Thursday, February 23, 2017

SSB/CW Transceiver Architecture

What is the best Homebrew Transceiver Architecture?

This question is almost like your XYL (or girl friend) asking you: "Do you think I am getting fat (or fatter as the case may be)? How in the world do you answer that question (in an honest manner) without getting a lot of blow back.
In some cultures a "plump" woman is highly prized while in others, almost looking anorexic enjoys the same level of adulation. But being the best begs the defining of what is a transceiver architecture?
Having built dozens and by that I mean maybe 4 or 5 dozen, describes the number of transceivers built by me over a nearly sixty year period and thus I have used various schemes. Here are a few:
  1. The first is a bit of a hedge where the project is not really a transceiver but a trans-receiver which means you build essentially a separate transmitter and receiver that share but a few common elements such as a VFO, BFO and some TR switching. In the December 1989 and January 1990 issue of QST, Wes Hayward W7ZOI featured a project that was a 20 Meter QRP SSB/CW trans-receiver. This approach allows you the freedom of customizing and optimizing the individual transmitter and receiver blocks without being constrained by having many common elements such as the Bitx40. The W7ZOI hycas IF/AGC is definitely better suited to a standalone receiver and may be a bit more difficult to implement in the Bitx40. I did not say it could not be done as I am sure I would get a flood of comments that I was bashing the Bitx40. I actually replicated the 89/90 project and have two such trans-receivers. What comes to mind on customization is one of the rigs has an 8 Pole KVG SSB Filter for receive and a 5 pole KVG for the transmit side --now that is a nice receiver. The customization could be extended further to having separate and switchable CW and SSB filters in the receiver. Frequently SSB transceivers with a single filter when pressed into service in the CW mode are a bit wide. The down side --lots more parts, lots more circuits, more costly to build and lots more potential for errors or circuits requiring maintenance. The chance for making it a small box is somewhat less by just the sheer number of additional circuits needed.
  2. Next is the transceiver which as the name implies is more unitized in that there are more common circuit elements that are used on both transmit and receive. A common receive and transmit mixer is almost standard as is the use of  say a double balanced mixer that on receive is the product detector and on transmit is the balanced modulator. In between are IF stages and a filter that are used both on transmit and receive and only the signal steering is changed --this is the Bitx40.This now is available as a built unit for the amazing sum of $59 shipped to you--and even includes the Si5351 and LCD display.
  3.  My transceiver approach that I use most frequently is to have a double balanced mixer on the front end which is always fed by the VFO and on the back end, after the bidirectional IF amps and filter, another double balanced mixer that is always fed by the BFO. On receive the signals go from left to right and out to the audio amp. On transmit the microphone amp inputs a signal to the DBM, the signal is steered to the other direction right to left through the IF amp/Filter to the mixer stage and now we have a signal that is sent on to the further transmit stages. A bit of signal steering and powering on either the audio amp or microphone amp and we have a bare bones transceiver. See the 1st figure below which is the shirt pocket transceiver block diagram.
  4. A variant to the approach in 2 above is to have the same modules DBM, IF/Filter, DBM. Only in this case on receive the a band pass filter is connected to the RF port the VFO to the LO Port and the mixed signal passes through to IF port on to the IF amps and filters. The output of the 2nd IF amp is fed to the RF port on the 2nd DBM and the BFO is connected to LO port and the IF is at audio and on to the audio amp. Now on transmit there is a bit of signal switching frequently done with diodes or relays. The rig goes into transmit and the RF port that was formerly connected to the Band Pass Filter is now connected to the microphone amp and the port connected to the LO is now connected to the BFO . This mixing process creates a DSB signal that goes on to the IF Amp /Filter chain. After passing through the filter we have either a LSB or USB signal depending on the BFO frequency which now enters the second DBM via the RF port. The LO port is now connected to VFO and the IF port now connected back to the band pass filter. This essentially means the IF amp is working in one direction --but for me too much steering of the VFO and BFO signals and too great of an opportunity for signal leakage given the routing of the RF carrying signals.
  5. Another variant to the trans-receiver is what I did in my JABOM transceiver (Just A Bunch Of Modules). See figure 2 below. This circuit use three double balanced mixers --here we go more parts. We start again with a double balanced mixer stage that on the front end is fed the LO input so that it is both a receive and transmit mixer. On receive the output of this  stage is fed to a post mixer amplifier with a 6 dB pad and then on to the crystal filter. But on transmit the input to the post mixer amp is connected to the balanced modulator DBM so here the filter is only operating in one direction.. After the signal passes through the crystal filter on receive it  goes on to the hycas IF amp and AGC stage and on to the product detector (3rd DBM) and finally the audio stages.  But on transmit the output from the crystal filter is routed to a 2N2222 amp stage and back to the original Rx Tx Mixer stage. You can think of this like a hybrid trans-receiver and transceiver.
  6. No transceiver topology discussion can be complete without paying homage to another Peter and that is VK3YE and his Knobless Wonder.VK3YE's approach was no LO which essentially means a single channel SSB transceiver and only uses 10 transistors. Basically the homebrew filter is on a frequency in the 40 Meter Band (I think 7.190 KHz) and the BFO is tweaked (slightly higher) to give LSB. There is no need for a transmit mixer as the output from the filter IS 7.190 KHz. Thus you always know what frequency you are on, no undesirable mixing products and minimum part count. I think he has a BD139 in there for a final which gives several watts to the antenna. What a great emergency rig! I did find 7.2 MHz crystals and have purchased a batch --so I may someday have a KW in my shack.
The real answer to my posed question is that it depends on what are the initial expectations. If you want absolute control and a great deal of functionality the trans-receiver may be ideal. If you are looking for bare bones, low cost, carry in your pocket then the Knobless Wonder is the best. Middle ground would be the Bitx40 Bilateral approach.
Pete N6QW
W7ZOI Trans-Receiver

W7ZOI Second Build



Figure 1

Figure 2