Ham Radio didn't start with Appliance Boxes: The Direct Conversion Receiver. 12-18-2022 Update

No Apologies! This Blog is for serious homebrewers and not for those who lack the knack!

Here is a prime example of the four circuits mentioned in an earlier post plus the bonus feature of the makings of a DSB transceiver, or a Superhet transceiver or 1/2 of an SDR Module. Yes, these circuit modules can do all that. 

The 1st configuration as a DCR has an RF Amp stage, a Band Pass Filter, a Mixer stage, an LO and the Audio Amp stage. Just for a moment ignore the Microphone Amp.

Two key points about the circuit and those are:

1. The circuit can be bilateral depending on the devices used.
2. The LO is Digital. You should build at least one conventional Analog LO and one Glue Stick PTO. Build it, understand how it works and then move on! The digital technology has so many pluses and few minuses. The Analog stuff is touchy, prone to drift and places demands on the builder to try to accurately display frequencies. It is strange to see die-hard Analog types who resort to digital counters to know where they are at. A few lines of Code and a simple 2X16 LCD and you are there with the Arduino/Si5351. I suspect the Die-hard types are fearful of building something digital or fearful of doing something different.

The real thrust of the Digital LO is that it can produce the USB/LSB BFO carrier frequencies while simultaneously producing the LO input for frequency conversion to the IF Frequency. Additionally, when run at 4X the LO signal can be processed to produce quadrature LO signals at the operating frequency. That is hard to do with a Glue Stick PTO. A single 74AC74 can perform the divide by 4 and produce the Quadrature outputs. The long-term goal is a progressive build going from a DCR, to DSB to Superhet to SDR.

That is the why of the Microphone Amp. Bypass the Audio stage and connect the Microphone amp to the Mixer and measure the DSB signal at the output of the BPF. If we replace the single 2N3906 with the Plessey Circuit that has both the 2N3904 and the 2N3906 it will be a bilateral stage. A simple Bias change shifts it from Receive to Transmit.

Before you get too excited the DSB will work well with other stations running SSB -- But a DSB-to-DSB contact   presents some issues.

Again, move on from Analog to Digital and along the way check the block for Analog VFO's and be prepared for fun.

Some reading this post will jump to the MC1496 or the Tayloe Detector as the Mixer stage in the DCR. Several years ago, I received a childish email about the fact I didn't use a Tayloe Detector. Along the way all designs go through an evaluation process of engineering economics. You can certainly build a DCR using these aforementioned devices and they will work well. Here is the BUT, and that involves added parts (with attendant complexity) and they are not bilateral. "A Good Enough For Government Work DCR" can be built with a Homebrew Double Balanced Mixer like this below. (This stage is both a Product Detector and Balanced Modulator using the W1REX MePads.) 

Some special notes as the point marked "In" is both In and Out. The AF Port has an Audio Pi Network Filter, so the signal output on Receive only contains audio frequencies, and this also works on Transmit to limit the audio into the system so that the Crystal Filter down stream is not getting "banged" with audio frequencies outside of its bandpass. Even in electronics there we have that wonderful term "banged"!

A final point is the 100 Ohm Trimmer Cap that functions as a Carrier Balance pot. (Yes, this is Kosher, and you can even find it in EMRFD which I said I would not use. But this is an exception as that input to EMRFD came from this writer.) Noteworthy you can also add circuitry to that balance pot to purposefully unbalance the carrier as this would provide a Tune Up functionality. Read the Tea Leaves: it is homebrew, has a Carrier Balance pot, an Audio Filter, and as a Bonus is bilateral! Count it, 10 parts!

For those who can read schematics see above and for those that can't see the photo. Of note the Schematic show 12 parts as I add the 39 Ohm resistor and a contact closure (small relay, remotely powered on) to provide a "TUNE" function. Start 1st by having the 39 Ohm lifted from ground and then balance the carrier out. Then test the circuit with the 39 Ohm resistor grounded. So, the test question is where or how would you detect the carrier is balanced out. You need a scope! 

This is a Tribal Knowledge Tip (TKT), often how you connect the diodes gets hosed up. So, what you see in the schematic on the left is an equivalent in the pictorial drawing on the right. Take some time to convince yourself these two circuits indeed are equivalent and then say: Damn -- that works!

The reference to the green and red wires on the two transformers is another TKT in that in winding bifilar windings I use two different color wires (red and green). In the case of the third winding, I purposefully make the leads extra-long. First the bifilar is wound (red and green) with the next step being that the extra-long lead is twisted with the bifilar leads. By habit I make the extra-long lead winding from red enamel wire. Look at the Photo and you will see the green wires connected to Points D & C and the red wires connected to points A & B

Thus, we will know which wire is which! Decoded the LO and the RF In/Out is the long lead AND observe the phasing. 

Update 12-17-2022 RF Amplifier Stage

Most often NPN transistors seem to find their way into ham circuits. K8IQY used a passel of 2N2222's to make his famous rig and of course Farhan used the 2N3904 throughout the stages of his famous Bitx20. N6QW in the P3ST used 2N2222A's in five of the seven transistors in his minimalist SSB Transceiver.  These are all NPN, and their use is a reflection of the plain ease in grounding of power sources but not for the lack of capabilities of PNP devices. 

The PNP Complement to the 2N3904 is the 2N3906. Just to demonstrate how the 2N3906 can be used in a RF Amplifier circuit its use was tried in a DCR project.

Engage the "Confusion Siren" for those who have followed along so far, who would say the labeling would seem to indicate that the Band Pass Filter is connected to the Antenna and then the RF Amplifier between the BPF and Mixer stage. For just the DCR I would recommend that configuration. 

Think about it, as placing the BPF first then you are passing on to the RF Amplifier just the signals that pass through the BPF. If the order was reversed, then you are amplifying the whole spectrum and trying to narrow out a small chunk of the band of interest-- you are also amplifying the ride along noise. The positions of these two might revert to a BPF between the RF Amp and the Mixer in a transceiver configuration especially at the front end where you are eliminating a mixing product. 

Engineering choices are mostly dictated by the specific application.

Update 12-18-2022

The Audio Amp Stage uses packaged amplifiers. I have built discrete component amplifiers and see the use of devices like the LM380 and LM386 as commodities. Install them and use them. But I do prefer the LM380N-8 over the LM386. 

Now some in the homebrew community abhor the packaged amplifiers and that is fine as the tent is large. That said I would rather spend my time improving signal handling and adding capabilities with the Arduino versus fiddling with complementary 2N3904/2N3906 transistors.

For those who champion the discrete component complementary pair you might take a peek at the LM386 schematic, where you might discover some common ground with the currently popular discrete component schematics -- so why not use a drop in device?

TKT -- if you are in the camp of the discrete component builders that use the 2N3906/2N3904 complementary pair, you can swap out those devices with the TIP32C and TIP31C and make no other changes. This circuit will now produce over a watt of output.