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2017-04-27 Superregen FM BCB Receiver

I built this superregenerative FM broadcast band receiver today.  

Should say “https://rustybolt.info/wordpress/?p=11919” at bottom

Here is a list of parts values I used:

ANT – 12 cm insulated wire connected through 10 pF capacitor to FET source.

C4 – two 2.2 nF MLCCs from + to ground plane ( I built this on a small piece of copper circuit board.)

C5 – 18 pF silver mica capacitor 

C6 – not used – shorted.

C7 (A and B) – two 0.2 to 5 pF plastic variable capacitors 

C3 – 10 pF silver mica capacitor 

R3 – 2.7 M 5% carbon film resistor 

R4 – 22k 5% carbon film resistor 

Q1 – 2N3819 VHF JFET 

Q2 – C9018 high gain low noise NPN BJT 

L1 – 6 turns 20 AWG bare copper wire wound on drill bit shank, appx 6mm ID.  Coil stretched so it measured appx 120 nH.

L2 – 6 turns 24 AWG solid insulated wire on a FT37-43 toroid core, measured 14 uH

(Following copied from FB post)

My Superregen won’t quench.  I built the circuit found here, with a few changes.

http://www.vk2zay.net/article/129

I used a 2N3819 for Q1, and a FT37-43 for L2, measuring 14 uH.  At the end in his notes he gave some limits to the values of some parts, and the values I used seem to be within the limits.

For an antenna I connected a short 12 cm length of wire to the FET’s source, as he recommended.  The coil is 6 turns of 20 AWG bare copper wire, wound on a drill bit shank, and spread out untl it measured about 120 nanohenrys.  I used an 18 pF silver mica capacitor and two 0.2 to 5 pF variable capacitors in parallel with the coil.

 I used a dip meter to tune the coil and capacitors to a point within the FM broadcast band.  I measure a little over 2 volts at the source, so the JFET is conducting in the linear region.

I used a scope to check the R1 – C1 point and I see no oscillations.  I’m using 9VDC regulated for the supply.  I connected the Q2 output to an amplifier and I get noise like a hiss but no stations.  I can hear my cell phone burps loudly, and hum if I touch points in the circuit.  So I’m fairly confident that I have the circuit set up properly  – no major malfunctions.

I just thought of one thing I can do.  I can put the circuit on a variable supply and try various supply voltages.  I’ll try that and update this.

Update evening until 1:30 AM – I experimented with several different parts.  I put it on a variable supply and it seemed to do a little bit better at 6 volts.  At the beginning of this session I could receive only a single FM station, one that’s very close and powerful.  This was with the 12 cm wire connected through a 10 pF capacitor to the source.  I couldn’t hear any other stations.

I removed the L2 and rewound it, so it has 9 turns on the FT37-43 core and measured 33 uH.  The audio seemed to be louder.  But as I tune, the audio is garbled, seems that the level is too high for the audio amplifier I’m using.  Still only one station.  

I moved ‘center tap’ one turn closer to the positive end of the tank coil.  I also spread the turns a bit, and adjusted the variable capacitors a lot.  I think the coil could be longer and lower inductance so it will cover the high end of the FM band.

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2017-04-23 Linear Power Supply Renovation 

In the past I’ve built several of these small power supplies to use around the house, and they come in handy.  But those others used a 12 VAC transformer instead of an AC adapter, so I had to add a fuse and other stuff to prevent shock.  Using the AC adapter is much more convenient because all of that is included in the wall wart.  All I have to deal with is low voltage DC.  Those power supplies used old designs that didn’t have good regulation.  I wanted to replace them with a linear regulator chip which gives excellent performance.
Years ago I had built a 9 VDC shunt regulated power supply, low current, only 40 mA max.  A shunt regulator wastes the power that isn’t being used by the load, in this case it was built for powering a pocket transistor radio.  A Zener diode is a good example of a shunt regulator.  If none of the Zener’s current is used, then the current is wasted heating the Zener diode.  So I dissembled the shunt regulator and used some of its parts to build a new regulator using a LM317.  I also scrapped another power supply for the circuit board and some parts.  Both of these used an old transformer type wall wart with outputs about 13 volts no load.  So I used the adapter rated 9VDC at 600 mA.

