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Old 21st Mar 2022, 10:07 pm   #1
regenfreak
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Default 6-gang FM stereo tuner heads

I had the idea of building a 6-gang FM tuner a couple of months ago after making a 3-gang FM valve tuner. I acquired three sets of 6-gang silver-plated, air variable capacitors that are spanking new in original boxes. The seller said they were ex-MOD items. I guess they were used in 2 meter band military communication equipment?

I am going to build two versions: (1) valve version and (2) dual gated mosfet version (see attached schematics). The designs are inspired by the top-end FM tuners in the 1970s that had 4 up to 9 gangs. My schematics do not copy the designs of any particular tuner but are just a cocktail of ideas from many designs. I just hope they will work somehow. 6-gang valve FM tuners did not exist and it was rare to have more than 4 gangs. The topology here is:

single tuning + double tuning + doubled tuning

This is really a learning journey for me. I still consider myself a newbie, only taking up the hobby in 2018. I don't have a real spectrum analyzer and VHF sweep generator but I can make it work using NanoVNA and cheapo Chinese sweeper.

The first step was to design the 2nd order Butterworth filter. I did hand calculations based on the method by Zverev and validated the results using software. They give identical results. I measured the unloaded and loaded Q of the single and double tuned filters using the test rigs attached. The sweep results matched the simulation very closely.

Thanks to Synchrodyne in this forum, I was made aware of a 1965 IEEE paper about the performance of 3 coils versus 4 coils FM tuner. This is the only paper technical paper written on the comparative study on the effect of double-tuned gang vs single-gang front end.

The key considerations of high-performance FM tuners are (list below is taken from Pioneer TX8500 service manual):

1. noise figure,
2. spurious response,
3. image rejection,
4. IF rejection
5. sensitivity,
6. 50db quieting sensitivity
7. image rejection IMD interference
8. cross-modulation interference

I still struggle to understand concepts like quieting sensitivity and limiting sensitivity.

There will be lots of shieldings around the coils. Lets the battle begins!
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Old 21st Mar 2022, 10:49 pm   #2
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Default Re: 6-gang FM stereo tuner heads

Few FM tuners have enough gain in the limiter amplifier that limiting starts as or before a signal lifts above the noise floor, so there is a sensitivity figure relating to the onset of limiting. Reduction of the inter-station noise, which is inherent in FM really gets going as the amount of gain reduction through limiting gets stronger, so you have another sensitivity figure for a defined amount of quieting (pick how much!)

All this beggering around is because there wasn't a single figure of merit that was easy to test.

There is now. Signal to noise ratio at the output is what's wanted. SINAD in another word.

You could draw a graph of SINAD ('SINADDER' meter or HP 8903A/B) versus input signal level, and indeed draw a family of curves for different amounts of deviation. This chart would show the sensitivity across all combinations of input signal. As far as a single figure goes. Pin the tail on the donkey however you will. There is no unique condition which is right, whic gives the whole graph full of data, but if anyone wants to know what input level is needed to get x dB SINAD, ot what SINAD you get if you shove y dBm in, then you have it at your fingertips.

With multistage receiver sdesign, you have to get arty about gain distribution. Selectivity is lossy so the noise champion wants plenty of gain before selectivity. But amplifiers are non-linear, so the intermod champion wants plenty od selectivity before gain.

A dynamic range champion sees that a compromise is needed and it amounts to a strip with distributed selectivity and distributed gain.

But don't guess, there are spreadsheets written to analyse these structures. Commercial ones cost money and don't do everything you might wish, so most serious RF system designers write their own. Mine designed the Agilent Noise Figure Analyser family (well the firm was called HP when we started...) N8973A onwards. It has quite a few extra features and estimators which are somewhere between a trade secret and a very black art.

