufficient to produce a case and the rest is just electronics. I can take time over this project, and hope to keep a record in this here blog.

Just electronics?
It sounds casual; flippant, even. However, the circuitry involved is extensive, but each module consists of ideas and techniques I've successfully implemented before. I already use the Racal for PSK31, I've had it running on LSB (its native sideband is the upper, being of military origin) and I will be trialling cw transmission using a sidetone oscillator (sinewave) in the near future. The PA already exists (an IRF510 unit in a 2-ounce tobacco tin), and works well. Additional, switchable pi-filters for the three octaves will be needed; and the mixer-VFO will be the only 'new' thing.

Regen RX stillborn, 160m TX improves
The regenerative TRF receiver didn't work, and I'm not especially bothered. I'll fiddle with that when I have more time, but right now I want to get the 160m transmitter running right.

The SSB transmitter now has an output of 1.5W PEP. I'll accept this as it's final power level, because I wish to prove that such moderate power can be viable for communication. I've aready logged contacts at three miles using 100mW, and I expect to be able to cover a radius of ten with the 1.5W now available.

The figures for the drain current in the PA FET are quite ordinary. I've set the quiescent bias current at 20mA, and the drain carries 300mA average on full output. The FET used was an IRF510; in hindsight, this is a waste of a good HF FET, and an IRF640 or BUZ11 would have served just as well. However, I have a second VXO crystal aboard which covers the upper part of the 80m band, and here the IRF510 would do better than a motor-switcher FET. The radio is designed to work up to the 40m band, using an external oscillator, so all bases are covered with this PA.

I put out a CQ call on 1940kHz at 1910UTC, but nothing came back. I'll try later, after dark, and hunt around a little more avidly.

All quiet on the laptop line
I finished the laptop supply, and it does what I expected. I expected it to run hot, but give no interference. I put the LM317T regulator device on the front so I could directly monitor its temperature with my fingers; this would not do for a production equipment, but I'm not in a production environment. I can also easily monitor to output voltage (in spite of my trust of these legendary devices), by measuring at the case and the TO220 tab.

It gets stinking hot when the PC's battery is charging, so I make a point of not using battery power now. This supply will be useful beyond the lifetime of the PC, because with the internal trimpot I can set the voltage to anything from 1.25V to 30V, and supply up to 1.5A.

Ho-hum, telegraphy practice now. dit dit.

It's oscillating.
The 160m transmitter is now delivering 800mW of nonsense to the dummy load, and this diminishes when I modulate. I added another gain stage between the second mixer and the pre-driver, with the idea of increasing the output at the driver's collector to around one watt. However, the thing is taking off, and is not responding to extra decoupling, so I will have to remove the extra stage and just add the PA. I will see if the output is usable (I'll accept anything from two watts and up), and clean.

Another digression.
I've always had 'trouble' with regenerative TRF radios. Some work, but poorly; others scream their heads off at the slightest sniff of a carrier, others flatly refuse to do anything. I try all the traditional remedies (including the usual beginner's error of an upside-down regen coil), but they simply suck.

So, I'm having another go. This time, a 'Heard All Continents' Twin-Transistor Receiver. It's a mature design, which everyone who has experience of has only positive things to say. I'm sure I can break the canon. I've started the job by using stripboard instead of tagstrip; I have a lump of tagstrip somewhere, but modern components lend themselves to the smaller pitch of the 0.1" stripboard. I've made track cuts to simulate the layout of the tagstrip, and made the board no wide than necessary to avoid long lengths of strip giving stray capacitance. The strip board is today's image, at top-left. I've just wound the first coil, a 50-turn monster for the high-MF and low-HF bands (around 1.6 - 5MHz).

The coils in the original were Denco Green Range, but I've not seen these since my childhood. Nowadays, you have to make your own. Go to http://www.arar93.dsl.pipex.com/mds975/content/trfradios02.html for the details. You will find the circuit for the H.A.C. (and others) on the same page.

I've got a lovely old epicyclic-geared knob to go on the tuning condenser (this old tinned brass device can't be called a capacitor), made of Bakelite. It has a slipper-ring attached to an outer knob, so you can defeat the gearing for fast tuning. I also have the main tuning gear from an R1132A RN VHF set, but that would be wasted on such a radio as this.

I want to try a frame aerial with this radio, or perhaps an 'active' antenna. Let's get the thing running, first!