If I were to physically construct the LO tuning circuit using a 2N3904 on a breadboard with the same biasing configuration as given in the datasheet (from the internal schematic for the NE612), and probe it with my oscilloscope to see if it runs, would that be a reasonable test? Per your advice: I shall add a preselector consisting of a buffer amplifier (to get a known impedance from the antenna) followed by a ~300kHz bandwidth LC filter. My intention is to build this, have it "mostly" work for one or two stations, and then move on to a conventional (more complex) superheterodyne AM receiver. On Sat, Oct 15, 2016 at 8:03 PM, Sean Breheny wrote: > Hi Jason, > > I'm not sure about some of the detailed points of your schematic (such as > whether your oscillator tuning circuit will result in continuous > oscillation), but it looks to me like it will probably work to some degre= e > but I'd expect it to have very poor selectivity when faced with strong > signals. This is because the mixer will have some tendency to rectify > strong input signals, which for AM signals, will result in their being > present on the IF output even if the LO is not tuned close to the frequen= cy > of the strong signal. I would expect that there would be several strong > stations which you will hear all the time, regardless of the tuning, and > then weaker stations that would be tuned in and out by the LO. > > A preselector circuit would help greatly with this. > > Another pathology which DC receivers have is feedback where they receive > their own LO AND strong audio signals can modulate their LO through > unintended power supply coupling. This can result in motorboating or othe= r > oscillations in the audio. It is also very difficult to both have enough > gain for weak signals AND avoid picking up AC hum and even feedback from > the audio output back in to the audio input. This is a fundamental proble= m > when all of your gain is concentrated at one frequency rather than being > spread across RF, IF, and AF stages. > > I would suggest you add a simple LC preselector and then try your design. > There will likely be other issues as I have indicated above but they can = be > more or less fixed later on. > > Sean > > > On Sat, Oct 15, 2016 at 7:23 PM, Jason White < > whitewaterssoftwareinfo@gmail.com> wrote: > > > Hello, as a personal project to become more familiar with RF circuits I > > would like to build an AM radio receiver. Online, I found a schematic > which > > uses the NE612 mixer IC to create a Direct Conversion ("Syncrodyne") > > receiver. I have modified the design so that the local oscillator does > not > > need a custom wound transformer. > > > > In my case, I don't require the receiver to have very good performance. > I'm > > okay with the oscillator drifting and the audio "warbling" (due to > > frequency mismatch) under normal operation. > > > > I don't have any sort of matching network or filtering on the antenna > > connection; but I am hoping a 10m wire antenna will produce a > sufficiently > > strong signal for the mixer. I am unsure how strongly signals and noise > > outside of the AM band are going to affect the receiver. Ideally, I'd > have > > a buffer amplifier on the antenna input and a bandpass filter that > covered > > the AM band, but I want to avoid making this too complex. > > > > I have attached a picture of the original schematic, a picture of my > > ltspice schematic, and zip containing my ltspice simulation. > > > > I would like to know if my design (ltspice simulation) seems reasonable > > before I try to build it. I welcome advice on improving the design, but > at > > the same time I do wish to keep things a simple as possible. > > > > Thanks, > > Jason White > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .