Tuesday, December 15, 2015

SimpleTransReceiver ~ Part 21

The J310 Companion Transmitter ~ The Journey Continues!

Addendum: 12/16/2015 Simple Transmitter Board Layout
Addendum: 12/18/2015 NEWS FLASH 9A3XZ, has a working Simpleceiver! Bravo Mikele.

If it hasn't hit you right between the eyes, by now, then turn off your soldering iron. So far we have built what I consider a pretty decent receiver for not a lot of money. There is ongoing development work on that part of the project inclusive of the incorporation for the W7ZOI Hycas AGC. That addition is not quite ready for prime time; but we are close.
The building of that receiver involved what I call a common template, the J310's configured as a Dual Gate MOSFET. That has worked very well and you only need to listen to the you tube videos and you can adjudge for yourself. We also introduced the use of LT Spice as a simulation tool. Most of the time is spent at the computer and when we are ready to solder up a circuit it is pretty much the final configuration so there is minimum time wasted rebuilding hardware.
Now we are ready to proceed with the building of the transmitter portion of the transceiver. For those who tuned in late, the project involves building a separate receiver (done) and a separate transmitter with a sharing of the LO and BFO that commonly links the two to form a transceiver. Some would argue hey I can build a BITX and be done. Well for those so inclined have at it. But if you want the ability to fine tune the receiver and transmitter for optimum performance, including features not normally found in the BITX like AGC and maybe ultimately a color display then this project is for you.

There is much flexibility in my design --yes I can really say it is my design wherein you can employ different filters (both homebrew and commercial) as well as different IF frequencies. We have provide information for IF's such as 12.096, 9.0 and 8.0 MHz. The use of the LT Spice makes for short work in being able to do that. Below is the block diagram for the low level circuits for the companion transmitter. You will note that the very same circuits employed in the receiver are now used in the transmitter. So if you have built the receiver all you will have to do is to duplicate several of the circuit blocks and you are there. The one new block is the microphone amplifier which will be covered shortly.

This is our starting point and be advised there may be some alteration but it is a the first step. One possible change is to take the 2nd SBL-1 which is used between the IF amplifier and the Band Pass Filter and put another DGM stage in there wherein the J310's are configured as a mixer stage. That very likely will be the case but for now we have a placeholder with the 2nd SBL-1

 The one block on the above diagram we have not seen before is the microphone amplifier and we will next provide that schematic --any similarity to the Product Detector template is not accidental!

WE have added the filter on the output so we have a very nice audio spectrum being presented to the Balanced Modulator. R3 is a 10K Trim Pot and the center wiper has a 10 NF capacitor (called C3) connected to it and the other end of the capacitor is connected to the Pins 3 & 4 on the SBL-1.

The audio generator used for the LT Spice simulation presumed a 600 Ohm series impedance and thus the amp is predicated being used with a 600 Ohm low impedance microphone which seems to be the norm today. While not included in the simulation this would be a great opportunity to test one of those inexpensive electret microphone cartridges. A bit of bias for the microphone and it would simply connect into the input.

Below is the output plot for the microphone amplifier as above. See the note about where to connect the 10 NF capacitor referred to as C3. This plot shows a very nice output curve. Thank You Pete and Thank You LT Spice.

We decided to publish this schematic since some of the experimenter's wanted to get a head start on the build of the transmitter.

Addendum: For those who are itching to build the transmitter below is the board layout that will enable you to get most of the modules on a single board. The squares are 2/10 of an inch so you can count the number of squares and figure the size ( about 4 X 6 inches). I have decided to build a specific BFO carrier oscillator for the transmitter stage. There is a good reason for this -- loading on the BFO when you shift between Tx and Rx most  likely would result in a frequency change. I have  stock of about 30 pieces of the 12.096 MHz crystals . I ran all 30 through the Simpleceiver and found 4 that are "dead on" with the crystal in the receiver --so we now have BFO crystals for the transmitter. Look at the arrows as that will be the signal flow. Virtually all interfaces are at 50 OHms so there will be many matching transformer --- get a large stock of FT-37-43's (Toroid King is a good  place to find these. The BFO layout will be the same as is the IF amp block. The microphone amp block will be similar to the product detector.

I will cover the control systems/switching/TR early in 2016.

Pete N6QW

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