Start From Crystal (II)

2/12 2019

Crystal Set

Well, In episode Experience Crystal Set, I know several important things about the Crystal Set.  We must had a good antenna, and what we received is what we received.  It's not by design, especially for a newbie.  Middle wave broad cast is even worse, mainly depend where you are.

LC tank not function?

This phenomenon has explained by W0BTU.  

Directly connecting antenna to TOP of the tank is the WORST choice ever! My antenna is 15 meter for each arm,  this is the minimal length for crystal radio to recepte medium wave broadcasting per W0BTU's opinion. 

Directly connecting the antenna to Top tank,  ruined everything. The antenna is short ( 50 feet, 15 meter) , electronically speaking,  so it's capacitance is huge for Medium wave.   This capacitance might domain the hole tank resonant frequency. 

It's complex than i thought, this explained why only that strong station emerge on each tap.  But still can not explain why that station disappear if don't connecting the variable capacitor to inductor.

Connecting the Antenna to top is also an interesting radio demo to children. I  want more than that.  One of the Invert V arms use as crystal antenna,  and another arm use as ground connection.  

Reference for crystal radio:

http://www.w0btu.com/files/Crystal_Radio/antenna.htm

https://qsl.net/ve7sl/crystal.html

There is more important notes form W0BTU's blog. What important to antenna, is make it resonant somewhere. For medium wave receiving, it's better as long as possible. But we can not do that for so may different reason.  My invert V had 15 meters Arm,  to make it efficiency,  a LOADING COIL is suggested by W0BTU.  It might need as much turns as tune winding, even more,  a taped inductor is a good thing to play with.  

Another tricks is light capacitive couple to top tank. series a small capacitor, around 1 to 10 pF for electronically short antenna. For electronically long antenna, might be 100 pF, not my case though.  In this way the antenna capacitance is killed. I tried it, it useful, but for WM receiving  use another winding is better than this one.

Crystal radio experiment probably end here. There is more tricks to play, like wave-trap for strong local radio( Here do have one strong shortwave station ).  

More on crystal radio:

http://www.skywaves.ar88.net/Improved/Improved.html

http://theradioboard.com/rb/viewtopic.php?f=2&t=7784

0V1 TRF

Audio Amplifier

Testing crystal radio in house  is kind of torture, the signal is weak no matter whatever you do.  Adding Stages of Audio amplifier is the simplest and straight way to improvement the crystal set any way and so well known. 

TL431 Audio Amplifier

I  saw this novel TL431 amplifier on Teclib's website. And this little audio amplifier been used wildly in  theradioboard.com project. The gain of this little beast probably  40 dB and more.  This is a simple class A amplifier, and it's input impendance is very high,  so very suitable for crystal audio amplifier.

http://www.techlib.com/electronics/crystal.html

I try to setup a test bed with modules. The modules i ever built are almost  garbage, been thrown away while done with it.  There is  nothing left while i begin to exploring new stuff.  I decide  make the module pretty, and keep them for later use. 

Summary

• TL431 amplifier is simple,  high gain and high input /low output impendance, perfect crystal radio audio stage.

• Limited by small quiescent current,  it's easily over-driven and cause audio distortion, especially with a low impedance speaker.  Choose current limited resistor from 68 ohm to 1000K as your wish.

• Crystal radio to room volume !  beating LM386 (If you don't care power efficient).

Wild 1V1 TRF

Un-tuned wild RF Amplifier 

I wonder what would happen if adding a RF amplifier before couple to Tuner without preselect filter.

I suspect no band pass filter would be a totally disaster ,   tremendous noise would be amplified.  Ltspice suggest the amplifier  would have 15~25 dB voltage gain with a resistor collector load, use RF Choke increase the gain to 30 dB.  Gain roll down about 15-25 Mhz.

These two little piece amplifier is built. The result is amazing,  signal was strengthened enough to overloading the TL431 audio amplifier.  Even without the audio amplifier, the crystal detector itself could  drive the earphone into room audible volume at HF band.

