The Unsung hero of radio!

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
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sinus trouble
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The Unsung hero of radio!

Post by sinus trouble » Mon Aug 22, 2016 11:07 pm

Greetings Necks!
Id like to take this moment to pay tribute to the humble FM reciever that we all take for granted!
I believe it is the beating heart of radio and without it? A transmitter would be a useless piece of junk belching out sh*t that no one can understand!
It can search out signals, produce superior audio, decode stereo and RDS!!
The sad fact is that most users on this forum could not explain how a basic reciever works?? :(
Now before i face a barrage of criticizm? i am one those people!
In the past i have built many inferior circuits incorporating the diode detector style which in effect is actually an AM detector! it does work? but not great!
Digging deeper is where it gets complicated! A local oscillator is introduced as a reference frequency and the incomming signal from the antenna is added! Therefore the difference between them will show an output!
Makes sense? Its not that simple! to reduce the high frequency demodulation 10.7 mhz difference between local and recieved is used (IF frequency) this makes it easier for lower frequency devices to handle demodulation (I think?)
Anyways enough of my rambling on! If you have any circuits or comments to add? id be most grateful! :)
I am as stupid as I look! :|

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Re: The Unsung hero of radio!

Post by Albert H » Tue Aug 23, 2016 1:36 pm

The FM receiver is a fairly complicated beast, but it can be broken down into several pieces, to make understanding it a little easier:

Let's start at the aerial. Any piece of metal will act as an aerial. Ideally, it should have a length proportional to the wavelength of what you're going to receive, but in most cases it isn't - it's just a random length.

The next bit is the input filter and amplifier. Since the aerial isn't actually related in length to the wavelength (in most cases), the input stage will provide a crude (and inaccurate) impedance transformation to feed the input filter. Some cheaper receivers get away with omitting that amplifier, but it really can help if you're just using a telescopic aerial. This amplifier works at Band II and doesn't provide much gain - it's more to do with matching. Some more sophisticated receivers use a filtered, tuned amplifier here, for the very best sensitivity, but most don't bother.

The next two parts are intimately connected together, and in the case of really cheap receivers, may be done in just one transistor. We have a "local" oscillator (LO), which is running either 10.7MHz above or 10.7MHz below the incoming signal. This LO is usually PLL-tuned these days, but older receivers used VFOs (and some clever tricks to stabilise them). The mixer combines the incoming signal with the oscillator signal, to produce a number of mixing products. The strongest of these will be the 10.7MHz difference signal, and this is selected by a tuned circuit filter which is set to a fixed frequency of 10.7MHz.

This frequency conversion is done because it's easier to greatly amplify a signal at one frequency, rather than a range of frequencies. The incoming signal will be frequency-modulated - varying slightly in frequency to represent the audio - and this variation is maintained in the 10.7Mhz signal. 10.7MHz was chosen in the late '50s as the "industry standard" "Intermediate Frequency" - IF - because there was a conveniently unused bit of shortwave there and it was a low enough frequency to allow the use of cheap amplification components. Also, 10.7MHz filters can be reasonably small sized, allowing compact receivers.

Three things go on in the IF stages - lots of gain, filtering and - very importantly - "limiting". Usually, the signal is amplified to the point at which a diode clipper can go into heavy conduction. This slices the top and bottom off the signal, but the FM information is still there in the squarewaves that come out of the limiter. The best part is that it also slices the interference (the AM components) off the signal, leaving pure FM.

The last bit is the "discriminator". This turns FM back into audio (or stereo multiplex). Basically there are three ways of recovering the audio - the most popular is the "Foster-Seeley Discriminator" which uses a tightly-tuned 10.7MHz coil, and a couple of diodes measuring the amplitude of the signal across the coil - deriving a variable voltage that is the audio. The next option is to use a PLL (phase-locked loop) which is tuned to 10.7MHz and follows the incoming IF. The error voltage from the PLL is the audio! The last version is least frequently used and is called the "pulse-counting detector". This charges a capacitor at a rate determined by the incoming frequency, and measuring that voltage will provide the audio.....