David
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Old 21st Mar 2022, 11:56 pm   #3
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Default Re: 6-gang FM stereo tuner heads

Thanks David. Some terminology mentioned in your post are new to me. This is the first time i have heard of the limiting amplifier. Due to my ignorance on the subject, I dont understand fully lots of terms described in 1965 IEE paper. As a beginner, I cannot afford the cheapest spectrum anlayzer. A noise figure meter would be what they used by manufacturers to optimise the tuned stages.

Attached is the comparison between simulation and actual "cold" frequency sweep of the double tuned filter with the filter Q = 60. They match very closely.(note i cannot calibrate my cheapo sweep generator properly).

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.

The VHF measurement techniques of unloaded Q and filter Q is based on the work of Wes Hayward. The hardest part of project is getting the input and output impedance matching of the filter stages. The alignment and fine tuning will be quite challenging. The coupling capacitor, the input and output tappings will determine the bandwidth of the bandpass filter. According to Wes Hayward, the inductance of coil is not proportional to the square of turn number when the length to coil diameter ratio is bigger than one due to leakage flux. Therefore the tapping positions have to be deduced experimentally. I avoid the use of gimmick coupling capacitor( they are just two wires and mechanically unstable). Instead I use DIY capacitor or piston trimmers in series to get very precise value down to accuracy of 0.01pF which is the limit of my DER EE LCR meter.

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
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Old 22nd Mar 2022, 5:50 am   #4
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Default Re: 6-gang FM stereo tuner heads

Most FM detectors are also sensitive to AM on an incoming signal. Some have a minimum signal required for them to start operating. So it is routine to have a multi-stage amplifier in the FM receiver IF, with stages being driven into saturation. If you apply a signal and steadily increase its amplitude, at first the last IF stage (benefitting from the gain of all the earlier stages) starts to saturate and limit the signal amplitude. As the input level increases, this stage goes further and further into clipping, until the stage preceding it starts to saturate as well... and so on and so on. This gives the FM demodulator an essentially constant amplitude driving it over a reasonable range of input levels.

When you don't tune to an input signal, the IF amp comes out of saturation and the gain is immense. So noise from early stages is amplified greatly, and is demodulated to audio noise, at max level. To stop the sound blowing you out of the room, a signal level detector is used to trigger audio muting below a certin level.

One of the most common FM demodulator circuits, the ratio detector, was common because it was a bit less sensitive to AM on the signal and receivers could drop one IF amp stage and be a bit cheaper. It isn't one of the best detectors in other respects, and it is an absolute swine to try to explain how it works to people. It got left behind when things went towards ICs.

Re front end noise figure, you may find somewhat more insertion loss than that. To get best noise performance from your MOSFET, you need to transform the antenna impedance to suit the source Z which coincides with the device's minimum noise figure. This isn't the same impedance as gives the best power gain in the device. So you have to compromise. The match you wind up with then forces you into running higher Q in the input filter than you first thought.

Sometimes you just don't win.

David
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Old 22nd Mar 2022, 12:24 pm   #5
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Default Re: 6-gang FM stereo tuner heads

Thanks. Ok I was confused by the terminology of "limiting amplifier", not knowing it is the same thing as limiters in FM IF stages. I read a whole Rider book on the subject of FM limiters. I tried both ratio and discriminator detectors in homebrews. The ratio detector is good for low-cost radios. For most high-end stereotuners, discriminators combined with two to three progressive limiting amps are common. Interestingly, the much-hyped gated beam valve 6BN6 was rarely found in FM tuners as a standalone limiter. In fact, I have never seen them used without additional limiters. I guess it is to do with the fact that the 6BN6 has to be driven at fairly high signal voltage.

The muting function is commonly found in FM IC chips. I have never seen them to used in FM tuners using discreet components.

I am trying to find a way of measuring the "relative or indicative" noise of the antenna tuned circuit using a cheap 20 diode noise generator or my Rigol DSS signal generator with the TinySA. Some people suggested it is possible to tune a 2m pre-amp front end by ears to find the best noise figure. I doubt it is doable...

