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Pi Pico Rx – A crystal radio for the digital age?

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Re: Pi Pico Rx – A crystal radio for the digital age?

#21

Post by anonymousiam »

nine_k wrote: post #6

Indeed very simple and within reach of a beginner. Another illustration how RPi 2040 PIO is an exceedingly helpful thing. Without it, the RF-related circuits would have to use either bit-banging, or a bunch of small discrete components.

Certainly within reach of a beginner to build, but I wonder how many beginners would struggle to understand the details of a SDR. Knowledge of processors, memory, IQ sampling, DSP, etc. are not typical subject matter for beginners.

The original crystal radios were very simple, with a crystal (or later, a 1N34 diode), an optional (low Q) bandpass filter, and a piezoelectric earphone -- no batteries required. The principles of operation could be explained to a beginner in just a few minutes.

When I was a kid, I optimized my first "crystal" radio down to four components:

1) 1N34 diode.

2) 22k Ohm resistor (impedance matches the earphone).

3) Piezoelectric earphone.

4) Antenna wire (with alligator clip).

It worked well because there was only one radio station nearby. (KNX 1070, 50KW was about a mile away.)

Re: Pi Pico Rx – A crystal radio for the digital age?

#22

Post by nine_k »

anonymousiam wrote: post #21
nine_k wrote: post #6

Indeed very simple and within reach of a beginner. Another illustration how RPi 2040 PIO is an exceedingly helpful thing. Without it, the RF-related circuits would have to use either bit-banging, or a bunch of small discrete components.

Certainly within reach of a beginner to build, but I wonder how many beginners would struggle to understand the details of a SDR. Knowledge of processors, memory, IQ sampling, DSP, etc. are not typical subject matter for beginners. The original crystal radios were very simple, with a crystal (or later, a 1N34 diode), an optional (low Q) bandpass filter, and a piezoelectric earphone -- no batteries required. The princ…

By this token, the physics of semiconductors, which is required to explain how the crystal detector receiver works, is also uncommon knowledge among beginners!

Both computers and semiconductors can be explained qualitatively, bringing in the ideas like a one-way valve, or following written instructions. This, of course, would gloss over a ton of things, much like the optimized crystal radio blissfully lacks the RF filter in the audio frequency circuit: the mechanical properties of the earphone suffice.

"All models are wrong, but some are useful", as they say.

Re: Pi Pico Rx – A crystal radio for the digital age?

#23

Post by _paulc »

Animats wrote: post #14

(2023) Very nice. It's not a "crystal radio", though. It's a direct sampling receiver. That is, the processing takes place at the RF frequency directly. Most receivers use heterodyne conversion to down-convert a desired chunk of spectrum to a convenient working frequency. This doesn't. It just runs the raw RF through an A/D. As A/D devices and processing have improved, the frequency upper limit for direct conversion…

It looks like it is a Direct Conversion (Zero IF) rather than a Direct Sampling Receiver [0] - ie. it uses does a quadrature LO to down convert to baseband I/Q (in this case a Taylor QSD as a mixer). The RP2040 ADC bandwidth is only 250KHz.

[0] https://www.analog.com/en/resources/technical-articles/a-rev...

Re: Pi Pico Rx – A crystal radio for the digital age?

#24

Post by topspin »

The LTC6227, used in this design, is "last time buy." The replacements aren't as good: either higher noise or lower GBW, and they too will vanish in a few years. That is, of course, deliberate, forcing designers to climb the curve of ever more expensive, exclusive components. Another component in this design, the FST3253, is also a figment of history: no longer produced. Electronics is a nasty rat race, with AD, for example, having bought literally all of their competitors except TI, we have an perfect ADC/DAC/op-amp oligopoly, with everything getting worse with each passing day. So the notion of a cheap, easily reproduced "crystal radio for the digital age" isn't actually workable: nothing you can dream up will be buildable more than 2-3 years after you design it as the components you've chosen cease to exist.

Re: Pi Pico Rx – A crystal radio for the digital age?

#25

Post by Animats »

_paulc wrote: post #23
Animats wrote: post #14

(2023) Very nice. It's not a "crystal radio", though. It's a direct sampling receiver. That is, the processing takes place at the RF frequency directly. Most receivers use heterodyne conversion to down-convert a desired chunk of spectrum to a convenient working frequency. This doesn't. It just runs the raw RF through an A/D. As A/D devices and processing have improved, the frequency upper limit for direct conversion…

It looks like it is a Direct Conversion (Zero IF) rather than a Direct Sampling Receiver [0] - ie. it uses does a quadrature LO to down convert to baseband I/Q (in this case a Taylor QSD as a mixer). The RP2040 ADC bandwidth is only 250KHz. [0] https://www.analog.com/en/resources/technical-articles/a-rev...

Oh, you're right. Affordable direct sampling receivers for low bands do exist, but this isn't one.
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