I’ve been building these small RF circuits and I’ve been using a bunch of bench supplies to power them.  The circuits need very little power, but the voltage needs to be stable, so that’s why I decided to build a few of these with  adjustable regulator chips.  A few other parts and I’ll have a 1.25 to 9 VDC well regulated supply, that can put out about 200 mA to a load, up to 8 volts.  The wall wart output drops as the current goes above 150 mA and the regulated output drops below 9 volts.  If I had a better AC adapter that wouldn’t happen.  But I can live with the lower current.

This new power supply uses a LM317 with a 270 ohm resistor from Output to Adjust pins, and a 2.5 k wirewound pot from the Adjust pin to negative.  This would allow me to adjust the voltage up to 12V, but the adapter can’t put out that much under load, so I put a resistor in parallel with the pot to limit the voltage to 9 volts maximum.  But even at 9 volts, if I put a load of more than 150 mA on it, the voltage drops below 9 volts.

I added some capacitors and diodes to the basic design to filter ripple, and help protect the circuit from overvoltage or reverse voltage.  This is pretty much standard practice with the adjustable VRs.

I used several EMI / RFI suppressor sleeves on the input and output wires to prevent the RF from the circuits from radiating from the supply.  I added a 2200 uF, 16VDC capacitor on the circuit board to help the capacitor in the wall wart.  

The circuit fit on a perfboard the size of a large postage stamp.  I got the heatsink from an old PC power supply, but I should mount the heatsink on the aluminum lid of a project box for better heat sinking.

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2017-04-21 Signal Generator for 27 MHz

I need a source of signals on various frequencies to test how well my bandpass filters are aligned.  I built a Vackar Oscillator with tuning over a few CB channels, mainly 27.095 to 27.255 MHz.  These are radio control frequencies.  Like a Colpitts oscillator, the Vackar’s resonant circuit has two capacitors in series with the center point grounded.  The difference is that Vackar has the transistor’s parasitic capacitance somewhat isolated from the resonant circuit.

I’ve been dealing with building a stable RF inductor and trying to get it to be stable with changes in temperature.  Last night I put a coil on it wound with 26 AWG solid enameled copper wire, wound on and secured to a 1 inch length of bamboo skewer stick, and it measured 0.8 uH.  I tuned my Kenwood R-1000 receiver to it.  By this morning it had drifted 6 kHz.

I’m taking advice and adding capacitance to the resonant tank and reducing the inductance.  I changed the coil to 9 turns 20 AWG solid enameled wire on a T50-2 iron powder toroid core, measured at 0.72 uH.  In order to bring the frequency down, I added a variable capacitor, 8 to 50 pF, N750 tempco, in series with a 47 pF silver mica to the ‘cold’ (left) side of the resonant tank.

Update Apr 24 – several days ago I ordered some powdered iron cores, T37-7, which are supposed to have very low temperature coefficient.  I got them today, so I wound one with 23 AWG solid plastic insulated wire from a Cat5e datacomm cable.  I started with 11 turns, but I removed some until the inductance measured 0.62 uH.  The dip meter showed the signal was above 30 MHz, so I added another 100 pF in parallel with the 100 pF C1, and changed the 47 pF to 68 pF.  When I adjusted the 8 to 50 pF, I got the carrier on the receiver.  I tweaked the 0.2 to 5 pF, and got it to be on 27.145 MHz.  I also added a 33 ohm resistor between the collector and C5.

I left it alone while I was doing something else, and came back later and found that it was still at about the same frequency.  I have noticed that while I’m working on the circuit the radiant heat from the soldering iron and even from my body made the circuit drift.  But with the latest modifications the long term drift has decreased, and I can go away for hours and the circuit is still at about the same frequency.  Short term drift isn’t as good, the frequency goes up and down dozens of kHz over the period of a minute.  The toroid core moves around when I pick up the circuit, and the frequency warbles as the coil moves, but I can fix that by gluing the core down.

