Transition to the Superhet Configuration
Addendum: Schematic for the W7ZOI AGC Circuit.
Addendum #2: The Transmitter Block Diagram
Addendum #3: 12/15/2015 Simpleceiver Software located on http://www.n6qw.com Look for the Simpleceiver Software Link
In Part 19 we recapped the elements of the Direct Conversion Receiver and now in Part 20 we will move to the Superhet configuration. For the last week or so I have been listening to the Simpleceiver Superhet and there is a great deal of satisfaction to finally see all of the blocks and endless simulations come together. Cost wise there is about $60 in parts with the most expensive being the AD9850 followed by the LCD. I purchase parts in bulk and so my cost estimate may be skewed versus buying resistors one at a time. In summary --the sound and frequency stability outshines the $60 investment!
Let us start by looking at the Superhet block diagram as shown below. The darkened blocks are from the Direct Conversion Receiver. So to make the conversion only five additional elements need to be added plus you will need to reprogram the Arduino/AD9850 with the IF filter offset.
Four of those blocks (SBL-1, 1st and 2nd IF amp and the Filter) were built on a single board which is shown below and which I call the "Mainboard". The "island squares" are 2/10 of an inch so you can count the squares and get a feel of the size of the board. I used my CNC Mill to cut the squares but this could be made using the W1REX MePads and again count the squares and you are there. If you feel comfortable using nasty chemicals then the board could be built the old fashion way -- chemical etching. Or just do as I read in a recent GQRP SPRAT article, a ham used a wood chisel to cut his squares --Crude perhaps; but very Effective!
The final element to built following the Mainboard, will be the BFO. Being slightly conservative I used a single JFET versus the dual DGM approach. That is seen below and at the time I built the product detector I left space on the board for the BFO. Again count the squares (2/10 inch) and you get a feel for the size. The MePads will work nicely as will etching or use of the chisel. Look carefully at the Product Detector layout as this same approach was used on the mainboard! This keeps the leads short and direct! In place of the RF choke assembly in the product detector is the tuned network for the amplifiers and can utilize the same overall space requirements. Oh a bit of tribal knowledge here. Note that I made a crystal socket from a three hole SIP strip. This was done to enable finding the "just right" BFO crystal. I simply cycled about a half dozen crystals in the socket and by adjusting the small trimmer found the right one. Later you could remove the socket and permanently solder the crystal to the board
I did receive an email about "why did you use coax from the Product Detector to the input of the audio amp --grey colored cable"? That is not coax but small diameter microphone cable or at least that what the reel says.
Next are the schematics for the amplifier stages and BFO which in this design are at 12.096 MHz. I have also included a design for a 9.0 MHz version in the event that a builder does not want to undertake the somewhat arduous task of homebrewing a 12.096 MHz filter and choses to use either the INRAD Model #351 or the GQRP 9.0 MHz unit (and for that matter 9.0 MHz filters out of any radio).
All that is required is the proper matching transformer to go from 50 Ohms to the In/Out of the filter. For the INRAD it is 200 Ohms so a 4 turn to 8 Turn transformer on an FT-37-43 cores gives the 4:1 match. In the case of the GQRP it is 500 Ohms so you would need a 6 to 19 Turn match. If at this point you do not understand matching transformers and turns ratio squared -- you probably should not build this project as all interfaces are at 50 Ohms and that is achieved by liberal use of ferrite matching transformers!
The Schematic below is for the 12.096 MHz amplifier and includes a 2 dB resistive pad used only in the 1st IF amplifier stage and is omitted in the second stage. This pad is there to provide a constant load before going into the Crystal Filter. There is a notation about the matching transformer on the input side (Gate #1). The Four Pole Crystal Filter is 150 Ohms so a 3:1 match is needed. There are many combinations that put you quite close but a 4 turn to 7 Turn is probably the simplest and that is wound on the FT-37-43 Ferrite Core. (BTW a 8 to 14 turn transformer will do the same -- do you see a pattern here?) By design we repeat (but reversed) this transformer on the output side of the crystal filter so we have a 150 Ohm to 50 Ohm match.
On the output of the second IF amp there is a match of 50 Ohms to 2200 Ohms and that neatly is the input side of the Product Detector so no other matching transformer is needed --- 50 Ohms to 50 Ohms.
Note the 10K trimmer pot arranged as a variable resistor in series with a 3.3K resistor provides a convenient means of adjusting the amplifier gain.
For a 9.0 MHz IF amplifier only three parts are changed. L1 is now 6.985 uH and consists of 35 Turns of #24 on a T-68-2 core and C1,C2 are now 68 PF. The same output matching transformer would be used and the resistive pad is only used on the 1st IF amp stage. Below is a plot of the 9.0 MHz IF amplifier simulation. Kind of cool!
Next is the Crystal Filter and as was mentioned in a earlier posting there is a rigorous process to building a homebrew filter that starts with finding four crystals that have no more than a 50 Hz difference among any of the four. Nick Kennedy, WA5BDU has documented the information on the "how to do this process." I did not go through that total process but did select the 4 crystals that were closely matched. I guess if one has weak knees about building a filter and despite the dye in the wool hardcore build everything gang, there is absolutely no shame in having a commercial unit in your radio. The data has been presented to do this.
Finally we have the BFO stage and that is just something I found once and have repeatedly used it in projects.
The sketch for the Arduino/AD9850 will be uploaded to my website on December 15, 2015 and can be found at http://www.n6qw.com The wiring for the Arduino and AD9850 is shown below.
That completes the basic receiver portion of the project although in work is an AGC circuit but that may have to wait until early 2016. After the Holidays I will start with a discussion of the Transmitter portion of the project.
Addendum ~ Below is the baseline W7ZOI AGC circuit that will be modified for use with the Simpleceiver. With our studies of how Gate #2 responded with changing bias voltage our modifications will include setting the "no AGC" to about 5 Volts and "full AGC" to less than 1 volt. Thus this is a Reverse AGC --stronger signal less positive voltage on Gate #2. This is an "IF Derived" AGC" and the sample point is a couple of turns on wire wound over the inductor of the 2nd IF amp stage and connected to points "X and Y". [Wind in the same direction.]
The output of the AGC can be applied to the RF Amp Stage as well as the 1st IF amp. Once I have it built and adjusted, then I can further evaluate the optimum configuration. I will also add in the S Meter using an Analog meter, although if a TFT color display were used it would be possible to include a bar type S Meter right on the display (been there and done that). The transistors are 2N3904 and 2N3906 and the didoes 1N4152. Another modification would be to make R6 a 500 Ohm trimpot connected as a variable resistor.
The 50 Ohm Output R3/R4 is ignored since we are merely sampling the signal. In the original hycas this output was sent to the product detector for which we have used a different method to do that.
Addendum #2: The Transmitter Block Diagram --look closely as most of it is already built!!!! Now you know the why of a common template!
Note: For those wanting to use an 8 MHz homebrew Crystal Filter IF frequency the values for the IF amplifiers are as follows; L1 = 34 Turns on a T-68-2 using #22 Wire andC1, C2 = 100 PF. Note you will have to determine the Z in/out of the 8.0 MHz filter and build the matching transformers accordingly -- most likely the match will be in the 150 to 300 Ohm range. A 4 to 8 turn will work for the 200 Ohm and a 4 to 10 turns will work for 50:300 Ohms (1:6 where 16:100 approximately 1to 6).
Thanks for riding along --this has been a wordy journey but hopefully informational, inspirational and just a plain lot of fun. Here is wishing the N6QW blog readers the very best of the Holiday Season and a Healthy and Happy New Year
73's from the Left Coast