Sunday, September 30, 2018

RF Amp breadboarded and amplifying

I put the little amp on a breadboard and shot a signal through. It is 2Mhz. My digital scope doesn't like small signals. The input looks better with a little stronger signal but that is a scope problem. Anyway it shows a clean output and some gain. I used components on the bench and changed a couple of the values.

SA602A output options

Output options from the SA602A datasheet. It could be single ended or double ended. The signal level could be increased with an amplifier. If you want a DC you could use a simple diode rectifier circuit.

For example this field strength meter.

DBM data from Motorola

Thursday, September 27, 2018

made 2 mods and distortion is gone

I set the generator to 600 ohms and the filter resistor to 100k. this is with 2% mod
I made these two changes. Tapping the input resistor cleared the RF amp signal and raising the filter resistor cleaned the AF signal.
This is the signal with the 47k resistor and tapped input.
One last look at a 75% modulated signal
 Looks good I may have to build this one.

source of distortion

An over driven class A amp will clip and distort the signal as shown. The bias point needs to be centered or one peak may distort even at low signal levels. The
curve is not linear so some distortion is to be expected. In the case of the previous circuit it was distortion free at low signal levels and began distorting at high levels. Since the signal is AM the low percentage modulation is at the peaks and are the first thing lost. If the clipping removed 5% of the signal a 1% modulated signal would be lost.
An attenuator on the input might be a good idea. It could be adjusted for best operation. I borrowed this one from David White's beginner's radio.

It's a fairly simple circuit. You bias the diode on and it shunts the signal to ground. Some might say the diode resistance changes with current flow. 

Wednesday, September 26, 2018

Spice sim of an amp and detector

The circuit was designed for 300mv pk-pk input. Here is the result.
I divided the circuit into modulator, Amp, and detector. In this shot I have the modulator set to 300 mv pk-pk and am looking at the RF amp output. To see what's happening I will narrow the time line view.
The amp is doing to good a job and is clipping the signal. So I'll reduce the input.
I cut the signal to 30 mv pk-pk and now have better demodulated signal. The RF amp output is starting to clip but is still producing a decent output. What is the cause of the clipping?
One last shot with 30mv pk-pk showing the RF amp output and the audio output.
Looks like an envelope detector output should. This is with 10% of the designed input and I changed the AF filter resistor from 640 to 10k. I'm using a "high" percent modulation for the sim. What would happen if the modulation was only 1%? So the question is when does the signal get strong enough to over drive the amp and how to prevent it?

I can across this last night and wanted to store it before I lost it. I will have to look up the patent for the rest of the story.
Note the RF decoupling in the collector circuit. Keeping RF out of the power supply can be very important. I must do a new sim with this new found info!

300mv pk-pk input-Clean RF out-Good detected signal with low modulation.
I added a 200 Ohm resistor and 100n cap to decouple the power supply and the signal cleaned up nicely with the 10% modulation. It will probably go crazy at higher modulation levels. An RF choke would be better but that will be a later post.

Sunday, September 23, 2018

RF Amp references

Two Stage Amp
Cascode Amp

I think the next project should be CASCODE amp. Amps in cascade have separate DC paths. Amps in cascode share the DC path.
I'm focusing on the JFET version to put some of those 2SK2539s to use.
It would be fun to put a MOSFET version together too using the  2SK669.  (The K669 is used in FM receiver front ends so good for VHF.)

The AC circuit provides isolation between the input and output. The Miller effect is reduced.
This chart shows some capacitance comparisons.
This chart shows some gain bandwidth. The hybrid or BI-JFET version would need one.
An example from the book.
The book has labs too.
I'm thinking a DBM maybe a good circuit for the 2SK2539s. How much trouble to make the JFET version? I made this circuit with 2N2222s and it was a good performer.
If this has wet your appetite click the link at the top of the page and start studying the design.

Saturday, September 22, 2018

Detector design

detector design

It is a large file for so few pages but well worth the download.