DO NOT directly couple the signal Directly to the crystal tune tank ! The First rule for RF amplifier: extremely light couple the crystal tank!  (Gain is too high and the tap had too much turns.)

Connect a  wire to RF output,  it would act like a antenna.   The other end of wire need NOT connect to GND,  just put that wire around to the crystal tune. 

For HF band, connected the other end to GND help to reduce the gain hence make light couple easily. This RF amplifier had too much gain,  you could add 15-51 ohm degeneration resistor to improve the stability .

• Strong couple from RF stage would ruined the detector's selectivity

• Strong signal overwhelm the audio amplifier

• High rf Gain might cause oscillating

• Prefer inductance couple,  which is far more better than tapped couple. RF stage before tune circuits is wild.

Options available for curing these problem:

Audio.  TL431 could not handle large signal, it's class A.  Low Gain LM386 or similar chip could be alternative.  Personally prefer to use a simple push-pull  or a Class-A amplifier  match the impedance by a transformer.

RF pre-amplifier

An RF amplifier with too much gain  is  hard to tame. It easily overload next stages.  Spreading the gain in different stages is better. 

Another tradeoff is tuned amplifier or not tuned. The RF amplifier act as an active antenna instead of LAN. Such a RF stage amplify too much out-band signal, overwhelm the later stage.  The HF station is extremely strong ...   To cure the problem, couple between the RF stage to Tune Tank should be very light,  the high gain RF stage is no good.

Handle strong Audio Signal

MW signal might only  several mV after detector, but the HF signal reached to around 200 mV, this gonna overloading any simple Audio stage.

One of solutions might be feedback amplifier.  But simulating did not suggest  any  audio amplifier can handle such a strong signal and also keep sensitive to small signal.  

Looking for  an audio stages could:

• Handle large signal and keep relatively small distortion

• Handle Small signal in full power of Gain

• Possible solution:  AGC, Compressor and peaking-limit

First Stage input impendance is several Kohm,  not very suitable for directly connected to crystal style detector. Second stages input Independence is only ~500 ohm.

Audio transformer  has 3:1 turns ratio, only 50 mW available.  40mA quiescent current generated about o.5W heat dissipation,  half of  SS8050 power capacity, quiet hot.  

1N60 use to limit the maximum input signal delivery to the PA. 200 mV peak signal happens to be exactly the overdrive point. Output  is obviously  distortion. Hearing such a round top signal is not that bad, much better than overdrive.

Efficiency of  Class A Audio Amplifier

I thought use an audio transformer with ratio 5:1 or 6:1 could delivery more peak power on same quiescent current. Such a PA has at least 2 factors limited maximum available power:

• available current:  quiescent current, multiply the turns ratio is the maximum current could delivery to the load.

• available voltage swing:  transistor needs 2v bargain, the peak voltage is  (vcc-2),  then divided by turns-ratio delivery to the load.

The 3:1 ratio easily drain the quiescent current while the voltage swing still had potential.  I replace the transformer with a high ratio one,  5:1.  The output still distorted at almost same voltage level, nothing improved!! Stuned!

I tried several different audio transformer, Nothing improved !!  Go for Ltspice...  After play with everything,  just can not make breakthrough:  1 Vpp output swing is kind of hard limit! (with the same quiescent current)

Changing the impendance ratio would run into other limiting factor. The input signal could not exceeding 0.7 Vpp, it gonna reverse bias the transistor anyway.  To overcome this problem, changing the bias to resistor divider,  add a degenerate emitter resistor, and then it gonna use a  higher base voltage to provide same quiescent current.  Thus reduce the available voltage swing,  this is a endless loop. 

Suddenly It hit me, i actually trying to improve the efficiency instead of max output power.  25% maximum, i use a so-so transformer and not carefully design, so 10% efficiency! ( the transformer primary had 10 ohm DC resistance)

From power point of view, everything explained.  with the fixed quiescent current, max available power is not easy to improve. My 12V 40mA quiescent for a very good efficiency, let say  20%, we definitely could not reach the theory maximum efficiency(50%),   the output voltage only 1.2V if keep it not so distortion!!!!  And for a 6:1 ratio transformer the voltage available about only 1.6V peak (12Vcc - 2V margin ), this indicate even increasing the quiescent current, maximum 100 mW (1.3 Vpeak)  power is almost the hard limit.