Looking at the data sheet of BF981, the noise figure is 0.7 or noise temperature of 51k at 100MHz. However, any component placed in the circuit between the antenna and the input of the first Rf amplifier will cause a reduction in noise figure. it is a juggling act of balancing three things: the optimum noise figure, the best impedance match for power transfer and attainment of the required bandwidth for the antenna tuned circuit.

I often ponder the pros and cons of using doubled vs single-sided copper clad boards in VHF prototype of bandpass filters? (using either Manhatten islands or deadline bug techniques). If double-sided copper clad board is used, connecting ground wires must be used by drilling holes in every mm distance to minimise any ground loop. In the context of the VHF bandpass filters, I read somewhere that double-sided clad boards give steeper skirts of the filter response curves. However, I cannot see the logic behind it. Others claimed single-sided copper clad board is better for bandpass filter. So far I have only tried single-sided prototyping board, so far so good, not having any issue with tuning and skirt shape.
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Old 22nd Mar 2022, 1:13 pm   #6
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Default Re: 6-gang FM stereo tuner heads

One caution about basing what you're doing on dual gate MOSFETs.... Most semiconductor firms are stopping producing them, the quarterly obsoletion lists are whittling away at them. My last design using them is still shipping (aircraft VHF Comms radio) but it has to all be from life-bought stock.

Anything you do with them is therefore to be considered time-limited. This isn't important from the point of view of studying the technology, but you should be aware that it is a dead end.

That said, the best general range of MMIC amplifiers got canned a few years ago.

David
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Old 22nd Mar 2022, 6:30 pm   #7
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Default Re: 6-gang FM stereo tuner heads

I guess everything is time limited and a dead end. The same could apply to the end of analogue FM broadcast in x and y years from now. Since life is short, enjoy while it lasts I just notice the prices for older style of dual gated mosfets with legs are quite expensive, not a lot cheaper than VHF valves.


I wonder why some high-ended tuners use a combination of capacitive and inductive coupled bandpass filters. I speculated that sometimes the low-frequency skirt of capacitive coupled bandpass filter has a steeper skirt. While the inductive coupled bandpass filter has a steeper skirt at the high-frequency side. By combining these two, the overall bandpass response would have a more symmetrical shape. The attached schematic of a high ended tuner has 8-gang 3-pole filters that use such combinations. I dont know if alignment of such elaborated 3-pole filters would call for the Dishal's method. I find it difficult to understand the logic behind the Dishal's step-by-step procedure, giving me headache just to think about it.
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Old 22nd Mar 2022, 8:42 pm   #8
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
I still struggle to understand concepts like quieting sensitivity and limiting sensitivity.
Perhaps these graphs of typical (albeit quite old) quieting curves will help.

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As may be seen from the upper curves in each case, full limiting (usually I think measured at the 3 dB before full limiting point) occurs before the 50 dB quieting points. And the stereo 50 dB quieting points are the expected 20 dB or so beyond the respective mono points.

The ratio detector (RD) had higher distortion than a Foster Seeley Discriminator (FSD) of the same bandwidth. But, the RD bandwidth could be widened considerably, for lower distortion, whereas that for the FSD could not. So the wide bandwidth RD (WDRD) was a pathway to hi-fi performance. But it then required more IF gain and more limiting ahead of it, which cancelled one of the main reasons for using it. Nonetheless, the WBRD was sometimes found in valve tuners, for example the Radford FMT1. I think that the extra limiting was required because with wide bandwidth, noise spikes well removed from the centre frequency would demodulate quite strongly.

