A New NE602 DC Receiver

For the most recent details on this receiver, see changes to the audio chain/power supply and front-end filter.

While researching homebrew DC receivers for my “Polyakov” style receiver board, I stumbled across Dave AA7EE‘s various projects which combined the NE602 Gilbert-Cell mixer IC and some form of IC audio amplification to make simple, direct-conversion receviers. In particular, the Rugster receiver (named after a dear and recently departed pet), which itself seems to take inspiration from a number of other receivers, including the GQRP Sudden. VK2AWC has also published some ideas along similar lines, as I’m sure have others.

So, taking inspiration (aka stealing ideas!) from the above sources, I’ve started putting together my own little receiver based on the NE602 as a demodulator and an LM386 as an audio amplifier.

As of yesterday, 1/14/16, I’ve heard the very first (faint!) signals coming out of the headphones, so I think the project deserves the start of a write-up. I think it’ll make the most sense to show the schematic as it currently exists and discuss it afterward. So, here’s the circuit, as currently built:

NE602 Receiver Schematic.
Straight out of the notebook – once this receiver has a little more guts, I’ll be working on a cleaner, digital version.

Mixer/demodulator: The heart of this receiver is an NE602 doubly-balanced mixer. It has a an input impedance of ~1.5kΩ, but for a receiver I think this will be less troublesome than it would be for trying to interface with other circuits. (But see Front end for other thoughts.) The mixer is accepting a single-ended, untuned input on pin 1, while the other RF input is simply AC coupled to ground through a couple caps.

Oscillator: Currently, the receiver is rock-bound to whatever frequency of crystal is connected to pin 6 of the NE602. I installed a bit of 0.1″ header in this position to make it easy to swap crystals in and out for experimentation. Down the road, I think I’ll need to swap this for a bit of machine-pin header for more mechanical security.

The crystal and a couple of supporting capacitors are the entirety of the external oscillator parts. I’ll admit I grabbed whatever capacitors were available in my kit – the NE602 data sheet shows a 10pF cap between pins 6 and 7 and a 22pF cap in parallel with the crystal, so I just got close enough. The crystal seems to resonate just fine, but I’ll admit I don’t know what I’m doing I this area. Likely, I’m pulling the crystals off frequency; I’ve got a frequency counter on its way from eBay, I’ll have to check this out when the counter arrives,

I mean to experiment with some deliberate frequency-pulling once I get the rest of the receiver working a little better.

Front-End: A simple 40m pre-selector filter sits right behind the antenna jack – the filter values are taken straight from the GQRP Sudden 40 and the Rugster. I formed my 5.3μH inductors as 33 turns on a T50-2 toroid.

AA7EE has an impedance transformer in his front end, with a 1:9 turns ratio, presumably to help a 50-ohm antenna to the ~1.5kΩ input impedance on the NE602.? Or perhaps just to present a higher voltage level to the mixer. VK2AWC has a 4t:38t transformer in the same spot. I may need to try this as well.

Audio Amp: The audio amplification section is a simple LM386 run in a medium-to-high gain configuration. The 10μF cap across pins 1 and 8 should lead to a gain of around 200, according to the datasheet. To this point, I’ve neglected to bypass pin 7 to ground (the datasheet suggests 0.1μF), and I think that must be necessary for a high gain-figure, since the circuit turned into a squealer when I first assembled it!

Replacing the 10μF cap with a 4.7μF cap and a 1kΩ resistor helped this problem, but I think if I can improve the stability and get that lost gain back, so much the better.
The LM386 datasheet shows a 250μF cap between the IC output and a speaker; AA7EE and the GQRP use a 100μF cap here. I’m using a 220μF electrolytic since I had one on hand.

Power: For testing, the rig is being powered by a little 9V battery, but I think may be a factor in limiting the audio output (see Known Issues below). Given that the NE602 is being powered by a 7808 regulator and the LM386-3 will take any supply voltage between 5V and 12V, I should have some flexibility in choosing a power supply later on.

I didn’t originally have a reverse-current protection diode (1n4001) nor a10μF decoupling cap in place across the power input, but in future, I think they’re a must.

Here’s an image of the circuit as currently built:

FullSizeRender
The NE602-hearted mixer, built on a piece of perfboard.

It’s not pretty, but it sure is small! I know copper clad would have provided a better ground plane, but I got impatient waiting for my shipment of copper clad to arrive from eBay.

So, with power applied and a pair of basic Apple earbuds in place, I can pick out some faint (to me) signals around 7.023! Very cool! Well, at least I could when I turned off the LED desk lamp, fluorescent work lights, and cordless-drill battery charger in my office/shack, which were all QRM-firehoses. I’ve now run these all to a single power strip, for easy on-off operation.

Nothing’s perfect, here’s my short list of known issues:

  • With a 10μF cap between pins 1 and 8 of the LM386, the unit squeals like a banshee whenever the 10k pot is advanced past about 15%. This was improved by better bypassing right at the IC’s Vs and Gnd pins, and made better again by replacing the 10μF cap with a 4.7μF cap and a 1kΩ resistor. Still not quite sure what was happening, or why this oscillation depended on the position of the input pot.
    • Update 1/25: see my later post for a solution to the squealing. Turned out, used 9V batteries are not a great power source.
  • For some reason, when I advance the 10k audio level pot past about 25%, the audio cuts out entirely – no squealing, no AC hum, nothing.
  • There is no front-end tuning at all a little more verification of the filter characteristics would probably help.

Some possible future improvements, besides addressing some on the issues above:

  • Better output overall – the audio output is quiet at best.
  • VK2AWC had some interesting ideas about the combination of NE602 and LM386 – notably feeding back a little of the output to pin 7 of the LM386 to provide higher gain, and some passive RC networks between the NE602 and the LM386 to provide some shaping to the audio, that I think will be worth checking out.
  • Mounting the project in a nice, metal enclosure.
  • Providing an interface for an external VFO. (Possibly the Si5351-driven VFO from an earlier project.

Problems are just un-implemented features.

73

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