No Virginia Square Law detection is not all there is.

Actually the author does an excellent job of explaining the the different types and include Z matching info.

The book I linked 2 post earlier has some good data on Z matching too.

Friday, September 21, 2018

RF Components and layout

If you downloaded the book from my previous post you may have seen this.
Mr Carr says the joints need to overlap by 1/4" to keep the RF in. The power is being fed by way of feed through capacitors like these.
He also gives some circuits which are more stable. For high frequencies the cascode is a good choice.

The BF2040 is such a device. The maker declares them to be "unconditionally stable".  At 28 cents each and free shipping it's a bargain.

The 2SK2539 Jfet at 3000 for $15 is worth a look also.

I had a small strip of each in an envelope ready to send to NZ and the postmaster said $13.50 was the minimum! Ouch!!

You can buy 48 BF2040s for less than that. Last time I had this happen I simply gave my friend the info and he got a reel of the Jfets for less than I could send them.

Tuesday, September 18, 2018

RF Circuit Design secrets

RF Design Secrets

All the secrets about stray reactances and Z matching that you need to know to build a stable radio without hand capacitance and such.

Saturday, September 15, 2018

remote base - early effect - point contact - MAT - diffused junction - NOT THE SAME!!!!!

Some food for thought with the different types of transistors. This book gives a test to ID transistors on pages 14 -15. The test was developed to determine between Point Contact Transistors (PCT) or Alloy Transistors (AT).
Gernsback Transistors
The PCT being a 3 junction device with a PN hook in its collector  It will have a negative resistance when the collector is "reversed biased".  Actually the collector PN hook is being forwarded biased. Look at the structure.
As you can see the emitter and collector are both forward biased BUT there is an element 'floating' in the middle of the structure. This is the same as a SCR.
Can you see the structure as the same? If not look closer.
Well They are not exactly like each other. The PCT is p-n-p-n with a p-n hook. The SCR is n-p-n-p with a n-p hook. Ge made a device called a SCS which actually had 4 leads and you could make the same connection. Anyhoo, The SCR can be used as an amp.
In this case it is a remote base transistor. Using the SCR cathode as the base allows the SCR to be an amp. When you run Gernbeck's test you see the junction breakdown as negative resistance.
In a junction transistor the base collector barrier will break threw but will not avalanche. Running Gernsbacks test on it will produce a reading of about 1/2 the reading before biasing the collector.
Here we have some PCT circuits. They depend on the p-n hook and the negative resistance to operate.
Dr Early defined the 'early effect' which is the base for the negistor operation. Let's take a look at that.
Base width modulation produces a negative resistance effect which is the negative resistance effect as found in negistors. A negistor (2N2222 , 2N3904) will pass the Gernsback with higher voltage. The PCT will break over at about 3-4 volts. The negistor will break over at about 8-9 volts.

Roger on TRB nailed the description of the PCT operation and then built the circuit being discussed to test his theory. The oscillator sang! Some one blindsided him with the question, "did you try a regular transistor?". He tried a negistor and the circuit sang! Sadly it did not disprove his explanation but was perceived as doing so.
Some times we need to not compare apples and oranges but fail to see that is what we are doing. One of the TRB members has declared a PCT circuit will work without forward bias. Mr Gernsback says while this is true adding a little forward bias will improve the circuit operation. My test circuit worked without bias. When I added a little bias the output picked up and the test was discarded the same as Rogers.

Monday, September 10, 2018

2N2188 transistor circuits

2N2188 Mfg. Process AM.  (Alloy Mesa) Sometimes called diffused base plateau.
Dalmesa - diffused  alloy mesa?
Some references give possible subs which may be worth researching.

Tuesday, September 4, 2018

the crystal set

If I can do it anyone can.
I do most of my work based on muscle memory. If I can anyone can. All that is left is a detector, antenna, ground and headphone.
This is the hookup.
The little ear phone Fair Radio sells works works very well.

Next step is to build an amp to use a low impedance earbud.