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Old 27th Mar 2022, 12:06 pm   #21
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Default Re: 6-gang FM stereo tuner heads

It's a perfectly serious question and has a simple answer.

On the HF bands, communication is limited by the levels of atmospheric noise and now the amount of man made crap over that. Communications receivers just need to be better than this noise level. Most development goes into getting better large signal handling.

On the VHF bands, noise levels are far lower. An antenna pointing at a quiet part of the sky sees a noise level equivalent to the thermal noise from a perfect resistor cooled to 3 Kelvin. Noise from most man made stuff has rolled off well before these frequencies. So on VHF, there is benefit from getting as low a noise contribution as possible in a receiver front end.

The front end of a VHF receiver needs filtering for two reasons.... firstly to attenuate the mixer image frequency. The image crosses the frequencies of other services you don't want to hear along with the station you've tuned to. Secondly the front end filter reduces the number of unwanted signals hitting your RF amplifiers and mixer. This is needed to be able to work amongst a band full of other signals without suffering crossmodulation and intermodulation and blocking effects.

Filtering is not without its costs. If I make a filter narrower, for a given technology and quality factor of components, its loss increases. Increased loss makes the noise performance of our radio worse, just like adding a normal attenuator on the input. So we have a compromise between noise and overload behavuours.

High Q resonarors like Quartz crystals offer much lower loss for narrow bandwidth, but we can't use them if we want to be able to tune to other frequencies, and it is very difficult making them wide enough to pass an FM broadcast signal. We are stuck with L-C filters. Mechanical variable capacitors give better Q than varactor tuning diodes, so the capacitor-string-and-pointer tuners have an advantage over electronically tuned ones.

So the RF section can't have much selectivity ahead of its first amplifier, without much selectivity the amplifier had better not have much gain. But we've helped the noise a little and got some selectivity in. Time for some more selectivity and then another amplifier.

So the best compromise splits the amplification needed into several low-ish gain stages, and splits the selectivity needed into several mild stages, and interleaves the amplifiers and filters.

So you wind up with expensive FM tuners having many tuned resonators in the RF section and a few RF amplifier stages.

Sitting at home I have a Revox B761 tuner. It is ex-BBC, who used them for two purposes: To monitor transmission quality, or to provide audio to modulate a VHF FM transmitter by receiving signal from another transmitter. This was used for some fill-in transmitters and as a back-up at the bigger sites. So for an FM tuner to work at broadcast quality levels receiving a distant transmission while based at the site of a working VHF/FM transmitter isn't bad going! It uses varactor tuning, several stages and a carefully designed RF amp and mixer. Incidentally, the IF/limiting stages are not all lumped into one chip, there are several discrete stages. It doesn't use ceramic filters, it uses L-C adjustable sections and the discriminator is a pulse-count type. It works rather well and shows good enough performance can be had by taking care without going silly.

David
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Old 27th Mar 2022, 12:07 pm   #22
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Default Re: 6-gang FM stereo tuner heads

Comtekk needs a decent VHF signal generator with an external modulation input jack. So my junky walkie-talkie FM generator is no good. There is a 100MHz version of FY6900 Dual Channel DDS Function (about £140 incl import taxes) that has this feature. Obviously what you pay what you gets.
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Old 27th Mar 2022, 12:12 pm   #23
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Default Re: 6-gang FM stereo tuner heads

I spent 30 years designing test equipment... at HP of all places, so some of those spectrum analysers, scopes, sig gens and an awful lot of specialised stuff has bits in it I designed. I originated the new family of noise figure analysers which went on sale about 20 years ago, and I did the smart noise sources to go with them.

After Agilent (renamed instruments side of HP) pulled development out of the UK, I spent 14 years working for a start-up avionics firm designing radar transponders and com/nav radios.

So my hobbies and jobs have been somewhat intertwined with the exception of the horses and rally cars.

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Old 27th Mar 2022, 12:23 pm   #24
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Default Re: 6-gang FM stereo tuner heads

Quote:
High Q resonarors like Quartz crystals offer much lower loss for narrow bandwidth, but we can't use them if we want to be able to tune to other frequencies, and it is very difficult making them wide enough to pass an FM broadcast signal. We are stuck with L-C filters. Mechanical variable capacitors give better Q than varactor tuning diodes, so the capacitor-string-and-pointer tuners have an advantage over electronically tuned ones.

