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Old 18th Jul 2022, 11:44 am   #101
G0HZU_JMR
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Default Re: 6-gang FM stereo tuner heads

[
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Originally Posted by nemo_07 View Post
]So you think you have measured the OPI3 for the MAR-1+ with this little-tiny-weeny toy made in China. And all this in a single step?
I believe regenfreak was just sharing the results of an interesting experiment using an ultra low cost spectrum analyser. Many years ago, before I owned a spectrum analyser I did various experiments using a diode ring mixer and a sig gen and an early PC soundcard to try and do (narrowband) OIP3 measurements. I achieved reasonably good results, certainly good enough to be educational at least. There are times when you have to make do with what tools are available to hand and then always allow a good margin for overall measurement uncertainty.

I'd be interested to see more results with the TinySA as I'm still tempted to buy one. I'll probably use it for RFI sniffing around the house but any tests that show it can do more than this are very interesting to me.
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Old 18th Jul 2022, 12:46 pm   #102
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Default Re: 6-gang FM stereo tuner heads

Plenty of devices don't give a nice, straight, 3:1 slope for the third order intercept level.

Quite often the10, 20, I've seen 40! dB can be erratic and sometimes not even re-traceable.

So you say to yourself that you're using TOI to determine SFDR and you slap a ruler along the linear section and use that to determine the intercept point. This is where I expect a one-shot TOI measurement is likely to go astray.

To do a real TOI test, the levels of the two tone generators need to be varied over a decent span and the resulting plot needs careful interpretation. A bit of software in an analyser might give you a number, but you need to know how much you can trust it.

Once distortion effects in quartz crystals started to be known about, there was a trend away from using quartz filters in critical receivers. The HP3745A and 3747A were hex-superhets for example. Feedback RF amplifiers and also mixers can give 'interesting' TOI plots.

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Old 18th Jul 2022, 1:25 pm   #103
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Default Re: 6-gang FM stereo tuner heads

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Originally Posted by G0HZU_JMR View Post
I agree that the classic crystal model is extremely good but it doesn't take into account capacitance to free space. This capacitance might only be a tiny fraction of 1pF but it is there nonetheless.

It doesn't really matter though as long as everyone uses the same classic model and they fudge the C0 capacitance ever so slightly to take into account the tiny effect of the package capacitance to free space. A 2cm long piece of wire still has some capacitance to free space so a crystal will be no different.
Your concerns are overestimated.
If you look at the typical configuration for parallel resonance you will find that the package capacitance as seen from the crystal's terminals can be considered as an equivalent of two approximately equal capacitances in series, put across the terminals, with their common point connected to the case. As such it will be simply a part of total load capacitance.
What's more important, the values of these capacitances are more constant and they are of better quality than most commonly used types for prescribed load capacitors.
If you now connect the crystal's case to the ground plane as is (or should be) the case in any practical circuit you will end up with a circuit where the capacitance of this ground plane to free space may be some tens or more picofarads (let's call it Ccmfs). But this Ccmfs is, as seen from the crystal's terminals, a capacitance connected to the ground plane and looking into free space, and as such it can be freely ignored, regardless of its absolute value.
Think about a screened crystal oscillator.
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Old 18th Jul 2022, 1:45 pm   #104
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Default Re: 6-gang FM stereo tuner heads

What originally caused this debate was the observation of different results when comparing 1 port S11 measurements against 2 port S21 measurements when trying to measure a crystal with a VNA.

The issue here is that the capacitance to free space of one of the crystal legs will effectively be added to predicted parallel capacitance of the crystal when doing an S11 measurement. In the case of a 2 port S21 measurement this capacitance doesn't affect the through 'null' frequency.

There's also capacitance to the holder and then to free space and this is also not accounted for in the classic model. So I would expect to see a difference here of typically >0.2pF for the predicted parallel capacitance when comparing S11 and S22. It's a tiny difference but even a perfect VNA will show a different result for parallel resonance when comparing S11 vs S21. None of this matters in the real world. The existing model for a crystal (although incomplete) is extremely good and well accepted.
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Old 18th Jul 2022, 2:47 pm   #105
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Default Re: 6-gang FM stereo tuner heads

See below for a model of a 4096kHz crystal. If I add some shunt capacitance then the null on the S11 measurement will shift sideways away from the 4102.663kHz null frequency seen on the s21 measurement in red.