While I was poking around I noticed that I could make the RF modulate with hum when I touched the point where the two R6 resistors, 10k and 22k joined.  So I connected a PSO to this point to modulate the RF with about 1kHz.  But when I tune the receiver across the band, I notice that I hear the tone at several places over about 10 kHz.  This points me to believe that the modulation is more FM than it is AM.

Update Apr 25 – I have been increasing the capacitance of the resonant tank.  I got to the point where the variable capacitors would not tune high enough, so I removed one turn from the core.  Then the frequency was too high so I had to add more capacitance.  According to what I’ve read the Vackar oscillator should have a 6 to 1 ratio of the divider capacitors, so I changed them to 10 and 47 pF which is 4.7 to 1 ratio, better but still not quite 6.

While I was juggling the capacitor values I used one that was 220 pF Y5P.  This worked but the frequency drifted a huge amount, varying tens of kHz in a few minutes.  So I replaced it with a silver mica capacitor, and the drifting came back down to a point that was much more stable.  The temperature coefficient of these capacitors is very important for good stability – the lowest is best.

Along the way I disconnected everything to the right of C5 and R7.  It just wasn’t effective at adjusting the frequency.

The toroid core is still a T37-7 with 10 turns, and it’s about 550 nanohenrys.  I determined the total capacitance using 27 MHz and 550 nH, and it came to 73 picofarads.  That’s better than approx. 44 pF I started out at.  The Xc = Xl = 80 ohms, which is well below the 100 ohms maximum recommended by others.

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2017-04-20 27.145 MHz Transmitter And Receiver Pt 1

I’m trying to make a simple wireless alarm link.  The transmitter will be in an unheated garage, so the only way to easily get it to stay within the -+ 10kHz channel will be to use a XO (crystal oscillator).  

But for the receiver, I figured the best way to go would be to put a bandpass filter right after the antenna to keep the high power CB stations from wiping out my signal.  I figured a Pi type bandpass filter coupled by a low PF capacitor between the two LC sections.  Or perhaps I should use a link coupling so I can move the link away from the resonant winding to get the passband to be narrow.  This is what I need to experiment with.

After that a LO and mixer to downconvert the signal.  The reason I built the tunable Vackar Oscillator is for tweaking the bandpass filter.  I don’t have a decent RF signal gen for this freq.  I have an old tube signal generator, a Realistic 22-040 clunker from the old days when they still called them Megacycles.  And I have a Heathkit Dipmeter, which I find very useful.

I have some old aluminum IF cans from tube radios that I can use to hold the filter and jacks, and connect stuff together with some RG-174 coax.  I need to make the bandpass filter narrow band.  I thought about using some crystals to get a narrow bandwidth.  I had a low profile crystal for 27 MHz, and I checked it with my crystal checker, and it checked intermittently then died.  I think the crystal element was damaged by too much power.  It has to be small to fit in the small package, and it most likely was broken by the power from the crystal checker.

I have looked at several downconverter designs and I liked one because it used a crystal that was half the frequency.  The converter used two 1N4148 diodes in anti-parallel, so it rectified both halves of the sine wave, thus doubling the frequency.  I also got some SA602 chips for downconverting RF.  There are several designs using this chip when I do a search.

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2017-04-20 Buying SMTs From Newark Element14

I think I’m set for life when I need SMT transistors – I bought hundreds of them from Newark Element14.  They are having a discount sale; some of these transistors are discounted to less than 2 cents apiece whether you buy 1 or a thousand.  Some are as low as a penny – $0.011.  The discounted ones have a red circled star next to the price.  So I bought a hundred or so of the most used transistors.  They have many other parts discounted, too, and shipping can be free, depending on the carrier.

Note:  I checked the datasheets.  Some of them were mislabeled, in other words the datasheet was for a SOT-23 package but the transistor is in a different package, even smaller than SOT-23.  So check to make sure you are getting the ones you can use.

What I bought.  I edited out some private info.