So the RF section can't have much selectivity ahead of its first amplifier, without much selectivity the amplifier had better not have much gain. But we've helped the noise a little and got some selectivity in. Time for some more selectivity and then another amplifier.

So the best compromise splits the amplification needed into several low-ish gain stages, and splits the selectivity needed into several mild stages, and interleaves the amplifiers and filters.

So you wind up with expensive FM tuners having many tuned resonators in the RF section and a few RF amplifier stages.
Thanks for that. What is the typical overall bandwidth of a stereo FM RF front end? Based on the RCA paper, the overall bandwidth of their FM test jigs are very wide ( 700kHz -900kHz) as I mentioned above. The paper omitted this crucial information in the texts and in the figure captions. But I estimate it from the graph. In the USA, the station adjacent channel spacing and channel width are 200kHz:

https://en.wikipedia.org/wiki/FM_bro..._United_States

Quote:
I spent 30 years designing test equipment... at HP of all places, so some of those spectrum analysers, scopes, sig gens and an awful lot of specialised stuff has bits in it I designed. I originated the new family of noise figure analysers which went on sale about 20 years ago, and I did the smart noise sources to go with them.

After Agilent (renamed instruments side of HP) pulled development out of the UK, I spent 14 years working for a start-up avionics firm designing radar transponders and com/nav radios.

So my hobbies and jobs have been somewhat intertwined with the exception of the horses and rally cars.
very impressive

Last edited by regenfreak; 27th Mar 2022 at 12:30 pm.
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Old 27th Mar 2022, 12:45 pm   #25
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Default Re: 6-gang FM stereo tuner heads

My bandpass Q's for the three stages are entirely based on this picture I found on makeradio forum a while ago. It is supposed to be from a german car FM tuner. But I do not know the model....this is the one and only one information on the individual bandwidths of a FM stereo front end multi-stages. The scarcity on the subject is unbelievable.
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Old 27th Mar 2022, 5:03 pm   #26
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Default Re: 6-gang FM stereo tuner heads

Probably I don't need any SINAD software at all. Recently I bought a two channel analogue voltmeter (see pic) rated up to 1HMHz.

For example, a receiver of sensitivity of 0.25microV at 12dB SINAD. With a 50 ohm system 0.25microV = -119dbm.

The cheap FM walkie talkie signal generator can go down all the way to -132dbm or 0.0563 microV. The following video shows the procedure how it can be done with an analogue voltmeter at 33 minutes:

https://youtu.be/C9ydYR9hlCk

The video is a bit confusing, he was actually calculating:

SINAD = 20log(S+N+D)/(N+D)
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Old 29th Mar 2022, 10:27 am   #27
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Default Re: 6-gang FM stereo tuner heads

Thanks for the answers to my question, but I'm still a bit puzzled. Yes you need to filter out the image frequency and any nearby strong signals, but most domestic FM tuners do OK without multiple stages of filtering before the mixer. Excluding the BBC's very special case of a receiver "based at the site of a working VHF/FM transmitter", when/why are those extra stages of filtering needed? Is it to receive very weak distant broadcast transmissions?
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Old 29th Mar 2022, 11:17 am   #28
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Default Re: 6-gang FM stereo tuner heads

Specmanship!

Some people spend hours gazing at specs of things they are buying in order to choose which one. Decisions get made on the value of a parameter even if both contending products are perfectly sufficient for the user. Reviewers reinforce this.

Everyone wants "the best". Most people lose sight of what they need.

David
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Old 29th Mar 2022, 6:13 pm   #29
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Default Re: 6-gang FM stereo tuner heads

The same applies to HF transceivers; V4 vs V8 sport cars, Rolex watches.... If people have the money, they splash out on the best they can buy, even they don't really need all the features. The number of tuning gangs in a tuner correlate with the price point and performance of the product.

