Control and Power Circuits
One of the most critical factors in a homebrew transceiver is the process whereby the rig is put into transmit (quickly) and then the process reversed whereby it returns to receive, again in a quick recovery back to the receive condition. In days of old many relays were involved and that in itself caused some issues with the physical time it takes to transition to transmit and the trip back to receive. If it is not a quick process in both directions, then the station at the other end might think you faded away and when going back to receive you may miss a few words from the other station.
Most of my DifX rigs (DifX = Different than a Bitx) have relays and in some cases many relays. But it is how those relays are used that is a factor in a quick R > T > R. I use proven communications type relays for the selection of appropriate networks used in the band pass and low pass filters. These are not used in the typical TR process. So once the band is set these relays are either engaged or not engaged regardless if you are trasnmitting or receiving. There are four such relays used for this purpose in the Big Kahuna. Pairs are used for selecting either the 40M or 20M BPF and a similar pair are used for selecting a 40M LPF or a 20M LPF. Typically I use the NC pairs for 40 Meters and the NO pairs for 20M. It is just a convention and not dictated by some design requirement. These relays are DIP type made by OMRON the G5V1 series. Most times I use the 12 VDC but the surplus source for these is drying up so I have switched to the 5 VDC version which are still available.
Next we have three relays which are toggled depending on whether you are in receive or transmit. These three are all on one board --the N6QW designed bi-directional amp board. Two relays are connected at either end of the two 2N3904 amps and route the signals in/out of the Band Pass Filter or in/out to a third relay where the NC is connected to the antenna TR relay and the NO is routed to the Transmit Power Amplifier chain. So these three relays which are again the OMRON type and are being regulalrly exercised in the R > T > R process.
A fourth relay is a DPDT power relay where 1/2 of that relay has one task and that is to switch the voltage "on" to the CCI amp. Since this amp can produce 20 Watts of RF at 13.8 VDC -and is maybe 50% efficient means that dictates greater than 5 amps must be switched by the power relay contacts. The other half of the power relay connects the antenna to the Low Pass filter on Transmit or the 3rd relay on the N6QW bi-directional amp board on Receive.
Thus we have many relays in the TR process. However there is a 4th OMRON relay used in the Big Kahuna to supply power to switch the signal directions on the Plessey bilateral amps. But in later variants including one now on the bench this is done with a PFET. Yet another small reed type relay is used to control an external linear amplifier, making this a total of six relays in the TR.
A simple toggle switch routes power to the four relays involved in the band selection on the Band Pass and Low Pass filters. But the power being applied to various circuits used only on receive (Audio Amp, RX RF Amp) or only on Transmit (Mic Amp, Plessey Bilateral Amps, Transmit bi-directional amps) is all done with an N6QW designed solid state control circuit which is shown below.
Basically this circuit uses a 4N35 Opto-Isolator to trigger the SN7400IC which in turn Triggers the approprite power transistor. The TIP32C transistors are good for a couple of amps so they easily handle the chore. My first use of a similar circuit was in a 2009 Tri-Band Dual Conversion SSB Transceiver project that initially used a 4PDT power relay for this purpose. The results were disastrous! The back emf was so large that spikes on the line caused the PTO to jump several KHz -- Yes it was an analog PTO. It might not be so much of a problem today with digital VFO's --but keeping the back EMF to a minimum is always a good practice. Using this power control approach resolved the frequency jumping.
An additional wiring diagram of the relay interconnections will be subsequently added to this page.