In the solid state era, the equation changed. IF gain and limiting was much easier to obtain, and as RCA was wont to say, the integrated differential pair made an excellent limiter, with low phase shift across a wide variety of input levels. Early FM IF ICs were either designed to work with RDs, or actually included the requisite diodes. Thus some tuner makers, e.g. Leak, who had used FSDs in the valve era, switched to the WBRD for the early solid-state models. Then came the integrated quadrature type. Fairchild did it first in very simple form, but Sprague was first with the definitive, double balanced form using a six-transistor tree, which then became legion. RCA at first avoided this, and came up with its differential peak detector. But it capitulated with the CA3089 IF sub-system in 1971. For lowest distortion, double tuning of those quadrature detectors was desirable, until National came up with a feedback system that achieved the same low distortion with single tuning.

The 6BN6 and various other valve quadrature detectors (see: https://www.vintage-radio.net/forum/...6&postcount=15) were little used for FM receivers and tuners. They had higher distortion than the FSD and RD, and not only that, did not provide a bias output suitable for AFC and/or centre-channel tuning indicators. Rather they were used for intercarrier TV sound, where this basic system anyway limited sound quality. But as you say, the 6BN6 was used as a second limiter in some American high-performance tuners.


Cheers,
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Old 22nd Mar 2022, 11:01 pm   #9
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Default Re: 6-gang FM stereo tuner heads

Thanks Steve. Now I get the idea of quieting curves. Quieting is a kind of like a squelch function in HF transceivers; a circuit detects the condition when there is no signal and it filters out the hiss. Attached is the SNR versus Rf input for different receivers from the Ofcom report on page 28 . It is essentially the same graph:

https://www.ofcom.org.uk/__data/asse...10/annex-f.pdf

This is the first time I heard of WBRD. You are right that Radford FMT1. It has a WBRD as there is a 8.2K resistor across the primary tuned circuit of the ratio detector (see attachment 2). I built a few cheap DIY FM radio kits with IC chips using quadrature detectors or PLL (e.g. sc3610d, cxa1691, SC1088 ).

I didn't know that 6CS6 heptode ended up in FM demodulation application. In fact, I built my first MW superhet using a 6CS6 as a frequency convertor.


PS: BTW it is possible to measure the gain, one db compression point, 3rd order intermodulation intercept point, noise figure of an amplitude using a TinySA:

https://youtu.be/HJKjrEZRAKg

Even it is nothing like the real deal, this is something I will try next.
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Old 22nd Mar 2022, 11:29 pm   #10
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Default Re: 6-gang FM stereo tuner heads

Not that much like squelch. Muting is like squelch.... killing the audio if thesignal level is below a set threshold.

Quieting is a smooth change. As an illustration, if you are listening to a good FM receiver with an input signal above the onset of limiting in the IF amplifier, and you smoothly increase the signal level, the level of the wanted audio does not increase, it stays fixed, but the level of the background smoothly falls.

So when listening to an FM signal and trying to estimate the strength of the incoming signal, try to gauge how muchnoise there isn't !

Is it possible to measure....? Possibly. I've never tried it with one of those, but you'll need a decent signal generator to measure gain and 1dB compression point.You'll need a variety of attenuators to play around with to determine if you're measuring your tuner or your test gear. Make that two decent sig gens and a combiner if you want to measure intermod products.For noise figure, you'll need a calibrated noise source. I've got one sitting on the desk here, an HP346B which has an excess noise ratio of 15.65dB at 100MHz. It has a 10dB APC-7 attenuator screwed onto it to reduce the ENR for measuring low noise figures. This noise source covers 10MHz to 18GHz. Easy to say you don't need all that range, difficult to find anything with less that is properly calibrated.

Look up the Y-factor method of noise figure measurement. (HP/Agilent/Keysight AN57-1 and AN57-2 these were updated a few times. The older ones are maybe the best for you, the later ones are more aimed to sell network analysers as the dedicated noise figure boxes went out of production.)

There are other ways: A Russian amateur has a horn antenna kept looking at a cold part of the sky as his reference. It works very well. NPL have resistive loads maintained at liquid nitrogen and melting/boiling water temperatures. These have to be corrected for the temperature gradient along the loss of their connecting cables. Commercial noise sources are traceable back to these standards. NIST in the US had let a building roof fail (saved money on maintenance...) and had lost the parentage of their standards, so we transferred our calibration trail to NPL and got lower quoted uncertainties as a result.

