single transistor receivers and transmitters designs?

[ssagerian]

Hi Guys,
Let me start out by saying that I am not an RF engineer, just a embedded firmware guy with some hardware design (some analog). so I have been thinking about if its possible to have a single transistor receiver that could decode some basic on off modulated  RF signals? I would like to start learning about this, perhaps do some low power designs after a bit of learning. Any links to design discussions, tutorials etc would be welcome.

thanks
Steve

[vk3yedotcom]

For receivers you can achieve the following in one transistor (OK some of the below use 2 or 3 but you can strip them down for reduced performance)

* Regenerative receiver.  Receives AM/CW/SSB signals up to about 20 MHz.  Even digital modes eg PSK31 if connected to a computer's soundcard.  Mostly free-running and unstable. Uses positive feedback so can be touchy.  Builders often add an audio and/or RF amplifier for better audio output and/or isolation from the antenna.  Eg:

 

* Reflex receivers.  Use the one transistor to amplify both RF and audio.  Can be combined with regenerative circuits but a bit touchy as the transistor is doing multiple things.

* Direct conversion receiver.  Receives AM/CW/SSB signals up to about 20 MHz.  Even digital modes eg PSK31 if connected to a computer's soundcard.  Can be controlled by a crystal or ceramic resonator for good frequency stability but limited tuning range.  A 1 transistor version uses the transistor as the local RF oscillator and a diode mixer. However audio output will be very low unless you add audio amplification. Maybe even 'cheat' and use a sound card. Eg

 

* Superregenerative receiver.  Receives AM and wide FM signals.  Suitable for 27 MHz and above (ie VHF).  Broadband.  A 1 transistor version has just a regenerative detector but more transistors allow better audio output and isolation from the antenna.

 

For transmitters you can achieve the following

* Crystal controlled HF CW transmitter.   Can be heard several hundred km away. Can be modulated to transmit AM or FM.



* As above but controlled by computer for digital  modes eg WSPR.  This can be heard world wide.  Eg



* FM bug. Transmits voice to an FM receiver. See the Talking Electronics website for designs and theory.

* VHF crystal controlled transmitter. Uses crystals in the low VHF range this allows stable operation over several kilometres. Frequencies like 30 - 40 MHz are fun to play around with.  Eg



There is an elegance with doing it all in one transistor. People have even made a transmitter AND receiver in the one transistor. 

However performance increases exponentially when you go from 1 to 2 transistors and again to 3 transistors.  And oddly if you want to learn about stuff a 2 or 3 transistor design can be easier to get going than a 1 transistor circuit that tries to do everything (often poorly).

More on my website but that's just the tip of the iceberg.

[tggzzz]

Bear in mind that if you transmit any signal then you must be very certain you can demonstrate that you are only transmitting in bands for which you hold a current licence.

That's very necessary since otherwise you might stomp all over and disrupt other legitimate users, e.g. air traffic control, emergency services, military and many others. That is a criminal offence, and the laws are enforced. Beware.

You would be well advised to talk to you local radio ham society.

[ssagerian]

I am definitely interested in building a hybrid regenerative RF receiver mated to a low power micro. The idea would be to have the micro wakeup every so often, turn on the receiver and try to detect a specific tone or frequency for some period (maybe 10 cycles? ). Then take some action..similar to a garage door opener.  I guess a garage door opener listens all the time since its powered via mains but this device would use battery (or possibly a pv cell) so power consumption would be critical.
I found a set of notes from Charles Kitchin, and will also scrutinize the schematics in the videos you have provided..

[vk3yedotcom]

Many countries have frequency allocations for short range devices, low interference potential devices, FCC Part 15 etc where an individual licence isn't required if its power output is low.

Another possibility, if you're not wedded to building every part of the unit, is to use UHF transmitter modules.  These are suitable for applications like slow speed data or remote control.

[tggzzz]

Many countries have frequency allocations for short range devices, low interference potential devices, FCC Part 15 etc where an individual licence isn't required if its power output is low.

The problem for RF beginners is that often they don't realise the practical consequences of their design, in particular that harmonics and intermodulation means they will be transmitting on frequency bands other than their expected/intended band.

A useful sanity check for beginners is to ask themselves whether or not they know why IP2 and IP3 specifications are important. Then, knowing a device's IP2/IP3 specification, what will be the frequency spectrum of their signal when it hits the antenna. If they can't answer that then they don't know enough to be sure they've built a legal transmitter.