Slowly increasing this capacitance up to 0.25pF shifts it down by about -500Hz as shown in the blue trace below where it crosses zero degrees on the graph. Note that I've shown ang[zin3] rather than the angle of the reflection coefficient as it graphs better and is easier for the reader to see the resonance point.

I would expect a real crystal to show a similar result when measured with a VNA because it will have distributed capacitance within the overall structure. It's much harder to model this distributed capacitance but it will cause a similar effect as seen in the plot below. The resonance seen with an S11 measurement will not be at the same frequency as the null seen on the 'through' S21 measurement.
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Old 18th Jul 2022, 3:01 pm   #106
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Default Re: 6-gang FM stereo tuner heads

Here's a 2 port VNA measurement of the real crystal the model is based on. You can see the result is quite similar although the VNA data is a bit noisy.

I'd expect to see the S11 resonance shifted to the left when compared to the frequency of the s21 null and this is because of the distributed capacitance within the crystal leads and its package.
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Old 18th Jul 2022, 3:22 pm   #107
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Default Re: 6-gang FM stereo tuner heads

[
Quote:
Originally Posted by nemo_07 View Post
]So you think you have measured the OPI3 for the MAR-1+ with this little-tiny-weeny toy made in China. And all this in a single step?
You should perhaps look for informations supplied by reliable sites, not by wanna-be experts (remember the "negative group delay"?) like these:
https://tinysa.org/wiki/pmwiki.php?n=Main.OIP3
So they start with "After activating the OIP3 measuring mode ..."
They even didn't bother to realize the meaning of the term "intercept point" (which was explained in the math lessons in elementary school, but they obviously were absent).
So, what their gadget actually does when they talk of "OIP3 measuring mode" is visualizing 3-rd order intermodulation products (measuring their levels and calculating exact locations).

There is laking understanding of some fundamentals. And it is pandemic on the internet these days.
This exposes the simple truth, that the idea of "learning by doing" is fairly incompatible with the very basic principles of electronics (which is in essence the teritory of physics); and any careless and sloppy idea will be rigorously penalized.

To make the long story short: the OIP3 (Output related Intercept Point for 3-rd order distortion products) of any device is not its physical property, but merely a calculated value. It is an abstracted figure of performance in terms of linearity for given technology/device, not more, not less. It involves a number of power measurements (at least four).
Unlike the P1dB parameter, the OIP3 in most cases cannot be assesed through direct power level measurement. This is simply because the OIP3 power levels for most modern quality active devices lie way beyond the power handling capabilities of devices itself. This is clearly ilustrated in the Figure 6 here (you hate maths, so skip it):
https://www.maximintegrated.com/en/d...ls/5/5429.html
By now you should understand, that trying to get the real OIP3 power level of +35dBm (which is 3.16 Watt) from your https://www.minicircuits.com/pdfs/GALI-51+.pdf
operating even at its absolute thermal limits of around 0.5W supply power would be in violation of fundamental laws of physics.
Sort of "inventing free energy".

Anyway, I can't follow the sense of your investigations into the OIP3 of MMICs. Do you distrust the specs in the datasheets?

BTW: The simplest and cheapest way to make two-tone tests would offer two DDS chips from Analog Devices, with their outputs shorted together, and terminated into single 50 Ohm resistor. No combiner, and no attenuators required.
... And do not confuse "ceramic filters" https://www.vintage-radio.net/forum/...9&d=1657447624
with "crystals".
I knew it is hypothetical concept and previously read the article in the link you gave. There is no need of this belittling and patronizing tone in your language. I dont know what you are on about. If you read my post carefully I wrote I could not trust the results of the TinySA and has doubt about how he measured it in his youtube video. I didn't take the measurements seriously. I knew it is a toy but hey many people here are hobbyists and do not have a degree in electrical engineering. There is nothing wrong playing with the RF toys and trying to learn.