Construction of good receivers used to be quite difficult, but the advent of mass-produced hi-fi gear meant that ICs for various parts of a receiver can be bought cheaply. My basic Band I or IV receiver uses a dual-gate FET input stage, dual-gate FET mixer, bipolar transistor crystal oscillator, a bipolar transistor for the first IF amplifier (and to match the ceramic filters I use), followed by a CA3189 IF amplifier / limiter / discriminator chip. The final stage is an op-amp to provide a good low-impedance modulation output. I also add an audio frequency PLL looking for either a 19kHz stereo pilot or a sub-audio tone to operate a relay to switch the main rig.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by sinus trouble » Tue Aug 23, 2016 9:18 pm

Many thanks Mr Albert! :)
For something that was invented decades ago, It is a true marvel of tecnological acomplishment!
I find them fascinating, moreso than transmitters!
Once again you have taught me a few things! I did not know about the IF limiting? It makes perfect sense! :D
I also like the concept of your band 1 recievers, it reminds me of the similar version shown on the old NRG workshop!
band_1_circuit.jpg
I have to add the classic TDA7000 tuner IC too! :D i had one many years ago but they are hard to source these days :(
mine had a large trimmer for setting the local oscillator but this one utilizes a varicap which is a nice addition!
TDA7000.jpg
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I am as stupid as I look! :|

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Re: The Unsung hero of radio!

Post by Maximus » Tue Aug 23, 2016 10:07 pm

I remember an old timer telling me about how 'super hetrodyne' was all the rage when it first came out lol


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Re: The Unsung hero of radio!

Post by sinus trouble » Tue Aug 23, 2016 10:43 pm

LOL To my grandparents it was known as the 'Wireless' and they loved it!! :D
They say technology has changed dramatically over the past few years? but its all based on what the innovators of the past produced!
Every device has some sort of transmitter and/or a reciever on board these days!
Companies bang on about lithium battery technology, its been around since the 70s
So called 'Digital' brushless motors! Technics had them in thier SL turntables for decades!
I guess when the first radios were invented? it must have been such a surreal moment, like the future had arrived lol :)
I am as stupid as I look! :|

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Re: The Unsung hero of radio!

Post by Albert H » Tue Aug 23, 2016 11:34 pm

sinus trouble wrote:I have to add the classic TDA7000 tuner IC too!
TDA7000.jpg
The TDA7000 was a clever design. It had a very low IF (around 100kHz) so that it could use (effectively) audio processing for the IF part of the receiver. It also made use of a pulse counting discriminator, and eliminated the need for most of the coils usually found in a receiver.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by Albert H » Tue Aug 23, 2016 11:37 pm

Maximus wrote:I remember an old timer telling me about how 'super hetrodyne' was all the rage when it first came out lol
Almost all modern receivers are "super heterodyne" types - these are the type that convert the incoming frequency to an Intermediate Frequency. The IF stages are where all the sensitivity and the selectivity of a receiver comes from.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by sinus trouble » Wed Aug 24, 2016 12:36 am

I couldnt agree more Albert!
The super het was a major breakthrough in radio communications! The local oscillator essentially provides a 'clock' to compare any type of modulation which is the basis of most digital designs today
I am as stupid as I look! :|

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Re: The Unsung hero of radio!

Post by Albert H » Wed Aug 24, 2016 2:53 am

Of course, the very latest technology in receivers (like the Tecsun jobs) is "DSP". There are now digital ICs that can convert an incoming radio signal directly to a digital facsimile and do all the processing in the digital domain.

