6-gang FM stereo tuner heads

[G0HZU_JMR]

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.

[Radio Wrangler]

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.

Ever get the feeling you just can't win?

The root cause is that the varactor diode needs to operate n a low-signal-voltage environment, and you want to use diodes needing plenty of tuning volts to scale the problem down. So the trend to low voltage varactors so tuning voltages can come from 5v supplies is very undesirable.

Varactors can be embedded in low impedance tanks and can produce reasonable Q and not suffer magnified signal voltages.... they get magnified signal currents instead, but that's not so devastating.

An R&D engineer at HP in California, Bart McJunkin, invented what he called a 'cartwheel oscillator'. It used a printed inductor looking like a very short very fat piece of coax. say 50mm in diameter, 1.6mm long! So it was a 2 inch ring of track on each side of the PCB with hundreds of stitches connecting them. This formed the outer of the coax, and the inner was a lily-pad of copper in the middle of each side, also connected together with loads of stitches. His oscillator circuit amplifier lived on the lily pad.

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.

David

[regenfreak]

Originally Posted by Radio Wrangler

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.

David

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:

https://ieeexplore.ieee.org/document/1324731

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

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.

[regenfreak]

I have tested the Nelson Jones's IF and detector circuit. It works OK but it does not sound that great. My homebrew valve IF strip is staggered tuned and it sounds great at the expense of poorer selectivity.

I use 10.7MHz Toko IF transformer from ebay as the quadrature detector coil for the TAA661B chip. It gets lots of distortion if the coil is not fined tuned at its sweet spot. Maybe the audio quality gets better if I change its 4.7k resistor across the tank circuit...anyway I am going to try LA1222 IF chip based on the IF stages of the Kenwood KR-9600 (1600Watts per channel, I mean who can play that level of output power before the neighbors complain and police turn up at your door step)

[regenfreak]

I have replaced the lossy cores by T50-6 for the 5-pole 10.7MHz Chebyshev with a bandwidth of 300kHz and 330ohms I/O impedance. The insertion loss is about -6bd including some mismatch losses with the L-pads. Tuning is very sharp and it is very difficult to make it perfectly flat and symmetrical in practice. So this is the best I can get so far. The S21, S11 and group delay are attached.

I will compare the overall responses between the following IF Amp and bandpass configurations:

1) 280kHz ceramic filter ---->CA3053 cascode IF amp--->280kHz ceramic filter

2) 280kHz ceramic filter ----> CA3053 cascode IF amp------> 5-pole 300kHz Cherbyshev

3) 280kHz ceramic filter --->LA1222 IF amp--->280kHz ceramic filter---->LA1222 IF AMP--->two 280kHz ceramic filter in cascade (total 4 ceramic filters like the Kenwood KR-9600)

4) 280kHz ceramic filter --->LA1222 IF amp--->280kHz ceramic filter---->LA1222 IF AMP--->5-pole 300kHz Cherbyshev

LA1222 is two IF amps combined in one package. I am building the TA7303P IF amp and discriminator IC using homemade discriminator coil with a blank Toko style transformer. I can add a signal strength meter for the TA7303P later. The MPX decode is TA7343AP.

Further experiment with the TAA661B chip has been futile. I swapped different 10.7MHz tank coils for the quadrature detector with different impedance and Qs. I can only get mono with some audio distortion, potentially due to the mismatch in impedance and Q. I do not bother to mess about with the value of the damping resistor which affects the loaded Q of the LC quadrature detector coil. There is plenty of gain and selectivity though. I strongly believe the TA7303P IF amp and discriminator IC will be much better.