Last edited by regenfreak; 18th Jul 2022 at 3:42 pm.
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Old 18th Jul 2022, 5:41 pm   #108
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Default Re: 6-gang FM stereo tuner heads

[
Quote:
Originally Posted by nemo_07 View Post
]o you think you have measured the OPI3 for the MAR-1+ with this little-tiny-weeny toy made in China. And all this in a single step?
You should perhaps look for informations supplied by reliable sites, not by wanna-be experts (remember the "negative group delay"?) like these:
https://tinysa.org/wiki/pmwiki.php?n=Main.OIP3
So they start with "After activating the OIP3 measuring mode ..."
They even didn't bother to realize the meaning of the term "intercept point" (which was explained in the math lessons in elementary school, but they obviously were absent).
So, what their gadget actually does when they talk of "OIP3 measuring mode" is visualizing 3-rd order intermodulation products (measuring their levels and calculating exact locations).

There is laking understanding of some fundamentals. And it is pandemic on the internet these days.
This exposes the simple truth, that the idea of "learning by doing" is fairly incompatible with the very basic principles of electronics (which is in essence the teritory of physics); and any careless and sloppy idea will be rigorously penalized.
Clearly one of your assumption was that Erik Kaashoek (the creator of the TinySA) and myself are clueless about the definition of the OPI3. I dont care if you call the TinySA a piece of cheap chinese junk, we deserve a public hanging, good beatings or whatever. Whether he is right or wrong, Erik deserves better respect. I thought this is a civilised forum. Erik should get a pat on his shoulder instead of being bashed and labelled as "wanna-be experts". His TinySA makes RF experiments accessible to students and hobbyists at a very low costs. It is a RF toy, so want? The NanoVNA proves itself to be valuable and serious piece of instrument that is a must-have for hobbyists wanting to learn RF.

The following guy is a university lecturer in electronics, he uses the TinySA to teach his university students in receiver design course in six episodes in his channel:

https://youtu.be/He0-X6FCLMo

You can accuse him being wrong again, shouldn't use that piece of junk for teaching, blah blah, who cares? It is just a hobby for me.
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Old 20th Jul 2022, 6:18 pm   #109
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Default Re: 6-gang FM stereo tuner heads

I did a few quick measurements of a GALI-51 eval board this evening (when biased at 65mA) to look at how the OIP3 changes when the GALI-51 is driven quite hard.

I got the results I predicted back in post #82 where the OIP3 degrades at very high output levels. It looks like the 35dBm figure only applies when the output level of each test tone from the GALI-51 is below about +3dBm. If each output tone seen on the analyser is at +10dBm the OIP3 degrades to about +32dBm and with GALI-51 output tone levels each of +13dBm the OIP3 degrades to about +25dBm.

My spectrum analyser is quite old now but it does have a built in OIP3 measurement function so it helps speed up the whole process. I used 60dB internal attenuation for the highest output levels and I also checked the result using an external attenuator. The distortion performance below is definitely that of the GALI-51 and not the analyser. This may explain why you only saw an OIP3 of +30dBm with some of your tests. You may have been driving it quite hard.

All of the tone levels are measured at the output of the GALI-51 as seen on the spectrum analyser.

o/p tone levels OIP3

13dBm 25.1 dBm
12dBm 28.2 dBm
11dBm 30.8 dBm
10dBm 32.3 dBm
9dBm 33 dBm
8dBm 33.4 dBm
7dBm 33.6 dBm
6dBm 33.9 dBm
5dBm 34.1 dBm
4dBm 34.4 dBm
3dBm 34.6 dBm
0dBm 34.9 dBm
-10dBm 35.1 dBm
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Old 20th Jul 2022, 7:25 pm   #110
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Default Re: 6-gang FM stereo tuner heads

Thanks, Jeremy.

That table shows OIP3 varying. At lower levels, it seems to stabilise close to +35 dbm and this is the linear 3:1 gradient region. At higher levels you can see the extrapolated OIP3 result degrading due to curvature of the line away from 3:1.

Most often TOI3 measurements are being made to assess SFDR, so the interest is in where the 3rd order products creep up out of the noise floor.

There are some rules of thumb for rough estimates of 1dB compression point from OIP3, but they don't always work very well. Then there is a rule of thumb for the reduction in compression as operation is moved below the 1dB point. I came across this from reports by Howard Swain, and it works reasonably well.