I've been playing with DSP "Software Defined Radio (SDR)" modules recently, generating a DAB multiplex using completely Open Source Software and really cheap computer hardware. I'm doing the basic audio streaming using Raspberry Pis, and the multiplexing on an old laptop, and cheap sound cards for the audio inputs. All I now need to build is some tight filters for the output of the SDR board and a big linear Band III PA, and we can have a pirate multiplex on the air for a few hundred quid!

Personally - having had lots of experience with DAB - I can assure you all that FM (and AM) isn't going to die out any time soon!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by radionortheast » Tue Aug 30, 2016 10:27 am

dsp isn't everything sony xdr wins on selectivity but on sensitivty it was kind of deaf, deffinatley had more senstive tuners specially ones where you replace the filters with narrower ones. lot of cheap sdr sticks dsp radios just overload when hooked up to outdoor aerial

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Re: The Unsung hero of radio!

Post by ronald001 » Tue Aug 30, 2016 12:06 pm

Albert H wrote:we can have a pirate multiplex on the air for a few hundred quid!
!

Albert, are you willing to share this info with us?
Really interesting stuff! :tup

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Re: The Unsung hero of radio!

Post by Albert H » Tue Aug 30, 2016 1:06 pm

There's a Google Group dedicated to the development of Open Source DAB encoding software. The only expensive part of the whole kit is the Hack One RF SDR board. The rest is cheap commodity computer hardware and sound cards (for Raspberry Pi).
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by ronald001 » Tue Aug 30, 2016 2:00 pm

Okay, i will look for this group.

Thanks!

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Re: The Unsung hero of radio!

Post by NOYB » Mon Sep 05, 2016 9:32 am

The group is called MMB tools.

https://groups.google.com/forum/#!forum/crc-mmbtools

You can run the encoding and multiplex on a Raspberry Pi or ODroid. Maybe even generating the IQ signals as well. The Hack RF SDR is not quite good enough quality to create a proper spec signal, but it works OK. Then you need amplification - because you only get a mW or so out of the Hack RF - and they are prone to going low output as well.

If you are good at Linux it's simple. If not, then it's challenging but not impossible.

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Re: The Unsung hero of radio!

Post by Albert H » Mon Sep 05, 2016 6:35 pm

The Hack RF can be made good enough to drive a big amplifier - there are details in a couple of the postings in that Google Group. I used a MAR6X to drive a cavity filter, then the PA followed by a further cavity filter.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by reverend » Fri Oct 29, 2021 3:02 pm

Maximus wrote:
Tue Aug 23, 2016 10:07 pm
I remember an old timer telling me about how 'super hetrodyne' was all the rage when it first came out lol
The 'heterodyne' principle was invented in 1901 by a guy called Reginald Fessenden. He has another claim to fame, which is that he also invented Amplitude Modulation. Now there's an unsung hero for you...

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Re: The Unsung hero of radio!

Post by Albert H » Fri Oct 29, 2021 8:26 pm

There are a few other "unsung" heroes too: John Ambrose Fleming (my Great-Great-Uncle!) invented the thermionic valve! Without these, we'd still be using iron-filing "coherers" for reception!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: The Unsung hero of radio!

Post by reverend » Wed Nov 24, 2021 4:22 pm

Albert H wrote:
Tue Aug 23, 2016 11:34 pm
The TDA7000 was a clever design. It had a very low IF (around 100kHz) so that it could use (effectively) audio processing for the IF part of the receiver.
I used the TDA7000 for some link RXes and then gave up very quickly. The low IF is fine if the band is empty but it means that it has an image at around ~200 kHz from the tuned frequency which is impossible to filter out. Dreadful IC.
if it ain't broke, keep tweaking

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Re: The Unsung hero of radio!

Post by Albert H » Thu Nov 25, 2021 3:42 am

The TDA7000 also freaks out in the presence of any strong RF field.

I used to use the TDA1062 in my link receivers for Band I or III. It was OK, but I got much better results when I reverted to ordinary discrete transistors and FETs (dual-gate for the mixers!).
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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