Who'd have thought that all the national standards labs was a competitive industry?

David
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Old 23rd Mar 2022, 1:06 am   #11
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
I didn't know that 6CS6 heptode ended up in FM demodulation application. In fact, I built my first MW superhet using a 6CS6 as a frequency convertor.
There’s an interesting story there. Zenith and GE developed the 6BN6 for the FM job. Then Zenith found that it made an excellent noise-gated sync separator for TV receivers, except that it was rather costly. The existing 6BE6 radio frequency changer valve was found to do a reasonable job, so was used instead. But the market was big enough to justify the development of sharp cut off heptodes for the job, and that resulted in the 6BY6 and 6CS6. These were also intended for use as colour subcarrier synchronous demodulators in colour TV receivers, but that was a much smaller market at the time. In the usual manner of things, both of these valves found application in HF receivers as mixers, I think (but I am not sure), the 6BY6 by Drake and the 6CS6 by Heathkit. In Europe, Philips used first the ECH81, then the ECH83 for the noise-gated sync separator job, eventually developing a dedicated valve, the ECH84. But when it wanted an FM quadrature demodulator, for some reason it chose the 6CS6, renamed as the EH90. Zenith and GE went on to develop the 6BU8 combined noise-gated sync separator and gated AGC generator for TV applications. Mullard used the 6BU8 in at least one FM stereo decoder circuit, and the ECH84, alternatively the EH90 in others. All of that is evidence in support of the “circular flow of valves” theory.

By the way, notwithstanding that Zenith and GE had worked well together on several prior projects, the Zenith-GE FM stereo broadcasting system was not the result of a co-operative development project. Rather, it was the FCC who merged what were two similar proposals into one. I think that there was a little cross-sniping between the two afterwards.

Re the FMT1, Radford gave the RD bandwidth as 1.5 MHz.

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And here is the quieting curve:






Cheers,
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Old 23rd Mar 2022, 12:07 pm   #12
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Default Re: 6-gang FM stereo tuner heads

Quote:
There’s an interesting story there. Zenith and GE developed the 6BN6 for the FM job.
Thanks for the informative history. The attached schematic is the discriminator for Harmon Kardon that uses two 6BN6. It also looks like a wide bandwidth tuned circuit for the first 6BN6 due to the presence of 8.2K resistor across it. The second 6BN6 is connected to a discriminator coils and diodes. Therefore the 6BN6 is not used as a sharp cut-off "chopper" originally designed for a quadrature detector.
The 12AX7 is part of the muting circuit.

The Fisher FM1000 is a very interesting tuner. It uses a total of 6 IF stages; two IF amp and four IF/progressive limiters. One 6BN6 is used as a limiter as shown in the 2nd and 3rd attachments. Interestingly, it opted for ratio detector instead of discriminator. Here ECC88 serves as a muting circuit.

Quote:
Is it possible to measure....? Possibly. I've never tried it with one of those, but you'll need a decent signal generator to measure gain and 1dB compression point.You'll need a variety of attenuators to play around with to determine if you're measuring your tuner or your test gear. Make that two decent sig gens and a combiner if you want to measure intermod products.For noise figure, you'll need a calibrated noise source. I've got one sitting on the desk here, an HP346B which has an excess noise ratio of 15.65dB at 100MHz. It has a 10dB APC-7 attenuator screwed onto it to reduce the ENR for measuring low noise figures. This noise source covers 10MHz to 18GHz. Easy to say you don't need all that range, difficult to find anything with less that is properly calibrated.

Look up the Y-factor method of noise figure measurement. (HP/Agilent/Keysight AN57-1 and AN57-2 these were updated a few times. The older ones are maybe the best for you, the later ones are more aimed to sell network analysers as the dedicated noise figure boxes went out of production.)