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Old 20th Jul 2022, 10:15 pm   #111
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Default Re: 6-gang FM stereo tuner heads

Quote:
did a few quick measurements of a GALI-51 eval board this evening (when biased at 65mA) to look at how the OIP3 changes when the GALI-51 is driven quite hard.

I got the results I predicted back in post #82 where the OIP3 degrades at very high output levels. It looks like the 35dBm figure only applies when the output level of each test tone from the GALI-51 is below about +3dBm. If each output tone seen on the analyser is at +10dBm the OIP3 degrades to about +32dBm and with GALI-51 output tone levels each of +13dBm the OIP3 degrades to about +25dBm.

My spectrum analyser is quite old now but it does have a built in OIP3 measurement function so it helps speed up the whole process. I used 60dB internal attenuation for the highest output levels and I also checked the result using an external attenuator. The distortion performance below is definitely that of the GALI-51 and not the analyser. This may explain why you only saw an OIP3 of +30dBm with some of your tests. You may have been driving it quite hard.

All of the tone levels are measured at the output of the GALI-51 as seen on the spectrum analyser.
Thank you so much taking time to do this. Your results look very close to what I would expect.

What are the frequencies of your two fundamentals and their frequency separation? How sensitive are your measurements to the frequency separation (if you have control over it)? As I wrote before I could only get it working by trying different frequency separation of the two fundamental frequencies.

The use of external attenuators is a good way to check if the results are not coming from the built-in signal generator of your spectrum analyzer. Also the intermodulation junks could come from the power combiner itself if there is not adequate isolation between ports.

I thought I drove the Mar-1+ and Gali-51+ both with about -35dbm (can't 100% remember if this was the case)two-tone measurements at the input of the MiMIC preceded by a 10db attenuator, power combiner and the 20 db attenuators to the outputs of the signal generators. Also I was not sure really why the result for the Mar-1+ is ok but Gali-51+ is abnormal using both -35dbm drive levels. I am taking the TinySA results with a pinch of salt, having a bit of fun really.

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Old 20th Jul 2022, 11:10 pm   #112
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Default Re: 6-gang FM stereo tuner heads

I used test frequencies of 99.00MHz and 99.05MHz.

The GALI-51 has about 18.1dB gain at 100MHz so to get two output tones at +3dBm each I would have been driving the GALI-51 with two tones each at 3 - 18.1 = -15.1dBm at the input of the GALI-51 eval board.

My combiner has about 6dB loss and there's some more loss for the LPF. I also had a 10dB attenuator after the combiner. Normally I can get in the ballpark of -100dB IMD3 terms after the combiner with this setup at VHF.

My sig gens have a narrow ALC option so when I select this there is very little crosstalk between the sig gens at a 50kHz spacing. I'm not sure what the ALC bandwidth is in the narrow mode but I think it is much less than 1kHz.
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Old 21st Jul 2022, 8:02 am   #113
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Default Re: 6-gang FM stereo tuner heads

You might have been better with two 10dB attenuators ahead of your combiner because this gives an extra 20dB drop in the ,egel of one generatr getting to the other one. Any close-by signal getting in backwards to the levelling-loop detector guarantees intermod creation.

If the combiner is very sensitive to its output load then yoou maybe do need a pad after it. Perhaps split the difference and have 5 in each port?

In serious cases, I've used minicircuits power amps and then arge attenuation after each sig gen into a purely resistive combiner. I was developing mixers in the >+45dBm TOI area.

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Old 21st Jul 2022, 9:07 am   #114
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by G0HZU_JMR View Post
... The issue here is that the capacitance to free space of one of the crystal legs will effectively be added to predicted parallel capacitance of the crystal when doing an S11 measurement. In the case of a 2 port S21 measurement this capacitance doesn't affect the through 'null' frequency.

