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 degree
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 frequency
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 other
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 problem
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 whi=
ch
> uses the NE612 mixer IC to create a Direct Conversion ("Syncrodyne")
> receiver. I have modified the design so that the local oscillator does no=
t
> 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 sufficientl=
y
> 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 hav=
e
> a buffer amplifier on the antenna input and a bandpass filter that covere=
d
> 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 a=
t
> the same time I do wish to keep things a simple as possible.
>
> Thanks,
> Jason White
>
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