There are other ways: A Russian amateur has a horn antenna kept looking at a cold part of the sky as his reference. It works very well. NPL have resistive loads maintained at liquid nitrogen and melting/boiling water temperatures. These have to be corrected for the temperature gradient along the loss of their connecting cables. Commercial noise sources are traceable back to these standards. NIST in the US had let a building roof fail (saved money on maintenance...) and had lost the parentage of their standards, so we transferred our calibration trail to NPL and got lower quoted uncertainties as a result.
Unfortunately my Rigol DG1022 DSS function generator can only go up to 20Mhz. To measure third order intermodulation intercept point, i would need two sine wave generator sources going up to VHF. I have a walkie-talkie FM generator that can go up to 450MHz and a cheapo sweep generator up to 200MHz. But i would need two channels with a combiner for this kind of measurement..

Referring to above video from the designer (Eric) of the TinySA, he used a standard SMA 50 ohm load as a noise source reference ( -174dbm/Hz ) assuming it is standard room temperature. It is not good enough for professional lab but surely it is ok for us hobbyists.
At this moment, there is a listing of a used HP346B noise standard, going for 1500. I read about the y-method in the HP manual previously.

I have many SMA attenuators and a 80db RF step attenuator. Therefore gain and one db compression measurement can be done with my TinySA. I have two these cheap toys, they can be used as VHF signal generators but they only output square waves full of harmonics. The same with NanoVNA, they can be used as sweep generator with appropriate DC isolation capacitors and resistors in the port 1 and port 2. I tried to use NanoVNA to align FM IF transformers and it worked well.

PS: Maybe IEE produced some sort of sky noise map at different frequencies?

PS i
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Old 26th Mar 2022, 12:14 am   #13
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Default Re: 6-gang FM stereo tuner heads

My bad i made mistakes in the dual gate mosfet schematic in my 2nd post. The gate 1 and gate 2 were connected in the wrong way round.

Attachment 2 is the overall bandpass response (Q=149) of the BF981 with 1st stage Butterworth bandpass of Q =60, and 2nd single-tuned stage Q=30.

I have done some simulations, for a bandpass filter centred at 100M, Q = 60 bandwidth = 1.667M:

At 88M Q = 76.5 bandwidth = 1.15M

At 108M Q = 51.4 bandwidth = 2.1M

So the bandwidth increases with increasing frequency for a fixed coupling capacitor set at the design point of 100M.
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Old 26th Mar 2022, 2:45 pm   #14
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Default Re: 6-gang FM stereo tuner heads

To measure third order intercept points you first need a pair of very good signal generators.

The issue is any signal from one making it to the other, backwards into its output connector will cause the intermod products you are looking for in the output amp of that signal generator... and this works both ways.

So you run the sig gen at a high level so you can have plenty of attenuation before the combiner so that there is plenty of loss from one sig gen to the other. In some cases I've added minicircuits medium power linear amplifier modules to the outputs of a pair of HP 8662s.

THEN you need a combiner which provides as deep a null as you can get in the source-to-source path. Such couplers tend to be transformer-based and you need to find good ones so that they don't create the intermod products you are after. THEN you want some more attenuation so that the balanced transformer bridge type combiner sees a good load impedance to help it null cross-coupling, and to bring the level down to what you want to apply to the equipment under test.

If you're looking at RF or IF of the front end, then you need a good spectrum analyser, something with about 100dB dynamic range.

If this sounds terribly expensive and professional, then it's what's needed to measure the sort of RF front ends that hobbyists build.

Stewart of Reading have a number of 8662s in stock and were asking 750 each when I bought a couple for work-work. The minicircuits amps, combiner and attenuators came to about the same in total. Spectrum analysers were in the 2500 region. This is what it takes to build a rig capable of measuring the TOI of a reasonable RF amp or mixer.

The same gear will measure compression and blocking etc as well, so you get those for free.

Noise figure is easier, you need a calibrated noise source, and a very linear receiver/detector.