There's also capacitance to the holder and then to free space and this is also not accounted for in the classic model. So I would expect to see a difference here of typically >0.2pF for the predicted parallel capacitance when comparing S11 and S22. It's a tiny difference but even a perfect VNA will show a different result for parallel resonance when comparing S11 vs S21. None of this matters in the real world. The existing model for a crystal (although incomplete) is extremely good and well accepted.
The crystal model assumes a reference plane placed to it terminals at zero leg lengths. In this sense and only in this it is repeatable and thus complete.
Proper fixturing and calibration should account for stray impedances of terminal legs and anything connected to them. Ideally a calibrated VNA removes this strays and shows corrected results. Calibration standards need to feet into crystal socket.
It is important to know, what and how your VNA processes the raw data, prior to presenting results, otherwise the results shown can be misleading.

The graphs you posted in #105 and #106 are not conclusive to me (both are labelled "DBS21" but look like admitance charts, and the markers show no shift); the circuit diagrams attached
https://www.vintage-radio.net/forum/...1&d=1658155336
imply that the crystal will be pulled by 0.125pF in the S21 set up, and by 0.25pF in the S11 set up.

The question is, why do you take pains of measuring parallel resonance of a crystal (well known to have extremely high impedance) where any 50 Ohm system will run into resolution limits, instead of looking at series resonance?
At this end, however, you have to know the C0 to be able to mathematically remove it (remodelling the crystal as two arms: the reactance XC0 connected in parallel with the motional arm C1, L1, R1) to get the clear picture: minimum Z (being then R1) coinciding with zero phase, indicating series resonance.
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Old 21st Jul 2022, 9:57 am   #115
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Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
[
Quote:
Originally Posted by nemo_07 View Post
]o you think you have measured the OPI3 for the MAR-1+ with this little-tiny-weeny toy made in China. And all this in a single step?
You should perhaps look for informations supplied by reliable sites, not by wanna-be experts (remember the "negative group delay"?) like these:
https://tinysa.org/wiki/pmwiki.php?n=Main.OIP3
So they start with "After activating the OIP3 measuring mode ..."
They even didn't bother to realize the meaning of the term "intercept point" (which was explained in the math lessons in elementary school, but they obviously were absent).
So, what their gadget actually does when they talk of "OIP3 measuring mode" is visualizing 3-rd order intermodulation products (measuring their levels and calculating exact locations).

There is laking understanding of some fundamentals. And it is pandemic on the internet these days.
This exposes the simple truth, that the idea of "learning by doing" is fairly incompatible with the very basic principles of electronics (which is in essence the teritory of physics); and any careless and sloppy idea will be rigorously penalized.
Clearly one of your assumption was that Erik Kaashoek (the creator of the TinySA) and myself are clueless about the definition of the OPI3. I dont care if you call the TinySA a piece of cheap chinese junk, we deserve a public hanging, good beatings or whatever. Whether he is right or wrong, Erik deserves better respect. I thought this is a civilised forum. Erik should get a pat on his shoulder instead of being bashed and labelled as "wanna-be experts". His TinySA makes RF experiments accessible to students and hobbyists at a very low costs...
Would I try to call the TinySA a junk or discredit its creator? Certainly not.
I like all these tiny/nano toys and find the ideas standing behind brilliant. We've had yet few words on TinySA https://www.vintage-radio.net/forum/...1&postcount=69
I did not mean to insult anyone but this text https://tinysa.org/wiki/pmwiki.php?n=Main.OIP3
as it stands there reads like written by a wanna-be expert, I can't help.
What I actually intended was to point out, that their single-step "OIP3 measurement" is misguided.
Firstly, there is no mention that the basic uncertainty of this feature with the TinySA amounts to +/- 4dB.
Then, there is no warning that the errors of this method are generally not systematic and not predictable, thus turning the "OIP3 measurement" into a Random Number Generation when applied to a DUT with poor linearity.

This (over)simplified method can be safely used with restricted range of device types (some Variable/Programmable Gain Amplifiers, X-Amps, FET devices, vacuum tubes) for which the assumption that the slope of 3-rd order products will be a pretty straight 3dB/dB line is justified.
Taken for granted as general, this assumption is illegitimate, especially in the case of bipolar transistors, where these slopes will most of the time depart more or less from 3dB/dB, depending on bias, drive level and frequency.
In worst case this departure can generate results missed by many, perhaps tens of dB.
Can it be a "learning experience"? Definitely maybe.