58K9429 MMBTH81 Tape and Reel Cut 1 100
$0.027 $
Customer Part Number: Customer PO Line Number: 001
Description: BIPOLAR TRANSISTOR, PNP, -20V SOT-23-3; Transistor Polarity:PNP; Collector Emitter Voltage V(br)ceo:20V; Transition Frequency ft:600MHz; Power Dissipation Pd:225mW; DC Collector Current:50mA; DC Current Gain hFE:60hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 100      Final Expected Ship Date: 04/19/2017     
Line No:2 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
67P3530 KST10MTF Tape and Reel Cut 1 25
$0.021 $
Customer Part Number: Customer PO Line Number: 002
Description: RF TRANSISTOR, NPN 25V 650MHZ TO-92; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:25V; Transition Frequency ft:650MHz; Power Dissipation Pd:350mW; DC Collector Current:-; DC Current Gain hFE:60hFE; No. of Pins:3Pins
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 25      Final Expected Ship Date: 04/19/2017     
Line No:3 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
45J0609 BC858CLT1G Tape and Reel Cut 1 200
$0.011 $
Customer Part Number: Customer PO Line Number: 003
Description: BIPOLAR TRANSISTOR, PNP -30V SOT-23; Transistor Polarity:PNP; Collector Emitter Voltage V(br)ceo:-30V; Transition Frequency ft:100MHz; Power Dissipation Pd:225mW; DC Collector Current:-100mA; DC Current Gain hFE:420hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 200      Final Expected Ship Date: 04/19/2017     
Line No:4 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
88H4790 MMBT4401LT1G Tape and Reel Cut 1 250
$0.013 $
Customer Part Number: Customer PO Line Number: 004
Description: BIPOLAR TRANSISTOR, NPN, 40V; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:40V; Transition Frequency ft:250MHz; Power Dissipation Pd:300mW; DC Collector Current:600mA; DC Current Gain hFE:250hFE; No. of Pins:3Pins
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 250      Final Expected Ship Date: 04/19/2017     
Line No:5 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
10N9479 MMBT2222ATT1G Tape and Reel Cut 1 300
$0.019 $
Customer Part Number: Customer PO Line Number: 005
Description: BIPOLAR TRANSISTOR, NPN, 40V; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:40V; Transition Frequency ft:300MHz; Power Dissipation Pd:150mW; DC Collector Current:600mA; DC Current Gain hFE:100hFE; No. of Pins:3Pins
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 300      Final Expected Ship Date: 04/19/2017     
Line No:6 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
09R9378 BC847CDW1T1G Tape and Reel Cut 1 200
$0.024 $
Customer Part Number: Customer PO Line Number: 006
Description: BIPOLAR TRANSISTOR, NPN, DUAL, 45V, SOT363; Transistor Polarity:Dual NPN; Collector Emitter Voltage V(br)ceo:45V; Transition Frequency ft:100MHz; Power Dissipation Pd:380mW; DC Collector Current:100mA; DC Current Gain hFE:270hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 200      Final Expected Ship Date: 04/19/2017     
Line No:7 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
75R0777 MMBT3904,215 Tape and Reel Cut 1 300
$0.013 $
Customer Part Number: Customer PO Line Number: 007
Description: SWITCHING TRANSISTOR, NPN, 40V, 200MA, 3-SOT-23; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:40V; Transition Frequency ft:300MHz; Power Dissipation Pd:250mW; DC Collector Current:200mA; DC Current Gain hFE:100hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017     Expected Ship Quantity: 300    Final Expected Ship Date: 04/19/2017     
Line No:8 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
67P3546 PN2222ATFR Tape and Reel Cut 1 200
$0.028 $
Customer Part Number: Customer PO Line Number: 008
Description: TRANSISTOR, NPN, 40V, TO-92; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:40V; Transition Frequency ft:300MHz; Power Dissipation Pd:625mW; DC Collector Current:1A; DC Current Gain hFE:35hFE; No. of Pins:3Pins
Shipping Via: UPS


Expected Ship Date: 04/19/2017  Expected Ship Quantity: 200   Final Expected Ship Date: 04/19/2017     
Line No:9 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
98H0755 MMBT5089LT1G Tape and Reel Cut 1 100
$0.026 $
Customer Part Number: Customer PO Line Number: 009
Description: BJT, NPN, 25V,SOT-23; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:25V; Transition Frequency ft:50MHz; Power Dissipation Pd:225mW; DC Collector Current:50mA; DC Current Gain hFE:1200hFE; No. of Pins:3Pins
Shipping Via: UPS