I am lusting after a Siglent SSA3021X Spectrum analyzer ( 2.1GHz,161dBm/Hz ,-98dBc/Hz @ 10kHz, RBW = 1Hz) but i hear this conflicting voice constantly in my head: " Do you really need this? Too expensive! Don't pull the trigger!"
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Old 13th May 2022, 9:49 pm   #30
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
The 2nd and 3rd pictures are my test rig for "hot" frequency sweep of double tuning+ single tuning with a dual gated mosfet BF981. The overall gain is 19db and bandpass Q = 159. The "cold" Q (no amplifier) of the double tune stage is about 85 and single tune stage Q is 30.
...
In the Pioneer TX8500 service manual, it stated that "the noise figure is determined by the insertion loss of the antenna input tune circuit",

insertion loss = ( 1 - QL/Qu)**2

In my measurement Qu = 100, QL = 30, IL = 0.49db
I'm sorry to say that, but your interpretation of result is incorrect.
The cited formula gives an absolute value of insertion gain (here 0.49).
Insertion loss expressed in dB should therefore look as follows:

IL[dB] = -10 log {( 1 - QL/Qu)^2]} or
IL[dB] = -20 log ( 1 - QL/Qu).

The minus sign is to denote insertion loss as opposite of insertion gain.
For your data we have
IL = -20 log (1 - 0.3) = 3dB.
This is not a drama in terms of S/N ratio, as the noise present at the antenna terminal will experience the same treating.

Adam
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Old 14th May 2022, 5:24 pm   #31
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Default Re: 6-gang FM stereo tuner heads

I've got myself a few nice toys. Something quite off topic outside and a few spectrum analysers inside. The star is an HP 8566B 200Hz to 22GHz 10Hz to 1MHz BW The portable one I assembled myself, an HP8591E with tracking generator and a lot of options - some options which never got released for sale. This does 9kHz - 1800MHz with BW down to 10Hz (crystal) Does up to 5MHz BW and has a fast digitiser as well as AM and FM demods.

Yes I had a job I loved, but due to recent changes, the UK is no longer a member of any civil aviation certification body, so the firm has had to move most operations to a country which is still a member of a certification body.... there went my job! The IET reckon 600 companies were affected and have either moved out or closed, Rolls Royce aero engines being the best known, now a German business. So I never intended retiring, just a long tapering off. But t'was not to be. No discussion, please, the base cause is definitely OT for the forum.

David
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Old 14th May 2022, 6:03 pm   #32
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Default Re: 6-gang FM stereo tuner heads

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Originally Posted by RichardGM View Post
Thanks for the answers to my question, but I'm still a bit puzzled. Yes you need to filter out the image frequency and any nearby strong signals, but most domestic FM tuners do OK without multiple stages of filtering before the mixer. Excluding the BBC's very special case of a receiver "based at the site of a working VHF/FM transmitter", when/why are those extra stages of filtering needed? Is it to receive very weak distant broadcast transmissions?
Those are not only required to enable reception of weak stations but also to suppress spurious signals, created at any gain stage, and capable of blocking weak transmissions. Typical gain stage is told to be linear, but this holds only for really small input signals, say up to the limit of few mV at best (for a BJT; FETs are more immune in this respect). There doesn't have to be a BBC transmitter site to make a trouble. In any crowded urban area you will find signals from local FM stations and/or FM relay nodes, various RFI sources, etc., with levels of several mV, perhaps even 100mV RMS a simple dipole can deliver. Such strong interfering signals, with frequencies lying several channels away from desired will
a) mix themselves in the RF gain stage into a "ghost" RF or IF signal,
b) another ones will reach the mixer (amplified by the gain stage, not much weakened by poor selection),
c) 2-nd harmonic of unwanted one (Fi1) will mix with another one (Fi2, spaced by IF/2) into IF (IP3 issue),
d) yet another ones' second harmonics produced by the gain stage will mix with the second harmonic of LO.

Whenever any one of such mix product fits into the IF bandwidth, you will possibly hear a station, where there is none (a "ghost"). Such a ghost, when stronger than desired weak transmission will quiet the last one in the process of IF limiting.

To any desired frequency Fd you can find multiple of "unwanted" frequencies that fit into one of the mentioned schemes.
In case b) for example, we have at least twenty candidate pairs of following unwanted frequencies: Fu1 = Fd + IF/2 +/- n*(channel spacing) and Fu2 = Fd - IF/2 +/- n*(channel spacing), n = 1 to 10. Easy to see that, in a typical "economy" class front end they will encounter regular amplification (~26dB), and some 12...26dB attenuation before reaching the mixer, and will all mix into IF bandwidth.
A rough calculation can show, that immunity limits to the listed phenomena are for typical 3-gang front end ... let's say, "not outstanding at all".
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Old 14th May 2022, 7:35 pm   #33
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Default Re: 6-gang FM stereo tuner heads

Even if images, intermods and other ghosts have no FM on them, they tend to cause swirling noises, pumped-up noise floors and other things which spoil the signal to noise ratio of your wanted signal.

