6-gang FM stereo tuner heads

[dmowziz]

Originally Posted by regenfreak

Thanks. The 4 rectifier diode idea is still in my mind but I am concerned about the large diode junction capacitance which may call for a matching network to tweak the S11 and S21 at high frequencies.

I have taken another potshot at the PIN diode. I ordered a Skyworks SMP1330-005LF last night. It is a PIN-Schottky pair, having a threshold about +10dbm, C=0.7pF, 14db attenuation at +30dbm working up to 4GHz (see power characteristics attached). I speculated that the cheapo pin-limiter does not work well because it uses only a single diode PIN diode.

https://www.skyworksinc.com/-/media/...es_200050N.pdf

I am going to add two 1kV 1206 10nF and 250mA SMT fuse plus a 70V 4532 SMD ceramic GDT (capacitance = 0.5pF) to safeguard against "catastrophic event". I will house them in RF shield instead of using a metal box.

I have been looking at the integrated SKY1662-632LF datasheet:

https://www.skyworksinc.com/-/media/...LF_202945L.pdf

It is kind of interesting to see how they put a simple series LC network to optimize the S11 and S21. With L = 1.8nH, C=15pF for 2.45GHz, they transpose the peak and trough for S11 and S21 to the right. The equivalent circuit of a PIN diode (in open state) is like a small inductor in series with the paralleled capacitor and a very large resistor. The S21 and S11 look a bit like a very broadband bandpass filter.


PS: I have received the ATC caps and other goodies that you sent. Thank you so much. I am building a "Q detector probe" with your ATC 1pF caps. It will use FET MMBF310 and MMBF3906 as impedance convertor. The device will be used to find approx Q and resonance frequency of air variable tuning gangs and IF transformers for MW, SW and FM radios in unpowered state.
The ATC100 series caps have Q about 10,000..quite badass!

Just wondering,

Please the big things that look metallic.

Are they the capacitors? 1 of them is 1 capacitor?

[regenfreak]

[QUOTE=dmowziz;1525766]

Originally Posted by regenfreak

Just wondering,

Please the big things that look metallic.

Are they the capacitors? 1 of them is 1 capacitor?

They are low-profile Faraday cages/ metal shields for SMD circuit boards that come with mounting clips. They are handy for high-frequency or microwave circuits and are cheaper alternatives to the metal housing.

[Radio Wrangler]

They aren't Faraday cages, the metal is solid, so they also act on magnetic field components. Faraday screens are special, they are aligned grids of wires designed for purely electric field screening and are pretty much useless for general screening duties.

The idea behind the wire grids is that there is no path for circulating eddy currents.

This is a very common confusion, but it's wrong. You'll find real faraday screens inside some transformers to block capacitive coupling between windings.

These things are simple metal screening boxes, often plated steel or brass. They get soldered onto printed circuit boards over an area of componets doing something sensitive. Often the PCB has a groundplane area which acts as the lid, completing the screening box. They are a right pain for fault finding and access. They need a bigger soldering iron to remove them than they take to fit!

David

[regenfreak]

Originally Posted by Radio Wrangler

They aren't Faraday cages, the metal is solid, so they also act on magnetic field components. Faraday screens are special, they are aligned grids of wires designed for purely electric field screening and are pretty much useless for general screening duties.

The idea behind the wire grids is that there is no path for circulating eddy currents.

This is a very common confusion, but it's wrong. You'll find real faraday screens inside some transformers to block capacitive coupling between windings.

These things are simple metal screening boxes, often plated steel or brass. They get soldered onto printed circuit boards over an area of componets doing something sensitive. Often the PCB has a groundplane area which acts as the lid, completing the screening box. They are a right pain for fault finding and access. They need a bigger soldering iron to remove them than they take to fit!

David

I see. The real Friday cage is often found in tesla coil demo where people stand inside;

https://youtu.be/Lqwj5bACKbU

The shields i got have clip-on bits that you are supposed to solve the de-soldering problem you mentioned.

[Radio Wrangler]

Some people suggested that the magnetic induction transformer was actually coupling energy capacitively. So Faraday invented his special shield, which blocks electric fields only to prove that the coupling had to be magnetic.

Unfortunately, some people automatically prefix the word screen with Faraday's name. Maybe they think it makes the screens appear screenier? But in reality from the point of screening radio signals, Faraday screens leak like a seive. It's comparable to prefixing every mention of the word 'Tyre" with the word "flat". But Faraday screen/cage/shield seems to have entered folklore. Lexicographers have their dictionaries change to follow newer usage, but this is science and we are talking specific and defined terms here where we can't let meanings change without trashing knowledge.

Not being grumpy, just trying to defend something we need for accuracy.

David

[regenfreak]

Originally Posted by Radio Wrangler

Faraday screens leak like a seive. It's comparable to prefixing every mention of the word 'Tyre" with the word "flat". But Faraday screen/cage/shield seems to have entered folklore. Lexicographers have their dictionaries change to follow newer usage, but this is science and we are talking specific and defined terms here where we can't let meanings change without trashing knowledge.

Not being grumpy, just trying to defend something we need for accuracy.

David

The levels of H- and E- fields penetration vary significantly with frequencies, see attachment. Above 100KHz, the W- and H- field converges into plane waves over a wide range of frequencies...The rule of thumb is that the grid spacing is about 1/10th of the wavelength.