Expected Ship Date: 04/19/2017   Expected Ship Quantity: 100   Final Expected Ship Date: 04/19/2017     
Line No:10 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
75R0736 BC847C,215 Tape and Reel Cut 1 200 $0.016 $
Customer Part Number: Customer PO Line Number: 010
Description: BIPOLAR TRANSISTOR, NPN, 45V, 100MA, 3-SOT-23; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:45V; Transition Frequency ft:100MHz; Power Dissipation Pd:250mW; DC Collector Current:100mA; DC Current Gain hFE:420hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 200      Final Expected Ship Date: 04/19/2017     
Line No:11 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
79R5028 PBSS303ND,115 Tape and Reel Cut 1 25
$0.059 $
Customer Part Number: Customer PO Line Number: 011
Description: BISS TRANSISTOR, NPN, 60V, 3A, 6-SOT-457; Transistor Polarity:NPN; Collector Emitter Voltage V(br)ceo:60V; Transition Frequency ft:140MHz; Power Dissipation Pd:1.1W; DC Collector Current:3A; DC Current Gain hFE:570hFE
Shipping Via: UPS


Expected Ship Date: 04/19/2017      Expected Ship Quantity: 25      Final Expected Ship Date: 04/19/2017     
Line No:12 Stock No: 
Manufacturer Part No: 
UOM:
Quantity: Price: 
Extended Price: 
09R9448 MMBT2907ALT3G Tape and Reel Cut 1 200
$0.013 $
Customer Part Number: Customer PO Line Number: 012
Description: BIPOLAR TRANSISTOR, PNP, -60V, SOT-23; Transistor Polarity:PNP; Collector Emitter Voltage V(br)ceo:60V; Transition Frequency ft:200MHz; Power Dissipation Pd:225mW; DC Collector Current:-600mA; DC Current Gain hFE:100hFE
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2017-04-17 Lost Blogs

I had problems with the blog and I had to reinstall the app.  All of the local copies got deleted from Jan 21 up until today.  I haven’t been able to do any blogging or moderate comments since Jan 21.

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2017-01-22 FM Regenerative Receiver

I’ve been building another FM regenerative receiver. This one is supposed to use a BF244 JFET. But they were declared obsolete years ago, and aren’t available in quantities at a reasonable price. So I ordered some 2N3819 JFETs from Mouser and got them a few days ago. The 2N3819 is also a VHF JFET, so it should do the job.

I used this circuit. This is a superregenerative circuit – it self quenches at a supersonic frequency, as shown by the waveform.
image Here’s the website where I obtained this.

The antenna is shown as a collapsible type from a portable radio, but I used a short piece of wire, abut 10 inches or 25 cm long. For the 1.2 pF antenna coupling capacitor I used a 0.2 to 5 pF miniature variable capacitor set to a low value. I will be using these a lot — I bought a bag of 500 of them for a few dollars.

I made L1 from 18 AWG wire, 5 turns wound on a Bic pen, about .36 in. diameter. It measured about 190 nanohenrys, but it’s less than that after I stretched it to lower its inductance. For C1 I used a 15 pF ceramic in parallel with one of the 0.2 to 5 pF mini variable capacitors (see changes below).

Added Audio Stage
I added a stage of audio amplification after the regen, in place of the ‘auriculare’ headphones that the schematic shows. It’s a SS9014D high gain audio transistor with the emitter grounded. The collector load resistor is 4.7k, and the base bias resistor between collector and base is 1 Megohm. It should be higher, probably 2.2 M. The DC blocking capacitors at both base and collector are 0.56 uF MLCCs. It’s loud enough to drive a 120 ohm earphone from a telephone handset.

Performance
After building a few of these, this is the first one that is sort of acting as a receiver. I got a little hiss and noise when I swept my dip meter’s frequency dial past this regenerative receivers frequency. I’m happy to find that it’s tuned to a frequency on the low end of the FM band. But I still haven’t heard any FM stations. It’s like these regen receivers are very insensitive. Some schematics I’ve seen have an RF preamplifier before the regen stage, but the impression that I got was this preamplifier was more for preventing the RF generated by the regen stage from going out from the antenna. I can hear this regen noise when I listen with a pocket FM radio, so it definitely radiates RF.

