NE-602A examples and a little data

 I like the DBM because I make the transformers and basically feel like it is a home brew. Take a look at the circuit.

Another MOSFET amp

With a supply of 3 - 9 volts it will drive and earbud or headphone.  It could be a good starting point. I think a 2N7002 would work without any circuit changes?
I'm using the K669 because that is what I have on hand in leaded MOSFETs. My 7002's are surface mount, I may try one with them. A match box amp would be good to have around.

* you can connect the earbud / headphone directly to the source. I don't like putting DC thru mine and I have a bin full of the little transformers so I use them. You can wind your own. A 1:1 or 1:2 will work.

NE602 update

This one has 2.1 MHz in and 100 KHz out.

DBM mixer

I'll let the sims do the talking. Here's the schematic. So shall it be a DBM or a JFET mixer?

JFET mixer

First a look at the schematic.

It is straight forward. Feed a signal to the gate another to the source and draw the output from the drain. Seems simple enough. The circuit is a bit to simple but we will see.

The display is the top of L1, L4 and L6. The inputs are 2.0 MHz and 2.1 MHz One of which is modulated with a 1 KHz tone.The green is the 100 KHz output. You can see the inputs are a higher frequency but the 1 KHz tone is lost because the sweep is to fast.

The input transformer is step up. I moved the probe to L2 and the input is a phasing is a little easier to see.As the red and blue go in phase the read peaks.

I changed the sweep to show the 1 KHz modulation better. As you can see the orange signal is constant in amplitude. The green is the modulated signal and the pink (difference signal) is carrying the signal too.
Next step will be to wind 3 cores and gather some test equipment and probes. First I should point out the mods I made to the original circuit. I did not put the resistor in the drain circuit. In the build I may use it. I put the bypass below the tank in the drain circuit. I wanted to bypass the tank with the high frequency signals. I used a different JFET simple because I don't have any MPF102.
Anyhoo, time for a build.

toroid frequency response and number of turns

In the short wave frequency range 1 turn can made a big difference.

The core material can make a big difference too.
The first choice to make is the core material?

The calculator give the frequency range for the material. The circuit under development is operating at 2 Mhz so the 77 material could be a good choice.
The next question is what will be the input Z. The rule of thumb is the primary should be 4 times the input Z. So with a 50 Ohm system we need a 200 Ohm primary. The next question is will we use a diode ring or feed a Jfet mixer?

Toroid core transformers - part 1

This calculator will provide the winding data for our transformers. It has the core data built in for many cores but in case your core is unknown it has a function for that too.

The calculator

The first step is to download and install the calculator. You can do the math if you like but I will use the calculator.

Since we don't always know the core type I will go thru the steps to determine the core parameter first.
(this requires an inductance meter or test procedure to determine the inductance.) The calculator also ID's cores by color code.

run the calculator and click tools.

Select the first option.

The program ask for turns, inductance and dimensions.

I made 4 turns and measured 17uh. The core is 0.5" so I look at T50 for the dimensions.

I transfer the data and the program gives me the AL and ui values.

The FT50-77 give me a fair match. The program displays 3 decimal places which could imply it is very accurate but a single turn will give a large step in inductance with this core. As you can see when I ask about 4 turns it gives 17.6uh. When I ask for a 17uh coil it answers 4 turns. There has to be some round off in the process. Wire spacing will cause some deviation in our results. Inter-winding capacitance will cause some variation.

Try inserting a frequency and you will get the reactance. In my case the coil has a reactance of 220 Ohms at 2 Mhz. This can come in handy as we look at the transformers.