While transformer's current drain out (highest voltage swing), the output became distortion, for a better signal, gonna need  few more  mA current,  efficiency become even lower. 

Back to Handle Large signal

Both amplitude limiter and audio compressor tested, none of them could handle 0.x mV to exceeding 200 mV audio signal. 

AGC is last straw.  Reduce the gain could make small signal disappeared, and then reduce  noise while received strong  signal.  Shunt type AGC could  be deployed easily and improve  dynamic range.

This little amplifier could provide 40 mW to 60 mW power. If it been over driven,  tremendous noise is generated, voltage spike on the transformer might be high, tens volt even hundred.  The Audio AGC prevent it from heavily over driving.  

Notes and results:

•  30 dB input signal  signal dynamic range without severe distortion, sound good.

• AGC time set long to prevent oscillation,  change 82k to larger value in case it is unstable.

• To improve AGC control, change the output transformer to 4:1 to 6:1 version.

• Overall gain  35 dB -40 dB, frequency bandwidth from 500 to 5k kHz (simulation result)

AGC Control Method

Use BJT as AGC Control device is rather common back to transistor ages. Use negative voltage reduce  the transistor current then reduce gain.

Use another transistor control the  quiescent current is much better than one transistor. In this way,wider control range is achieved with small AGC current.

But control the quiescent current  impact  the amplifier. The input impendance is changed a lot,  the very biggest problem is that a very strong signal gonna make the AGC pull out all current from transistor, the transistor then became distorted.  The result is worst sound under strongest signal.

The shunt type AGC is much better. Shunt type AGC gonna change the input impendance rapidly,  the lowest impendance defined by preceding divider resistor.  Use with a pre-amplifier or buffer is suggested.

Enhanced MOS-FET shunt type AGC control is very easy to use and have wider control range,and easily setting the time constant.  JEFT is another favor way but JFET is expensive than MOS-FET  nowadays.

Try to receiver MW signal from Strong HF background

Bigger antenna better signal,  but with most strongest HF signal instead of MW signal.  The simple receiver's tuned tank easily been destroyed, occasionally, the strongest Shortwave signal flooded everywhere. 

The strong out band signal ruined everything.... literally everything.  Out Band Signal must be eliminated in fist places,  a AM band LPF or BPF help a lot. 

2Mhz Butterworth  LPF

It's suppose use a band-pass filter. But below 500kHz, there is no stations.  Simple LPF did  the tricks.

 MW stations emerged, and soon,   I found the gain is insufficient receive weak MW stations. I need sensitive detector or add one more RF amplifier.

Sensitive envelop detector

V. Polyakov suggested a small detector circuits. which bias the transistor on it's starting up ramp, works well for detecting the AM  signal.  Sensitive Envelope Detector.

V. Polyakov said this detector has good efficiency works well and  voltage gain is around 60.  Input impedance is not too high, and output impedance is not low as expected, thus need a audio amplifier better have  dozens kilo ohm.   

After tank RF amplifier

With is configuration,  the receiver now received the AM signal well .  One stage tuned circuits is enough to separated adjacency stations, peak one signal is hard without speed reducer. 

Insert An audio LPF after the Detector

The terminator impendance is not that critical as source impendance.  300 ohm to 1k won't impact too much, only little bit group delay change.  Instead, the source impendance is critical.  The L6 value choice is 'random', from  15 - 25  mH,  the band pass changing not too much.

This Audio amplifier already had a RC type LPF, which cut off frequency is near 5Khz, and it might overkill, lost too much high pitch sound. 

It's time say goodbye to crazy mad TRF.  The most problem due to the uncontrollable RF amplifier. Too much gain and tend to oscillate, cause  tremendous noise.