If you try to make a narrow tuned RF circuit, its loss increases the more narrow you try to make it. So you soon need extra RF stages to make up for the loss. Analye this carefully and you'll begin to see that because the amplifiers also need selectivity to protect their inputs form out of band signals, the optimum structure becomes a series of alternating selectivity and gain stages to give the best compromise between noise figure and non-linearity effects.

So the old fashioned IF with multiple gain stages with selectivity between them was actually quite good if done properly. What supplanted it, having all the filtering in one block right at the beginning of the IF works, but has disadvantages in broadband noise, so filters got added in later in the IF amplifier chain to stop the detector convolving-in noise over the wider bandwidth of the IF amplifier stages.

Limiter amps are not perfect and don't strip away all AM problems. AM to PM conversion is a vice of theirs and converts any AM to a little phase (frequency) modulation, which gets demodulated and adds into the audio.

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Old 14th May 2022, 11:36 pm   #34
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Default Re: 6-gang FM stereo tuner heads

The attached RCA paper provides a simple “worked example” of the benefits of extra front-end selectivity for FM receivers.

That showed that in the case of bipolar transistors, it was better to place the bandpass at the input and sacrifice some noise performance in favour of better spurious response rejection. When dual-gate mosfets arrived, it was more common to place the bandpass at the interstage, with a single-stage at the input. Presumably the better inherent linearity of the mosfets allowed this. Although there were some mosfet examples with a bandpass input, such as the D&W four-gang front ends (https://vintage-radio.net/forum/show....php?p=1153245).


Cheers,
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Old 15th May 2022, 1:21 am   #35
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Default Re: 6-gang FM stereo tuner heads

The high impedance inputs and outputs of a dual gate MOSFET have a strong influence on the sorts of resonator Q's you feel steered into using with them, and the antenna connection is the only low impedance around. So for a front end with 4 tuned coils and variable/varactor capacitors to tune them, although you can allocate the coils before and after an RF amplifier as wanted, the pattern of the bandwidths tends to be foisted on you.

More recently, with dual gate MOSFETs rapidly becoming obsolete and NRND, I was looking at alternatives for an aircraft transceiver I'd designed. This meant that the RF amplifier would need to become an MMIC low impedance sort. With two coils before it and two coils after it, a low-Z mixer and a low-Z antenna, then all four coils would want tappings. This then doesn't sit well with a management edict to use off-the peg VHF inductors. A pseudo-tap can be arranged by having a 1t inductor mounted at the earthy end of a longer one, with the common connection becoming the tap. Instead of messing with tiny coupling capacitors (or potted down ones) then just spacing the second resonator parallel to and a controlled distance away in the same screening compartment will effect field coupling.

It's all a set of nested compromises. There is no single 'right' solution. Optimisation, whether mathematical or empirical (Kanban) gets stuck on local optima and can't be relied upon to find the best one.

You don't value the convenient impedances of the MOSFET until you lose them!

David
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Old 15th May 2022, 6:54 pm   #36
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Default Re: 6-gang FM stereo tuner heads

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Originally Posted by Synchrodyne View Post
The attached RCA paper provides a simple “worked example” of the benefits of extra front-end selectivity for FM receivers.

That showed that in the case of bipolar transistors, it was better to place the bandpass at the input and sacrifice some noise performance in favour of better spurious response rejection. When dual-gate mosfets arrived, it was more common to place the bandpass at the interstage, with a single-stage at the input. Presumably the better inherent linearity of the mosfets allowed this. Although there were some mosfet examples with a bandpass input, such as the D&W four-gang front ends (https://vintage-radio.net/forum/show....php?p=1153245).