These are non-trivial measurements that stretch test gear to the limit. Get it wrong, and you still get a result, and unless you are suspicious and take precautions, there is nothing to tell you that the result is badly wrong. The internet has a lot of people who've mislead theirselves and don't know it.

David
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Old 26th Mar 2022, 6:14 pm   #15
regenfreak
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Default Re: 6-gang FM stereo tuner heads

Thanks David for the advice. I was looking at Stewart of Reading stuff after recommendation from Ed Dinning. No doubt HP makes the best RF test equipment in the world. A budget of 750+2500 = 3500 seems out of my league. It would be like firing a NLAW and then a Javelin into my savings against the backdrop of costs of living crisis. I had many hobbies, i probably spent significantly more than 3.5K on other hobbies in my younger years. The older i get , the more prudent i become with disposable income. I have a big mortgage to repay. Probably it is a question of how justifiable to spend that amount on a serious hobby and how many more years i will stick to this hobby before i get bored of it. I am not that crazy about Ham radio and restoration of antique radios. I just read about RF stuff and make homebrew radios to take my mind off from stress at work and the pandemic.

Realistically I would go for the entry-level offerings by Rigol or Siglent. I have been eyeing for the Siglent SSA3015X PLUS with tracking generator (resolution bandwidth of 1Hz, -161 dBm/Hz Displayed Average Noise Level ) at about 1265, and the Rigol DSA815-TG with trackng generator at 1000 ((DANL) <-155dBm/Hz,RBW = 10Hz. I don't have a bench, any HP or Macroni beasts would be too bulky and heavy for my small PC desk. The modern spectrum analyzers are very light and compact.


The more I read about the IP3 measurement, the more I realize it needs very good equipment. The TinySa video gave the over-simplistic illusion that it is a simple measurement. This Texas Instrument article is probably the best explanation why the slopes of the two straight lines (fundamental and IMD3) in the power log-log graph are 1 and 3 respectively:

https://www.ti.com/lit/an/slyt090/sl...oogle.com%252F

Last edited by regenfreak; 26th Mar 2022 at 6:23 pm.
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Old 26th Mar 2022, 8:24 pm   #16
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Default Re: 6-gang FM stereo tuner heads

IP3 needs TWO decent sig gens at once.

Ouch!

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Old 26th Mar 2022, 8:54 pm   #17
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Default Re: 6-gang FM stereo tuner heads

Stewart of Reading checkout price in shopping basket: 2x750+2500=4000 Maybe I consider selling my kidney!
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Old 27th Mar 2022, 7:50 am   #18
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Default Re: 6-gang FM stereo tuner heads

Well, it was the firm's money, for work-work. I'm still intact.

For audio testing, there are programs which use the 'soundcard' of a computer as both source and spectrum analyser. You'll need an attenuator on the source in order to go down to low levels for sensitive inputs. Don't use 'digital' level controls as you will go down into the quantisation noise of the DAC and the computational word-width. The analyser response is also limited by the anti-alias filter, so you don't get to see any higher frequency products... you aren't supposed to be able to hear them, but finding any can give clues to what's going on in the circuitry you're testing.

For RF testing, you're stuck with the expensive stuff. I visit radio rallies and keep an eye out for good quality test gear which is either needing to be fixed, or is too heavy for the trader to want to cart it back at the end of the day.

There is one way round the cost... build a pair of crystal oscillators your chosen separation apart, then your own moderate power amps, combiner and attenuators as needed. This gives you only one test level and spacing but it's a quick one-shot assessment of intermods. I built one years ago into a small diecast box. At an amateur radio show I could feed a test signal into a new model radio being demonstrated. Tune the set to the product frequencies above and below my pair, and see how strong the responses were as a combination of looking at the S meter and by ear. Inevitably someone borrowed this little box and accidentally transmitted into it. End of game. Remarkably few dealers twigged what I was doing, and the ones who found out weren't best plaesed. Obviously they lacked confidence that their product was much good?