RF amplifier after tuner is good choice,  avoid troubles to make a wide bandpass filter , avoid much more trouble to terminated the filter. 

Stable the RF amplifier

RF amplifier had too much gain is disaster. I dip into that wild amplifier several months, so impressed .  RF voltage Gain > 25 dB (17x voltage gain) is almost a hard limit before it's tending to oscillate. Of course, this Gain limit depending what transistor you use. 

Per experiment,  one stage RF Amplifier deploy 10dB to 20 dB voltage gain, i.e, voltage 3x to 10x,  is easy and stable. Use RFC as load and capacitor couple. In this way, RF voltage gain is around 20 dB @1 Mhz/per stage. 

 To cure the unstable RF amplifier,  2 methods deployed to RF amplifier:

• Insert a 15 Ohm to 33 Ohm degeneration resistor. low overall voltage gain, while coupled to next stages, it's gain varied from 20 dB to 30dB (around 1Mhz),  input impendance around 200 Ohm, and decrease quickly while frequency increase.

• Couple the stages with a small enough capacitor, so the low frequency gain decreased.  NOTE: this is just pushing the self resonant frequency higher,  and if luck enough, there might no gain anymore.

It's now stable while coupled to next stage. 

Couple Huge  antenna to small TRF receiver

Finally, i notice this: connect the wire antenna via a 22pF capacitor to the top of the LC tank, everything works very well.   22pf isolated the huge capacitance introduced by the long wire.  

Connect the wire to tap or another winding not a good method for this antenna.  This probably because the wire capacitance and the tap inductance resonant at HF.  

Another possible reaseason, might be the shield of the rod antenna is poor,  rod itself is antenna after all. I winded another Toroid inductor, use a standalone one turn  winding as antenna couple point, that one works well. 

TRF, especially this rough  one,  selectivity is poor, any stray couple from that HF friendly antenna is gonna make the TRF slide to HF band.

Capacitive ground plane

Recently, a good laboratory breadboard builted.  It's not intend to making RF modules. That won't scare me,  I built a TRF receiver on it.  Yeah,  a very noise TRF,  it does not oscillating, though.

I try to cure the problem, there is no ground plane, and the GND is routed everywhere, totally not conform any RF rule.

But, while i connected one GND joint to the bellowing copper clapboard, magical thing happened, the TRF silent as it built on a ground plane.  I guess this clapboard make a capacitance with corresponding wire on the breadboard, make it partly ground plane, a capacitance ground plane. 

It's works well on MW, HF? don't know yet.

How about this sensitive detector

Sound much more noisy than diode detector.  Diode detector gonna clipp the weak signal,  let only the strong signal pass, but transistor amplified weak signal. Transistor detector's threshold is much lower than diode. 

Remove one stage RF amplifier help a lot.     BTW:  use 15m long wire attenna.

Outdoor Testing

To take the receiver outdoor, i need duplicate a smaller platform for the modules.  The 2V386 version is lack of sensitivity, only one station received. but lately i realized the 365pF capacitor is problematic, it's rotator soldering pin is missing, contact resistance is high and then,  selectivity is poor.

 With a new small testing receiver, this 3V386 receiver is under testing.   3 RF amplifier crowed in  a small board ,  it's disaster,   and tending to oscillate.  Adjust the ferrite rod antenna help a little, at least most of the AM band is quite.  

3V386 could  received 3-5 station in downtown.  the loudspeaker volume is acceptable and audible in a relatively quiet environment.

Unstable RF amplifier

Put 3 RF amplifiers on  the small clapp board is disaster. One step forward, deploy Polyakov  transistor detector leads to unavoidable oscillate.  The overall feedback might  not enough to make it became a stable oscillator,  but enough to make a regenerative receiver.   

60 ~ 80 dB RF amplifier gain is 'hard' limitation to the simple, rough TRF.  And with  3xRf amplifier and a diode detector,  3V386 could deliver a audible TRF receiver with perceptible selectivity.