Another possibility, if you're not wedded to building every part of the unit, is to use UHF transmitter modules.  These are suitable for applications like slow speed data or remote control.

IMNSHO, for RF beginners using pre-built and pre-certified modules is almost always the best route.

[Z80]

Many countries have frequency allocations for short range devices, low interference potential devices, FCC Part 15 etc where an individual licence isn't required if its power output is low.

The problem for RF beginners is that often they don't realise the practical consequences of their design, in particular that harmonics and intermodulation means they will be transmitting on frequency bands other than their expected/intended band.

A useful sanity check for beginners is to ask themselves whether or not they know why IP2 and IP3 specifications are important. Then, knowing a device's IP2/IP3 specification, what will be the frequency spectrum of their signal when it hits the antenna. If they can't answer that then they don't know enough to be sure they've built a legal transmitter.

Another possibility, if you're not wedded to building every part of the unit, is to use UHF transmitter modules.  These are suitable for applications like slow speed data or remote control.

IMNSHO, for RF beginners using pre-built and pre-certified modules is almost always the best route.

Whilst I understand where tggzzz is coming from I think it is a bit far fetched to expect a beginner to understand intermodulation figures let alone have the necessary test gear and experience to make the measurements.  Hands up anyone who didn't build FM bugs and the like as a kid, I know I did and never had any issues, the signal bareley went more than a few metres.  I would say sticking to a small signal low frequency jellybean transistor like a BC548 and a low energy power supply like a 9v battery would produce a circuit incapable of generating any significant power.  As long as no external aerial is used it would be very unlikely to cause any problems.  With those 2 limitations I would positively encourage experimentation, it's the only way to learn.

[dmills]

I even managed to get into 'trouble' with a super regenerative RX as a 12 year old.

My dad had announced that he was going to take us to an air show, so me being me I threw a VHF super regen set together in a small cake tin so I could listen to the planes, IIRC the valve was a QQV03-20 or similar as I knew they were good for VHF.
Well, turns out that using a superrgen set to listen to the planes while stood right under the tower causes the tower to keep asking the planes to change frequency, (My LO turned out to be radiating the best part of 100mW while squegging like a bastard in the ultrasonic)....

A man with a yagi and a field strength meter turned up, asked if he could examine my radio, then suggested that he might be able to get me into the tower to listen on one of their radios, and that this would be better (As it was raining), smart man.

I did tend to get lucky in my interactions with the wireless authorities back then.

Regards, Dan.

[Z80]

Like the story Dan.  No wonder you caused them problems standing under the tower with a 100mW valve set singing away.  Good on the ATC to invite you in, nowadays you would probably get carted off under some bs anti terror law.  This is quite relevant to the OP as a regen / super regen will be high on the list of simple one transistor receivers.  As you need to hook them up to a good aerial, it would be wise in this case to break the rule and use a second transistor as a preamp to reduce the emissions and get better performance into the bargain.

[mmagin]

Given that a superregenerative receiver also tends to radiate unless you isolate the detector stage from the antenna, I have wondered if a single-transistor superregen could be used as a full-duplex, low-bandwidth (maybe for slow data rates) transceiver.

[Kilrah]

I know I did and never had any issues, the signal bareley went more than a few metres.

The signal you wanted didn't, but you might have been radiating 100mW on some random other band without knowing it... his post is about knowing you could, and checking you don't do that.

There are infinitely more wireless devices and reliance on them nowadays than there used to...

[vk3yedotcom]

Given that a superregenerative receiver also tends to radiate unless you isolate the detector stage from the antenna, I have wondered if a single-transistor superregen could be used as a full-duplex, low-bandwidth (maybe for slow data rates) transceiver.

Potentially.  Eg http://www.vk2zay.net/article/156

But for a beginner the real minimalist designs can be harder to get going (and diagnose faults) than something that uses a few more transistors and more easily traceable stages and signals.

[mmagin]

Given that a superregenerative receiver also tends to radiate unless you isolate the detector stage from the antenna, I have wondered if a single-transistor superregen could be used as a full-duplex, low-bandwidth (maybe for slow data rates) transceiver.

Potentially.  Eg http://www.vk2zay.net/article/156

But for a beginner the real minimalist designs can be harder to get going (and diagnose faults) than something that uses a few more transistors and more easily traceable stages and signals.

Yep, not beginner, guess it's kind of a thread tangent.  I have a reasonable set of RF test equipment.  Always interested in weird minimalist weekend projects though