Cheers,
The thread you've linked was an interesting read.
In my young days I experimented with prototyping FM front end being capable of DX reception in a "hostile" environment, which was in my case: proximity of a local FM transmitter, FM relay towers, electrical tractions of train station (at ~150m distance), and sparks from tram line and ignition electric of cars passing the street in front of my windows.
Running through various topologies and technologies I dropped BJTs soon, as these were incapable of suppressing these annoying spark interference, even with elaborated filtering (which tended to worsen the noise performance).
The only JFETs I had in hand those days (Japanese 3SK41) performed somewhat better in this respect, but I felt, they had too small transconductance and/or too high NF to achieve the ultimate sensitivity I was after.
The real kick in performance was the use of RCA dual gate MOSFETs I got some day. The circuit was:
single tuned input -> RCA 40672(*) gain stage -> double tuned bandpass -> RCA 40673 mixer with LO injection to gate 2. The LO used a Japanese 2SC668 BJT.
Tuned with use of internally back-to-back connected varicap diodes which were a speciality of those days. These were varicap sets made extra for FM tuning, one type (case dotted green) for RF tanks, the other (case dotted blue) for the LO tank, both types made to have their Ct = f(Ut) characteristics matched to assure best possible tracking along the tuning range. The types I used were Japanese (of Type I don't remember, labels starting with ESM..., if I recall it correctly). Some European companies at that time came with similar device sets (BB204 and BB205, if I'm not in error).
Unfortunately these pearls had gone long ago, sacrificed to the "low cost" philosophy of modern world. (In this discussion therefore, it is worth to note, that today one has to live with an inferior tracking.)
This front end, despite its simplicity, worked exceptionally well. It boosted remarkably the sensitivity as compared to previous prototypes, suppressing the spark interference quite a lot. Good tracking RF vs. LO was possibly essential to achieve that performance.

(") The 40672 were then the types RCA recommended for use in RF stages, and 40673 for mixer. Especially for this project I've bought two sets of 40672 and 40673 from Radio Rim in Munich in 1979.
Interestingly, in the RCA book "RCA Solis State, Linear Integrated Circuits" from 1978 I acquired years later only the 40673 type is included. No spur of 40672 to find today, anywhere.

PS: Of the RCA paper you linked the pages 28-32 seem to be completely blurred. I found a clean copy here: https://ur.booksc.me/book/14578305/f04202

Adam
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Old 17th May 2022, 3:29 am   #37
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by nemo_07 View Post

PS: Of the RCA paper you linked the pages 28-32 seem to be completely blurred. I found a clean copy here: https://ur.booksc.me/book/14578305/f04202
Thanks. I compressed my clean copy (as a whole) in order to get inside the forum size limit, but it did not end well. As an alternative, I broke it apart, size-reduced each page individually, and reassembled it. I think that the result is an improvement.

Cheers,
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Old 17th May 2022, 3:41 am   #38
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Default Re: 6-gang FM stereo tuner heads

Following that RCA paper was a whole sequence on FM front ends in which the bipolar signal path devices were progressively replaced, firstly by single-gate mosfets, and then by dual-gate mosfets.

Initially, the RF bipolar was replaced by a single-gate mosfet:

RCA AN-3453 FM Tuner with Single-Gate Mosfet RF Amplifier.pdf


Secondly, the same was done for the mixer:

RCA AN-3535 FM Tuner with Single-Gate Mosfet RF Amplifier and Mixer.pdf


Thirdly, the RF single-gate mosfet was replaced by a dual-gate device:

(See the second part, p.80ff.)

IEEE 196707 Application of Dual-Gate MOS Field-Effect Transistors in Practical Radio Receivers (.pdf


Finally, the mixer single-gate mosfet was replaced by a dual-gate mosfet:

(See p.68)

RCA AN-4590 Mosfets in Linear Circuits.pdf


I reckon that the third stage, single-gate mosfet RF and dual-gate mosfet mixer, could have been where Leak got its idea for the Stereofetic front end.


Cheers,
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Old 17th May 2022, 5:18 am   #39
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by RichardGM View Post
I'm still a bit puzzled. Yes you need to filter out the image frequency and any nearby strong signals, but most domestic FM tuners do OK without multiple stages of filtering before the mixer. Excluding the BBC's very special case of a receiver "based at the site of a working VHF/FM transmitter", when/why are those extra stages of filtering needed? Is it to receive very weak distant broadcast transmissions?
If we consider only the British market, then before the VHF/FM band got the commercial stations added in the seventies, there wasn't much interest in long distance reception because there wasn't much any different to what you were already in the service area of.

