Our 12MHz IF Amp plot out to 100MHz
In yesterday's posting I shared a 9MHz IF Amp using the J310's configured as a Dual Gate MOSFET. Soon you will be able to purchase a J310 DGM module from K7TFC. Now that you have a Module what do you do with it? A good answer is to build an amp circuit that could be used in an IF Module.
In true N6QW fashion I included something in yesterday's posting that may have been missed by the Blog readers. Yes, Capacitive matching! So let us examine that a bit.
For that examination I refer you to the diagram which is for a 12 MHz IF amplifier. For those ever welded to Bitx circuitry and never try anything different-- this will work with a crystal filter from the Bitx 40.
A good deal of time has been spent in trying to come up with the turns ratios to match things like a TIA amp (50 Ohms) to a 500 Ohm filter. That is a 10:1 match so we have the 19 Turn and 6 Turn transformer (19^2 = 361 and 662 = 36 thus 361/36 = 10). But what if you wanted to do it another way as you hate winding all those turns on a FT-37-43.
Enter the capacitance match. Now a couple of things to observe from the git go.
1. Our Inductor L1 from an AC signal standpoint is grounded through C7. The impedance of C7 at 12MHz = 0.000132629 ohms (Z = 1/(2*Pi*F*C)). I would call that ground!
2. The combo of C1 and C2 is like resistors in parallel so the effective capacitance is the product over the sum = 23.7 picofarad. The resonant frequency of the tank circuit with the 6.63uH inductor is nearly 12MHz. With C1 being a trimmer -- you can put it dead nuts on 12 MHz.
3. Now a note: with a 10:1 valuation of the caps (265 and 26pF) you pretty much have a resulting value of the smallest cap. The effective tank capacitance is near 26pF. BUT the order of the capacitor installation is critical. Going back to the top end of the inductor being nearly at ground as shown, the voltage divider places a larger voltage across C2 to Ground. If you reverse the order, then you would see a smaller output relative to ground. It is all about the math. If you have taken the plunge with LT Spice you would see this in real time.
4. I included a small Pi Type Pad (2dB) on the output to provide a constant load to the amp and improve any possible distortion in the filter (A tip from W7ZOI).
5. The astute Blog reader will ask the question ... The Pi Type Pad is 50 Ohms in and out so if you are matching to a Crystal filter of 500 Ohms you will need that 10:1. But now you ask about the output impedance of the tank circuit where you are taking the output (to ground) of a 265PF Capacitor. So, if you do the calculation, you will see that at 12 MHz the impedance of 265pF capacitor is 50 Ohms. Z = (.159/F*C) [A trick is that 1/2*Pi = .159.] Thus, we are matched on the input side to the P Type Pad.
I see the look of dismay as you sit there with an IF other than a warmed over Bitx filter. You can easily translate this output tank to other IF frequencies by a bit of reverse engineering.
Start with C2 to find a value of capacitance that equals 50 Ohms at the IF frequency. Say you are using an 8MHz filter and then C2 = 379.3pF. With our 10:1 then make C1 = 37.9PF and that combo comes to 34.5 picofarad. So, the task is now to find an inductor that resonates with 34.5pF at 8MHz. Doing the math suggests a value of 11.449 microhenry.
Now what I haven't covered is the input matching to the Gate #1 on J2. Another subject for another day.
73's
Pete N6QW