View Single Post
Old 28th Jul 2022, 1:46 pm   #163
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 534
Default Re: 6-gang FM stereo tuner heads

Originally Posted by Radio Wrangler View Post
Q as we know it today, used to be called the Circuit Magnification Factor. Marconi Instruments' Q meter was called their Circuit Magnification Meter, making it sound rather grand.

What this means is that the circulating power in a tank circuit of reasonable Q is significantly larger than the input and output power.

In FM tuners, it's usual to use dual-gate MOSFETs and to match these circuit impedances are run high. This means that the signal voltage in the tank is much larger than the signal voltage coming from the antenna.

Varactors are very much voltage controlled devices. Voltage sets their capacitance, so the increased voltage from unwanted large signals is made even worse, modulating the tank's tuned frequency by a greater amount, dramatically worsening the strengths of intermod products.

To avoid overload, most people would think to narrow the input tank to try to filter them out, but this means higher Q and greater voltage magnification of unwanted signals close to the wanted channel.

So this needs a tuning capacitor... The cartwheel has spokes, each spoke being a back-to-back pair of varactors. So there is a dozen varactor pairs, all working in parallel. Our tank has only tiny inductance and very large capacitance. It is very low-Z. Signal voltages are very low, yet the varactors run with tens of volts of DC bias to tune them.

Coupling into and out of this monster tank was by coupling loops also printed in the PCB. A thin ring at half diameter distributed tuning voltage to the common cathode points of the varactor pairs.

It was patented as an oscillator, where it gave an advantage in phase noise, but I spotted that it was a route to low intermod varactor tuned agile filters. It would be superb for an FM tuner and would haul back some of the advantage that mechanical variable capacitors still have. It could be taken to extremes and beat the mechanical capacitors (unless you use butterfly variable capacitors, you wind up with a sliding contact in your tank - everyone forgets this!)

So, yes, there is still fun to be had in FM tuner design, and designers still have unused tricks up their sleeves. But FM radio seems to be in decline. The BBC doesn't broadcast my sort of music, and I don't see any commercial return from designing something a bit different. I could do it for the hell of it, but I might as well pick an area with some profit in it to spend the effort on.

Anyway, so now you know why mechanical variable capacitors currently have advantages in FM tuners.

Interesting! The cartwheel oscillator sounds like a Frankenstein kind of looking device! I can't find the patent for this.

In term of the equivalent circuit model, the high Q varactor diode can be idealised as the a capacitor (junction capacitance of the depleted region) in series with a resistor (low resistance undepleted regions). The Q is sensitive to the reverse biased voltage. As the bias voltage increases, the depleted layer thickens, reducing the junction capacitance and the undepleted layer resistance. Hence the Q will go up.

One of the key advantages of varactor tuning is the possibility of miniaturisation of receiver circuits. However, the Q of inductors suffer as their physical dimensions, getting below a certain threshold. I attached the photo and schematic of my Mitsumi FE-352 micro tuner. It is super cute but I have not got time to build circuit to make it working.

There is a kind differential tuned varactor L-VCO that has very low phase noise used in microwave applications. I dont know much about:

The demise of AM/FM analogue broadcast will be inevitable, like digital photography replacing film photography. The only difference is that the DAB is an epic technological failure that it is not fit for purpose,..a joke. I have a very diverse music taste; classical, pop, trance, hip hop, dance, heavy metal, blue, jazz...I listen to BBC 1, BBC 4 and London LBC most of the time.

Originally Posted by G0HZU_JMR View Post
I put an aerial on the input to one of my spectrum analysers this evening and set it to capture about 20 seconds of the signal from a couple of FM broadcast stations. Reception isn't that great here as I don't have a proper aerial up for FM.

See below for the spectrum on a 200kHz span and the demodulated stereo signal from 0 to 100kHz. This shows the 19kHz pilot tone and the demodulated L+R up to 15kHz and also the L-R modulated on a suppressed 38kHz carrier. I think the strange modulation up at 57kHz is RDS.

The first image captures a quiet moment and this clearly shows the 19kHz pilot tone, the suppressed 38kHz carrier and the RDS waveform up at 57kHz. The second image shows a classic instrument and the third shows some pop music. The analyser works in real time so the screen display is very fluid in terms of update rate so it can be quite interesting to watch it whilst listening to the station at the same time.

It looks like the music has about 15kHz bandwidth although some modulation effects can be seen around the 19kHz carrier. I was expecting the demodulated signal to be a bit cleaner than this in this region.
Having no external antenna, I do get multi-path interference in my flat a lot.
Two years ago, I read the book "Stereo FM Radio Handbook by Harvey and Boltman. For me, it is a classic text that introduces me all the basic concepts of multiplexing. At that time, I knew nothing about FM.
Attached Thumbnails
Click image for larger version

Name:	m‫itsumi FE-352.jpg
Views:	28
Size:	51.0 KB
ID:	261702   Click image for larger version

Name:	mitsumi FE352 FM tuner.jpg
Views:	36
Size:	79.1 KB
ID:	261704   Click image for larger version

Name:	differential varactor  VCO.jpg
Views:	28
Size:	67.7 KB
ID:	261705  
regenfreak is offline   Reply With Quote