When I finished building it, I used a 10 inch wire for the antenna. I set the variable antenna coupling cap to a low value, probably about 1 pF. With the 10 inch wire, I get a 120 Hz buzz, which stops when I turn off my soldering iron. When I add a foot long alligator cliplead to the wire, the receiver goes silent, like it stopped oscillating. I need to find out why it’s sensitive to the antenna length.

It looks like I’m going to have to do some tweaking to get this receiver to work better.

I varied the supply voltage from 9 down to 4.5 volts, adjusting the tuning as I lowered it. I think it is a bit better when the supply is 6 volts DC. After adjusting the tuning, I could hear an FM station weakly mixed with the hum, noise and sputtering. This “first hear” is a defining moment, but whenever I move my hands, arms or body, the signal starts splattering loudly, so right now I have a proximity detector more than a radio receiver.

Next Day
Jan 24 morning I found that no matter what the supply voltage was, it still had problems with hum, noise, and dropping out. It’s supposed to quench at several tens of kHz, but I didn’t see that with a DMM or ‘scope. The original schematic had a pair of piezo earphones connected to the output. These are very high impedance and a minimal load. This audio preamplifier may be too low an impedance load. So I inserted a 10k resistor in series with the coupling capacitor C4. See here for an example (uses 22k res.)

I thought that part of the problem might be the L2 choke was not high enough value, only about half a microhenry. I removed it and wound more turns on a short length of soda straw. It measured 1.5 uH, three times the original. I put that in and it seemed to be less sensitive to my movements, and I heard a radio station a bit clearer. But as I tuned the 5 pF tuning capacitor, I found that the frequency was below the FM band, as low as 85 MHz. I removed the L1 coil and wound on a smaller diameter shaft of a drill bit about 0.3 inches diameter. I cut off some of the extra wire and reinstalled it in the circuit. Now the tuning is covering much more of the bottom on the FM band.

I tuned a pocket FM radio to a station at the bottom of the band, 88.5 MHz. I can now tune the regen receiver to that same station. But the regen receiver radiates RFI and causes the pocket radio’s audio to become quieter. The regen receiver’s audio is very weak, too. These two problems may mean that the regen receiver should have a RF preamplifier before the regen stage to prevent the interference from escaping and amplify the signal.

An My earlier regenerative receiver project.

I’ll continue this on the next blog tomorrow Jan 24.

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2017-01-18 Simulating An Analog Meter

A detector or discriminator in an electronic device might need to be adjusted to a very few points on a scale of a hundred or more points.  A digital indicator typically indicates only a fraction of that, possibly 16 points.  This is why the analog meter is still useful: it can indicate a small change on a much larger scale.  I would like to make a device that is much cheaper than the analog meter, yet can indicate a small change in a much larger voltage or current.

I had a thought.  With today’s 3d technology and super strong magnets, it might be easy to do this.  Back in ‘the old days’ they used to make super sensitive meters by replacing the meter needle with a mirror.  Then a light went through a narrow slit and reflected off the mirror onto a somewhat distant surface, and movement that couldn’t be seen on the needle was easily seen on the light line on the surface. 

Another idea.  If the circuit is designed like a Wheatstone bridge, where the meter is showing only a small fraction of the total reading, then a 16 bar digital meter might be usable.  In other words, if the 16 bar meter shows 1/8 of the full scale reading, then it would be like looking at a part of the scale of a 128 bar meter.

But the problem is the measured quantity might suddenly change to a much different value which would no longer be in the small scale.  Therefore the small scale should be able to readjust itself to cover the new point.  There could be a ‘zero’ button to let the user recenter the 16 bar display on the changed value.

Some form of automatic feedback correction should be able to do this readjustment.  The adjustment could be done manually, but it seems that it can easily be done automatically.  The adjustment and settling times should be quick enough to not interfere with the user’s measurements.

One thing that occurred to me is that a good sensor of small level changes might be the ear. I could try connecting the voltage output to a voltage to frequency converter. The slight voltage changes would change the frequency and the ear could discriminate very small changes in the tone. A simple oscillator such as an astable multivibrator could be used.