Therefore not a quick-and-easy method, but a multipoint measurement with driving levels located around projected bias point with the frequency of interest, and then interpolation would be required to arrive at more or less plausible intercept level.
And then, this tedious procedure will show only one of multitude of possible plots for given device which would be necessary to reveal its true performance.
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Old 21st Jul 2022, 4:02 pm   #116
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Default Re: 6-gang FM stereo tuner heads

Quote:
You might have been better with two 10dB attenuators ahead of your combiner because this gives an extra 20dB drop in the ,egel of one generatr getting to the other one. Any close-by signal getting in backwards to the levelling-loop detector guarantees intermod creation.

If the combiner is very sensitive to its output load then yoou maybe do need a pad after it. Perhaps split the difference and have 5 in each port?

In serious cases, I've used minicircuits power amps and then arge attenuation after each sig gen into a purely resistive combiner. I was developing mixers in the >+45dBm TOI area.
If I understand you correctly, that was what exactly I did. I had two 20db "isolators" meaning attenuators in front of the combiner. Please see my sketch of the set-up in post 81

Quote:
used test frequencies of 99.00MHz and 99.05MHz.

The GALI-51 has about 18.1dB gain at 100MHz so to get two output tones at +3dBm each I would have been driving the GALI-51 with two tones each at 3 - 18.1 = -15.1dBm at the input of the GALI-51 eval board.

My combiner has about 6dB loss and there's some more loss for the LPF. I also had a 10dB attenuator after the combiner. Normally I can get in the ballpark of -100dB IMD3 terms after the combiner with this setup at VHF.

My sig gens have a narrow ALC option so when I select this there is very little crosstalk between the sig gens at a 50kHz spacing. I'm not sure what the ALC bandwidth is in the narrow mode but I think it is much less than 1kHz.
Thanks. I read that sometimes they use ferrite beads to wrap around the signal paths internally to enhance isolation and avoid crosstalk between the two channels.

I am moving on to the IF amp using ceramic filters in cascade using Nelson Jones' WW 1971 design (see attached schematic). I have used two 280kHz ceramic filters using the "cute" CA3053 IC in cascode configuration. If you compare the response curve of a single ceramic filter in post 85, the roll-off is much steeper. The overall gain is only is 9.3db due to the slight impedance mismatch of LC matching pads. Measurement has been conducted with a 15db attenuator at the input and a 10db attenuator at the output of the 50 to 330 ohms LC matching pads. Previously I managed to get the LC pads perfectly spot on at the centre of the Smith Chart(see post 85). This time I tried to adjust L-pads with the trimmers but I could not make it any better. I am going to build the TAA6611B IF Amp and detector stage....
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Last edited by regenfreak; 21st Jul 2022 at 4:10 pm.
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Old 21st Jul 2022, 5:06 pm   #117
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Default Re: 6-gang FM stereo tuner heads

Also the input impedance of the TAA6611B IC is 2K, Nelson Jones bumped the output terminating resistor from 330 to 390 ohms, hence it explained why I could not get the impedance matching exactly at the center of the Smith chart no matter how much i adjusted the 50 to 330 ohms L-pads.

I also got some TA7302P IF AMP ICs that has built-in limiting and 34db voltage gain. It will allow more elaborate ceramic filter cascade chain design.
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Old 21st Jul 2022, 7:57 pm   #118
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Default Re: 6-gang FM stereo tuner heads

With my RF sig gens I find the best isolation is achieved if I exploit the attenuator hold/lock feature. It's often possible to get an extra 10dB isolation per sig gen for free using this method. It locks in an extra mechanical attenuation stage at the sig gen output and then I can turn up the internal level to compensate. This only works up to the limit of the ALC hold/lock system but it usually allows an extra 10dB of mechanical attenuation in the step attenuator. The ALC bandwidth can also be set to narrow instead of the usual tens of kHz of ALC bandwidth. This really minimises any artefacts generated by the ALC.

One of my sig gens has an electronic step attenuator rather than mechanical but it still seems to perform very well for stuff like this. To get the best isolation from the 6dB combiner I generally add a 10dB attenuator at the sum port of the combiner. This combiner typically has >46dB isolation across the HF and VHF bands if well terminated at the sum port.