[regenfreak]

Incidentally I have been digging through the Black Hawk helicopter aerodynamic performance data ( to do with my job), I have noticed that there were 5 crashes caused by the Black Hawk flying close to radio broadcast towers. Subsequently they improved the EMI shielding of the avionics and flight controls. There were many similar incidents for other fixed wings and rotary wing vehicles in this NASA report:

https://ntrs.nasa.gov/api/citations/...9960009442.pdf

[Radio Wrangler]

Distance gets in on the act too. Far field in normally taken as 20 lambda from a source. Within that area, as you get nearer the source, you can find predominantly one field more than the other. Another factor is skin depth and then there is the permeability of the shield.

However, use of the prefix - Faraday takes care of all that and specifies a shield designed to pass magnetic components. And as we all learn in trying to screen things, electric components aren't too bad, it's the magnetic ones which are the devils.

David

[regenfreak]

Before Christmas, I wrote that I was going to build another 6-gang tuner with an unbalnced dual gate mosfet mixer because I have three of the same military, silver-plated 6-gang air variable capacitors.

The attached schematic is the finished version 2 tuner. The design is very similar to the first version with a BJT mixer except it uses one inductive coupled tunable BPF and one capacitive coupled tunable BPF in the RF amp stages.

Everything performs well as expected. The local oscillator has a buffer amp that drives gate 2 at a much higher signal level than the RF input from gate 1. The transconductance is amplitude modulated by the local oscillator "pump" voltage (see attachment 5). I installed a digital frequency display like the first version. It is getting harder and more expensive to obtain the original vernier tuning dial knob so I have used a cheapo chinese alternative which is over 180 degrees of rotation. It works Ok but not as good as the originals.

My next projects will be the experimentation with a balanced dual gate mosfet mixer which is more difficult to implement.

I consider building a 8-gang varactor tuning FM receiver...I have some ideas floating around but I nowadays spend much more time keeping fit since the start of the year and spend less time on electronic projects.

[regenfreak]

One of the most difficult challenges of making a multiple-gang varactor tuned broadcast receiver is the linearity and predictability of the tracking. I have done some measurements to characterise the Philips BB204B double varactor diode in terms of their frequency linearity, unloaded Qu in an LC tank and their actual performance in a tunable bandpass filter.

Based on the datasheet, I think its graph was measured with one of the two diodes. One diode Q is about 200 at 100MHz. In my case, I have measured diode capacitance with two diodes in series which would half the Q to about 100. The advantage of having the diode in series is that it minimises the effects of the fluctuating RF AC on the diode capacitance and less distortion products.

The mathematical model of the varactor is represented by the simple equation in attachment one. V is the sum of DC tuning voltage plus RF signal voltages. Phi is the contact potential of the diodes. By taking the logarithmic function of the C and V relation, it is possible to find the power index n using linear regression.

The varactor can be idealised as a capacitor (of high resistance depleted region) in series with a small value resistor (variable, low-resistance undepleted region+ constant contact resistance). Q increases with increasing reversed bias voltage, therefore its performance should be worst at the lower limit of the broadcast band.

Attachment 2 illustrates LogC vs log(V+phi), the power index n = 3.37 using linear regression. For ideal linearity, n should be 2 because the frequency is inversely proportional to capacitance squared. I have found there are variations of the measurements among different BB204B samples.

In the 3rd attachment, it shows the measurements of (1) frequency vs reversed voltage and (2) unloaded Q of LC resonator vs frequency for a single LC resonator (L =88nH). In this scenario here, the Q of the inductor is comparable in magnitude with that of the varactor diode (1/Q = 1/Qc + 1/QL). Usually, the Qc of the air gang capacitor is significantly bigger than the inductor QL.

AT 88Mhz, Qu = 55 and at Qu = 129, very drastic difference!


To evaluate the performance of the varactor diodes in a tunable FM bandpass filter with 50 ohms terminations, I have tried it with a Butterworth optimised at 100MHz. As you can see in attachment 4, it smashes at 50 ohms; bang on at the centre of the Smith chart. S11 = -45db, S21 = -2.5db which is very good (typo in my file name, it should be 98.8MHz and not 86.8Mhz)

Attachment 5, shows the changes in bandwidth and bandpass response at 88Mhz, 99Mhz and 108Mhz, respectively. At 88Mhz, the filter has about additional -2db insertion losses due to the lower Q of the varactor diodes at the low-frequency bottom end. At the top end of the broadcast 108Mhz, the selectivity of the bandpass degrades and therefore it is desirable to have multiple gangs of tunable bandpass filters in high-performance FM tuners.

[regenfreak]

I have been suspicious about the value of n =3.73 in my previous post. I would expect n to be between 0.3 to 2. I have checked a 1967 IEEE paper, the previous equation only applies to a single varactor diode. Therefore I should have repeated the measurement with only one diode to obtain the correct n from the slope of the Log(C) vs Log(V+phi) graph.

Based on R. De. Cola's IEEE paper "Varactor Tuning applied to Am-FM receivers", the model of two back-to-back varactor diodes are rather complicated as shown in the attachment. In this case, it is not possible to find n using linear regression.

BTW the IEEE paper is full of maths and quite difficult to understand.