Another idea. I’d like to investigate how sensitive the visual system is to flash rate. I found that the reason they use a starting pistol for races is that the mind processes sounds much faster than visual signals. That’s because there is much less brain used in processing sound, so there are less delays. But can a person do a visual comparison of the flash rates of a flashing LED? Or perhaps compare the flash rate of two LEDs?

Another possible solution, which seems to be very similar to the magic eye displays. The single LED would be mounted on a rotating disk. It could be mounted so the LED is seen on the edge of the disk. The disk spins and the led moves from Left to right. As the LED appears on the left, the circuit turns it on, and it remains on for the time determined by the voltage. Zero volts would have a very short time and the LED would turn off at the left side. Maximum voltage would turn off on the far right and other voltages would be in between. So the user would see something similar to a bar graph, with a varying length of light depending on the voltage. I realize that the mechanism might be complicated, but the rotating LED might be replaced with a stationary LED and a rotating mirror. Or the LED might be mounted on a lever that is driven by electromagnetics similar to a speaker’s voice coil. The whole idea here is to simplify and miniaturize the display.

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2017-01-15 Tuned Circuit Tester

I’m building a tuned circuit tester, which is a Franklin Oscillator without the parallel tuned circuit.  I based it on the schematic I found at this URL.  The first amp is a JFET in common drain or source follower configuration, which then drives the 2nd stage, a PNP transistor in common base configuration, so there is no signal inversion and the common base has voltage gain.  Some signal is used to drive the third emitter follower stage, and its output is used to drive a frequency counter. 

The following photo shows the changes I made.  Most are increased capacitor values, such as coupling and bypass, which don’t affect the circuit very much.

I used an MPF102 JFET for the first amp, and a 2SC3355 for the emitter follower.  Instead of a 1N4148 I used a 1SS98 UHF diode for the JFET’s gate.

After completing the basic circuit, I double checked it and found a bias resistor that needed to be soldered.  It pays to double-check!  I then soldered a coil of 6 turns of 20AWG bare copper wire in parallel with a 24 pF trimmer capacitor across the ‘unknown tuned circuit’ pins.  I applied 9 volts and checked with a dip meter and FM radio, and tuned the circuit to the bottom of the FM band.  It oscillates just fine – I got a strong signal.  I tried adjusting the trimpot, but it doesn’t do a lot, just changes the power.

I changed the tuned circuit to a 2.6 uH toroid coil in parallel with a 100 pF disk capacitor.  This should resonate at 9.7 MHz.  I put a short wire through the toroid and made a  single turn loop by soldering the ends together.  This allows me to put the coil of the dip meter inside of the loop to pickup the signal.  I powered it up, but I got nothing.  I thought it may be because the loop is acting as a shorted turn and stops the oscillator.

I removed the single turn loop from the toroid. I thought, what would be a good way to indicate that the circuit is oscillating? I decided that a half wave doubler and LED should be fine. I added two 1N914 diodes, a filter capacitor and red LED to the output. I checked the LED to make sure it would light up. When I powered it up, the circuit did not seem to be oscillating and the LED did not light. I decided to go back to the 6 turns and trimmer capacitor. I powered it up and adjusted the trimmer so I could hear the dead carrier in my radio. I got a strong carrier, but the LED still didn’t light up. My thoughts are the circuit is oscillating but the amplitude is not high enough to make the half wave doubler rectify and light the LED. Maybe I need an amplifier after the diodes? …

Things are turned around. Circuits typically oscillate easier at low frequencies, and harder at high frequencies, but not in this case. Weird.

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2000-01-01 watsonspics.blogspot.com

This is a link to my pictures I posted.

watsonspics

Pictures, for the most part, of schematics I’ve drawn, using ExpressPCB, free software. There is little or no text. A few may have been drawn by others. Those from Quantsuff (quantsuff.com) have a QS in the name. I’ve received permission from QS to use them. Solid State Systems is a name I used many years ago.

I can no longer access these pictures.  The evil blogger.com deleted my watsonseblog.blogspot.com account.

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