I've got lots of other combiners here including a wideband resistive combiner that has about 19dB port isolation and 9.6dB through loss. This performs a bit better than the classic 6dB resistive splitter in terms of port isolation.
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Old 21st Jul 2022, 10:15 pm   #119
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Default Re: 6-gang FM stereo tuner heads

Quote:
With my RF sig gens I find the best isolation is achieved if I exploit the attenuator hold/lock feature. It's often possible to get an extra 10dB isolation per sig gen for free using this method. It locks in an extra mechanical attenuation stage at the sig gen output and then I can turn up the internal level to compensate. This only works up to the limit of the ALC hold/lock system but it usually allows an extra 10dB of mechanical attenuation in the step attenuator. The ALC bandwidth can also be set to narrow instead of the usual tens of kHz of ALC bandwidth. This really minimises any artefacts generated by the ALC.

One of my sig gens has an electronic step attenuator rather than mechanical but it still seems to perform very well for stuff like this. To get the best isolation from the 6dB combiner I generally add a 10dB attenuator at the sum port of the combiner. This combiner typically has >46dB isolation across the HF and VHF bands if well terminated at the sum port.

I've got lots of other combiners here including a wideband resistive combiner that has about 19dB port isolation and 9.6dB through loss. This performs a bit better than the classic 6dB resistive splitter in terms of port isolation.
I would be interested to see the measurement of IIP3 and OIP3 of a ring mixer,e.g. ADE-1 or SBL-1 since you have the right equipment to do it. It will need +7dbm signal to drive the LO oscillator. Here is a rather long-winded article about IIP3 and OIP3 measurements:

https://www.markimicrowave.com/blog/...input%20signal.

I dont think the duplex low pass filters are necessary for quick and simple measurements.


I have built a 5-pole Chebyshev 10,7MHz IF filter and have realised that the ferrite cores of my inductors are super lossy at 10.7MHz. This is the same inductors i used for my LC impedance matching pads used in the measurements of the single and double ceramic IF filters (with CA3053 buffer amp). This explains why I got so much insertion losses in the mesurements. I really have to use the proper toroids suitable for 10.7MHz . I was lazy and used whatever I got in my junk box.

Last edited by regenfreak; 21st Jul 2022 at 10:32 pm.
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Old 22nd Jul 2022, 9:16 am   #120
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Default Re: 6-gang FM stereo tuner heads

Sig gens are usually somewhat more complicated than Marki suggests, though they're closer in terms of microwave band generators where most of their mixers are aimed. For the 100MHz band you're most likely to have an output level control loop with an RF detector on the output of the amp of the sig gen, certainly any generator with an output level controllable enough not to spoil any accuracy of these tests.

Cross-coupled signal from one generator to another will go through whatever value of output attenuator the generator has engaged and will beat with the generator's own signal in the levelling loop detector. This will AM modulate the generator's output forming just the third-order products you're trying to measure.

To get reasonably useable results coutermeasures like disabling levelling, or adding power amps and external attenuators are often needed.

I did a lot of measuring aviation COM and NAV radio designs through the last decade and they were not quite in the class of a basic packaged diode ring mixer (mini TFM-2 for example) and quite a bit of elaboration was needed to get specified TOI results. Quick tests before elaborations were introduced could easily be several dB out, and this was with low power transistor tree mixers (Called Gilbert cells by everyone except Barrie Gilbert himself and those who know the real Gilbert Cell analogue computing element)

The choice of spacing for a TOI test is also important in receiver tests. It sets how far down the structure of the receiver both tones will penetrate and how much removal of one or both happens due to the selectivity of different stages. Tone spacing sets which aread of the set get their linearity measured. For narrow spacing on an FM set, you'll eventually start measuring the linearity of the limiter stages! and they are intentionally not at all linear. Wide spacing can give too encouraging results because RF selectivity enters the scene and controls what reaches the mixer.

If you were measuring just a diode ring double balanced mixer alone, there is no selectivity at play, and so tone spacing ceases to be so important.

Just being a few dB off in TOI has a larger effecto on your SFDR calculations.

David
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