On 15 May, 2007, kranenborg opened a thread discussing the design of a wideband superhet receiver. His goal is
Jurjen, I've been down that road a number of times over the years, to varying degrees. In every case, I've found that I'm no Wes Hayward or Ulrich Rohde. I've expended lots of time and effort, spent lots of money, only to wind up with something that fell far short of my goals, and didn't come close to matching the performance of a commercial receiver that I could purchase for much less money.
Unless you have the equivalent of a commercial laboratory's worth of RF test equipment and the knowledge and experience to use it, and a commercial fabrication facility in your garage, the homebrew superhet project isn't worth it for anything other than the knowledge and experience you'll gain (and perhaps the odd piece of RF test gear that you'll fabricate or acquire.)
However... and that's a big however...
The situation has changed. A man named Dan Tayloe, with a stroke of genius, has re-opened the door for the amateur to build a no-compromise, high-performance receiver with unparalleled performance, equivalent to anything that the commercial laboratories can turn out, using only the limited resources that we amateurs might have available to us in our home laboratories.
Tayloe has invented (or possibly re-invented) a commutating detector, placed immediately after the antenna jack. The detector provides all the tuning and selectivity that a superhet would gain from its RF amp, first mixer, IF strip, and filter(s). Lost in the process are all the dynamic range, intermodulation, image, spurs, noise, and compression problems caused by the RF amp, first mixer, IF strip, and filters.
The antenna connects directly to a 1:4 analog multiplex driven by a clock that's effectively four times the tuned frequency. The output of each mux channel feeds a capacitor that acts as an integrator. Each capacitor is switched in at 0, 90, 180, and 270 degrees at the tuned frequency. The RC time constant of these integrators determine the basic selectivity of the receiver, and the tuned frequency is determined solely by the rate at which the mux is clocked.
The outputs of those integrators are combined, producing an I and Q channel (90 degrees out of phase) that can be processed using a Weaver phasing network or fed to a sound card for downstream DSP detection and processing.
This is a pretty exciting concept. The front end is quite simple; just a few high-quality chips, some even available in through-hole packages. The most complex part of the receiver is the LO, which can be provided by a commercial or kit DDS generator. (Click the picture of the "IQPro".) Since there are no RF components prior to the detector, general coverage (broadband) performance is inherent in the design.
And the very same process works in reverse for transmission, greatly simplifying transceiver design.
I first heard of Tayloe's detector a number of years ago. I was excited, but decided to wait until a few receivers had been built to see if his concept provided the promised performance, or whether there was a fatal "gotcha" hiding within his design. Well, I'm embarrassed to admit that, in those years, I'd completely forgotten to keep tabs on its progress.
I recently stumbled across a link to a radio implemented using the Tayloe detector. The receiver showed promise, so I did some more searching and found a veritable flood of receivers and transceivers that implemented the Tayloe design. And the promise of high performance and inherent simplicity proved true. Here's a link to a relatively simple CW transceiver built using a Tayloe detector. This paper presents a detailed analysis of its performance, showing that the Tayloe detector, properly implemented, outperforms the best commercial superhet designs.
First, how does this pertain to the Picaxe, and second, what are the implications for us amateur scientists?
Full control of a Tayloe receiver is well within the capabilities of a Picaxe 28X1 or 40X1.
For us amateur experimenters, we can implement and experiment with Tayloe receivers without a king's ransom worth of test equipment, or access to a commercial fab factory. There's very little RF-type circuitry in the design. Most of the receiver is implemented with technology well within the capabilities of the amateur experimenter to understand, build, and measure.
Some further links:
Dan Tayloe's white paper that explains his design.
A stand-alone receiver implemented with a Tayloe detector.
A series of articles, "A Software-Defined Radio for the Masses." Scroll down the links; these four papers will be found about two-thirds down the page.
In addition to a good description of the Tayloe process, these papers are a very good primer on PC-based DSP processing, with lots of "how-to" information.
I haven't begun actively experimenting with Tayloe detectors yet, but I've been studying hard. I just ordered a DDS-60 kit, which incorporates an AD9851 DDS chip. This doesn't include a processor, so I'll experiment with a 28X1 for basic control.
The AD9851 isn't really suitable for a high performance receiver due to its 10-bit DAC. It's probably going to produce a receiver rampant with spurs. However, the kit is dirt cheap. I think it will provide a lot of useful knowledge and experience without costing an arm and leg.
(My budget for my initial experiments is in the vicinity of USD $150. Unfortunately, a good deal of that figure will be shipping costs. Grrrr!)
If it all comes together, and I find that I want to actually build a high quality receiver, I'll consider something that uses an AD9854 DDS chip. That chip contains a 12-bit DAC, and should provide substantially better spur suppression than the AD9851 chip.
So, Jurjen, if you're still pursuing your HF receiver design, I hope that the Tayloe detector will provide you another option. If your project has stalled, maybe this will get it back on track. For those who have never contemplated a homebrew HF receiver, Tayloe opens the door to performance that you can actually attain, at home, and for not very much money.
Tom
(I'm going to address the following comments directly to Jurjen.)My final, ideal goal is the following type of receiver:
- Double-Conversion Superhet
- Receiving range 150KHz - 30MHz (LW, MW, SW)
- Receiving modes AM, SSB, FM, DRM
- selectable IF bandwidths (PICAXE controlled)
- SA602A first mixer at 10.7MHz first IF
- TDA1572 (or TDA1072A) amplifier, second mixer + detector at 455KHz (single-chip AM circuit)
- PICAXE-controlled DDS circuit for receiver freq. oscillator
- PICAXE-tuned preselector stage
- PICAXE-controlled RF pre-amplifier (amplification control dependent of frequency and time!)
