
Direct Conversion Receivers: Some Amateur Radio History [pdf] - privong
http://w7zoi.net/dcrx68.pdf
======
VLM
Ugh WRT NE602 based designs the article says it all "The circuits using diode
rings continue to offer the better performance."

Worst thing to ever happen to direct conversion circuit topology was
widespread adoption of the NE602 in hobbyist designs in the 80s and 90s
leading to the belief that all DC receivers have unusable problems with hum,
intermod distortion, miserable strong signal handing in general, etc. Given
non-awful components (not a NE602), DC is quite usable.

~~~
philpem
What would you suggest as an alternative?

Is the SA602 or SA612 any better?

~~~
VLM
Diode ring, just like the linked article.

The cool part of the gilbert cell ne602 series (and similar) is its one chip
with an oscillator and a mixer and the performance is barely good enough for
1980s analog cordless phones at 49 mhz with small antennas over a short range.
However intermod and strong signal performance is dismal when connected to a
large 80M dipole or whatever in some ham radio application. A diode mixer will
require some external (IC?) oscillator of course but performance will be
vastly better.

The "new era" of design as seen in some SDRs seems to be a nice DDS
synthesizer driving something like a Tayloe mixer (four very fast analog
switches in an IC).

I don't have the numbers in front of me but from memory the 3rd order
intercept point (higher means less self-generated interference for strong
signals) is ridiculously higher for even a cheap analog switch IC as a Tayloe
mixer.

Of course the power consumption of a DDS and a fast switch will be a little
higher (10x? 100x?) than an analog cordless phone chip. There's always
tradeoffs.

~~~
resters
more info on the Tayloe mixer for those interested:

[https://wparc.us/presentations/SDR-2-19-2013/Tayloe_mixer_x3...](https://wparc.us/presentations/SDR-2-19-2013/Tayloe_mixer_x3a.pdf)

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drmpeg
Almost all SDR's are direct conversion (although direct sampling is starting
to become popular now that reasonably priced high speed ADC's are becoming
available).

~~~
tonyarkles
True, although the cheaper ones have a pretty significant spike at DC and you
end up using a tuning offset and IF in software instead. Not the case across
the board, but pretty common.

~~~
rfdave
The significant spike at DC is an inherent issue with Direct conversion
receivers, driven by circuit level DC offsets and IP2 contributions. Offset
tuning works, but you're stuck with sucky image rejection (30 to 40 dB at
best)

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th0ma5
I built a simple receiver circuit called the Zeta SDR and then used a 192 KHz
sample rate USB audio device to do the A2D... Combined with an antenna tuner I
received PSK31 signals from France in Ohio... The antenna was a mains power
extension cord I had lying around alligator clipped in and strung up indoors.

Quite possibly the craziest and most satisfying electronics project I ever did
and it was like 10 parts or something.

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ngvrnd
Wes Hayward is an awesome guy. His "Introduction to RF Design" may be of
interest: [https://www.alibris.com/Introduction-to-Radio-Frequency-
Desi...](https://www.alibris.com/Introduction-to-Radio-Frequency-Design-Wes-
Hayward/book/3317798)