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Old 27th Mar 2022, 9:54 am   #19
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Default Re: 6-gang FM stereo tuner heads

Can I come in with an idiot's question here? I understand the rationale for one or two TRF stages in a communications receiver, but what is the rationale for several TRF stages in an FM tuner?
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Old 27th Mar 2022, 11:18 am   #20
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Default Re: 6-gang FM stereo tuner heads

Quote:
Can I come in with an idiot's question here? I understand the rationale for one or two TRF stages in a communications receiver, but what is the rationale for several TRF stages in an FM tuner
Strictly speaking they should be referred as bandpass filter stages and not TRF. Like TRF, the overall bandwidth of the cascade amplifier decreases with increasing number of stages. The attached images are taken from the 1965 IEEE paper written by RCA engineers. It had a double-tuned antenna circuit followed by a single tuned stage. The loaded Q of the double gang is QL=51.4, single gang QL=84.2 and overall Q just under 1Mhz (700-800kHz?) at 3dB. So it is much wider than the FM stereo IF bandwidth of 200kh-300kH. Since each bandpass stage incurs insertion losses, you would need to add more amplifier stages. The goal is not to make the bandpass as high Q as possible, but we want to create steep side skirts of the overall response curve to filter out the spurious signals and images. Beside the front-end RF filter stages, the IF-stages also contributes to significantly to the selectivity of the FM receiver.

Quote:
ell, it was the firm's money, for work-work. I'm still intact.

For audio testing, there are programs which use the 'soundcard' of a computer as both source and spectrum analyser. You'll need an attenuator on the source in order to go down to low levels for sensitive inputs. Don't use 'digital' level controls as you will go down into the quantisation noise of the DAC and the computational word-width. The analyser response is also limited by the anti-alias filter, so you don't get to see any higher frequency products... you aren't supposed to be able to hear them, but finding any can give clues to what's going on in the circuitry you're testing.

For RF testing, you're stuck with the expensive stuff. I visit radio rallies and keep an eye out for good quality test gear which is either needing to be fixed, or is too heavy for the trader to want to cart it back at the end of the day.

There is one way round the cost... build a pair of crystal oscillators your chosen separation apart, then your own moderate power amps, combiner and attenuators as needed. This gives you only one test level and spacing but it's a quick one-shot assessment of intermods. I built one years ago into a small diecast box. At an amateur radio show I could feed a test signal into a new model radio being demonstrated. Tune the set to the product frequencies above and below my pair, and see how strong the responses were as a combination of looking at the S meter and by ear. Inevitably someone borrowed this little box and accidentally transmitted into it. End of game. Remarkably few dealers twigged what I was doing, and the ones who found out weren't best plaesed. Obviously they lacked confidence that their product was much good?
Lucky you. Your work coincides with your hobby, killing two birds with one stone. Work becomes leisure. Pleasure becomes work, work subsiding for the expensive test equipment. The only freebie i have ever got from work is a Hameg 20Mhz oscilloscope that i never use. One day when this hobby will become too expensive and i decide to quite, I may take up trainspotting since i live close to Clapham Junction station. I am sure it is as cathartic as replacing capacitors and resistors. The total expenditure of train spotting as a hobby is the costs of a pen and a note pad. I think it is illegal to use a camera on platforms due to terrorism concerns. So camera is not needed.

I am not clever and experienced enough to build my own IP3 test jig. So i need to forget about IP3 from now. I have been looking at the SINARD DSP software $67, not bad huh:

https://comtekk.com/sinad.htm

This probably does something similar to a SINARD meter like the link below to measure SINARD, SNR, THD:

https://youtu.be/sBqjNLB6HuM

I am not sure how accurate it is. My junky chinese FM walkie-talkie signal generator can produce output -70--132dm with 800Hz tone. So it can be used be get a ballpark figure of SINARD using the Comtekk.
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