- Extreme user friendliness using PICAXE control circuit (fully automatic + full manual modes)
- Possibility to download and directly use SW broadcast schemes from internet
- Single PICAXE used (PICAXE-28X1)
Jurjen, I've been down that road a number of times over the years, to varying degrees. In every case, I've found that I'm no Wes Hayward or Ulrich Rohde. I've expended lots of time and effort, spent lots of money, only to wind up with something that fell far short of my goals, and didn't come close to matching the performance of a commercial receiver that I could purchase for much less money.
Unless you have the equivalent of a commercial laboratory's worth of RF test equipment and the knowledge and experience to use it, and a commercial fabrication facility in your garage, the homebrew superhet project isn't worth it for anything other than the knowledge and experience you'll gain (and perhaps the odd piece of RF test gear that you'll fabricate or acquire.)
However... and that's a big however...
The situation has changed. A man named Dan Tayloe, with a stroke of genius, has re-opened the door for the amateur to build a no-compromise, high-performance receiver with unparalleled performance, equivalent to anything that the commercial laboratories can turn out, using only the limited resources that we amateurs might have available to us in our home laboratories.
Tayloe has invented (or possibly re-invented) a commutating detector, placed immediately after the antenna jack. The detector provides all the tuning and selectivity that a superhet would gain from its RF amp, first mixer, IF strip, and filter(s). Lost in the process are all the dynamic range, intermodulation, image, spurs, noise, and compression problems caused by the RF amp, first mixer, IF strip, and filters.
The antenna connects directly to a 1:4 analog multiplex driven by a clock that's effectively four times the tuned frequency. The output of each mux channel feeds a capacitor that acts as an integrator. Each capacitor is switched in at 0, 90, 180, and 270 degrees at the tuned frequency. The RC time constant of these integrators determine the basic selectivity of the receiver, and the tuned frequency is determined solely by the rate at which the mux is clocked.
The outputs of those integrators are combined, producing an I and Q channel (90 degrees out of phase) that can be processed using a Weaver phasing network or fed to a sound card for downstream DSP detection and processing.
This is a pretty exciting concept. The front end is quite simple; just a few high-quality chips, some even available in through-hole packages. The most complex part of the receiver is the LO, which can be provided by a commercial or kit DDS generator. (Click the picture of the "IQPro".) Since there are no RF components prior to the detector, general coverage (broadband) performance is inherent in the design.
And the very same process works in reverse for transmission, greatly simplifying transceiver design.
I first heard of Tayloe's detector a number of years ago. I was excited, but decided to wait until a few receivers had been built to see if his concept provided the promised performance, or whether there was a fatal "gotcha" hiding within his design. Well, I'm embarrassed to admit that, in those years, I'd completely forgotten to keep tabs on its progress.
I recently stumbled across a link to a radio implemented using the Tayloe detector. The receiver showed promise, so I did some more searching and found a veritable flood of receivers and transceivers that implemented the Tayloe design. And the promise of high performance and inherent simplicity proved true. Here's a link to a relatively simple CW transceiver built using a Tayloe detector. This paper presents a detailed analysis of its performance, showing that the Tayloe detector, properly implemented, outperforms the best commercial superhet designs.
First, how does this pertain to the Picaxe, and second, what are the implications for us amateur scientists?
Full control of a Tayloe receiver is well within the capabilities of a Picaxe 28X1 or 40X1.
For us amateur experimenters, we can implement and experiment with Tayloe receivers without a king's ransom worth of test equipment, or access to a commercial fab factory. There's very little RF-type circuitry in the design. Most of the receiver is implemented with technology well within the capabilities of the amateur experimenter to understand, build, and measure.
Some further links:
Dan Tayloe's white paper that explains his design.
A stand-alone receiver implemented with a Tayloe detector.
A series of articles, "A Software-Defined Radio for the Masses." Scroll down the links; these four papers will be found about two-thirds down the page.
In addition to a good description of the Tayloe process, these papers are a very good primer on PC-based DSP processing, with lots of "how-to" information.
I haven't begun actively experimenting with Tayloe detectors yet, but I've been studying hard. I just ordered a DDS-60 kit, which incorporates an AD9851 DDS chip. This doesn't include a processor, so I'll experiment with a 28X1 for basic control.
The AD9851 isn't really suitable for a high performance receiver due to its 10-bit DAC. It's probably going to produce a receiver rampant with spurs. However, the kit is dirt cheap. I think it will provide a lot of useful knowledge and experience without costing an arm and leg.
(My budget for my initial experiments is in the vicinity of USD $150. Unfortunately, a good deal of that figure will be shipping costs. Grrrr!)
If it all comes together, and I find that I want to actually build a high quality receiver, I'll consider something that uses an AD9854 DDS chip. That chip contains a 12-bit DAC, and should provide substantially better spur suppression than the AD9851 chip.
So, Jurjen, if you're still pursuing your HF receiver design, I hope that the Tayloe detector will provide you another option. If your project has stalled, maybe this will get it back on track. For those who have never contemplated a homebrew HF receiver, Tayloe opens the door to performance that you can actually attain, at home, and for not very much money.
Tom
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