But designers of FM radios and especially hifi tuners had to develop models which could compete on a global market, and that meant countries where a much larger variety of broadcasts were available. S some listeners might want to listen to a station serving a town many miles away and not just the big signals of the broadcasters in their local town. So much more emphasis was on dynamc range in Germany and the US.

As Japan came to dominate hifi, they had little interest in models only suited to the British market.

So across the period of time where tuners were becoming more complex, several other changes were happening. The FM band was filling up with stations and progressively taking listeners away from AM. Globalisation of markets meant tuners had to have superset specifications to suit many countries so the same model could be flogged around the world, with just a switch for 50/75us de-emphasis.

So Britain started receiving receivers with more RF selectivity stages than it really needed, then it filled-up it's FM band and started needing them.

On the world stage, quintessentially British tuners like Quad FM3 and FM4 have some performance figures which are quite attractive, they are not very competitive in terms of managing to get reception of weak signals with strong local ones nearby, and the monster tuners with many tuned RF tanks start to win.

So by looking at a circuit diagram, you can see how far afield a company's marketing department was looking. And each is also a snapshot of the time it was planned.

David
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Old 21st May 2022, 12:37 pm   #40
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Default Re: 6-gang FM stereo tuner heads

Many many apologies that I have not responded to your messages. For some unknown reason, I did not get any email notifications about replies of this thread since my last post in March. I have not logged onto to the forum since mid-March and I have seen the ten posts for the first time today. Maybe I switched off the email nortification accidentally.

The reason I have been away from the forum was because I have been busy at work and I was lost in my little world of making different solid-state VHF tuned amplifiers and bandpass filters. I got side-tracked completely spending lots of time designing and experimenting both capacitive and inductive coupled bandpass filters. In April I bought a £1500 Siglent SVA1015X dual VNA and spectrum analyzer but it developed faults and I returned it for a refund.

This week I have returned to this project after a long period of "procrastination" because I have taken three days off from work. To my astonishment, the 6-gang valve tuner actually works very well the first time i powered it today. Murphy Law states that anything can go wrong, it will go wrong but this project seems escape it. This is very pleasing and encouraging! This is after doing some very crude alignments using the NanoVNA and NWT200. It receives all stations and track perfectly from top and bottom FM band.

Last night I aligned it with the NanonVNA plus 4 using 50 ohms matching capacitors in the input and output terminations using "cold" tuning. Now this is not really correct way of aligning the bandpass filters because the center frequency and bandwidth depend on the input and output terminations. I did calculations for the Butterworth filters using excel and software with both cases of 50 ohm terminations and high Z source and load .If I powered up the tuner, the NanoVNa indicates that the center frequency and bandwidth completely shift completely and there is a large Z mismatch. This is because the input and output impedance of the valves are really unknowns. I only have ballpark figures from the datasheet.

To do correct alignment of the 6-gang tuner, I will need some sort of Z probe with variable input and output potentiometers with very little stray capacitance and inducatance. I was working on some variable Z input and output FET probe last year for my NTW200. It did not work work well at VHF and now I am going to build it again.

For FM tuner, I would opt for slight under-coupling instead of the critical coupling with optimally flat bandpass response commonly used in fixed tuned HF filters. In the case of slight under-coupling, it introduces higher insertion loss but higher filter Q. A more round top response curve is desirable for FM tuner in adjacent channel selectivity. The Butterworth filter tuning is very sharp. A small out of alignment introduces massive insertion losses. This makes alignment very challenging.

The last screenshot shows the alignment and measurement of unloaded Q of the single tuned front end using the NWT200.


I have started working on the solid-state dual gate FET version but I am toying this idea of building a third version using SMD GAL-51+ and Gilbert cell mixer with 50 ohms terminations. It is much easier to align 50 ohms filters!

Quote:
ollowing that RCA paper was a whole sequence on FM front ends in which the bipolar signal path devices were progressively replaced, firstly by single-gate mosfets, and then by dual-gate mosfets.
Thanks for the papers. i will have a look.
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Last edited by regenfreak; 21st May 2022 at 12:45 pm.
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