Kitbuilding 101 Part 3 Completing the Transverter


We have built a working transverter and have a few specifications. Let’s list what we have and build on that in order to determine if support or peripheral circuitry will be required.

RF RX sensitivity = <-135dBm

IF input maximum = 0.5W

RF TX output power = 0.09W (19.45dBm)

DC input = 13.8V

That’s all we have right now and we will need more information about our transverter to correctly specify a fuse, whether the IF can safely key the transverter, and thermal assessment to determine if additional cooling might be required. Let’s do those measurements; Peak current demand is determined by putting our multimeter in line and reading the current while transmitting. Transmit usually requires more current and we have checked the schematic to make sure this is the case.  Rarely, in QRP projects, receive can require more current than transmit. TX current = 229.1mA It is interesting that this current remains the same when RF is not being applied and the transverter is in the transmit mode. Here is a picture of the current test.

A little more investigation shows RX current draw is 111.6mA. Let’s check the schematic to see if that makes sense. We have a MMIC (the SNA-586) that is specified at 65mA operating current at 4.9V, a 5V regulator, and the 78MHz oscillator being powered. TX current would include the relays and two MMICs. That seems about right considering the regulator does not have an input resistor and will be dissipating about a Watt as heat. It might make sense to put in a dropping resistor at the input of the regulator to reduce this.  We have about 175mA at 5V so a 35Ohm resistor at 1.5W would do the job. It is easy to see that a component of this size might be inappropriate but not impossible to place on a small PCB. An additional check with the digital temperature detector reveals a very warm to hot regulator. I am concerned this may negatively impact the long term performance of the transverter so a dropping resistor makes sense. Some scrounging in the junque box yielded an old carbon 1/2W 27 Ohm resistor. The resistor will run hot but I prefer the resistor running hot to the regulator. Let’s give it a try. I lifted the input of the regulator with a bit of solder wick and a jewelers screwdriver (aka eyeglasses screwdriver) and mounted the resistor vertically between the 13.8V input and the airborne 5V regulator input. Time for another smoke test. 5V appears at the output of the regulator and 10.65V is on the input. We have about 3.25V dropped across the resistor and about 0.4W dissipated. That should shed enough heat from the regulator to improve the MTBF of the transverter. Again utilizing the digital temperature detector we find the resistor is warm to very warm and the regulator is also quite warm. (By the way, I bet you figured this out already, the digital temperature detector is your finger.) Receive is ok. Next we put the unit in transmit to see if the modification can withstand one of my time out timer deficient transmissions. Placing the transverter in the transmit mode 5V appears at the output of the regulator and 8.22V at the input. The 27 Ohm resistor is dropping 5.66V (my power supply is 13.88V). The resistor will be dissipating over a Watt during transmit. This is OK for temporary ICAS operation but the right resistor for this application is a 1W 27 Ohm resistor and I make a note in my engineering log to get one the next time I am at Mountain States Electronics. I also replaced the onboard 5V regulator with a upright unit with a bolt on heat sink. This is an interesting sub project and the first of several I suspect.

Modification summary: Added one 0.5W 27Ohm resistor from the 13.8VDC supply to the input of the 5V regulator and replaced the 7805 D-Pak with a conventional 7805 installed upright on its leads and equipped with a slip on heat sink. The resistor lead was insulated to avoid a possible short to the heat sink.

I contacted Bruce (W7BAS) and he is in the process of updating the board to address this issue. He suspects it will be a simple component change and will update purchasers as soon as the update is finalized. It should be noted that the regulator is capable of handling the current and heat as the board is now.

What were we doing? Oh yes. The specifications. Let’s list what we have now.

RF RX sensitivity = <-135dBm

IF input maximum = 0.5W

RF TX output power = 0.09W (19.45dBm)

DC input = 13.8V

RX current = 112mA

TX current = 230mA

We will need to key the transverter and measuring the key line voltage and current will help us decide if a keyline interface buffer will be required. The transverter is set up for RX operation and the multimeter employed to measure the keying voltage. PTT voltage should be the full input supply voltage and reads 13.86V. Keying current is read at the PTT contacts and is 61.6mA. Add a T/R relay and this is more than seems wise to key with an FT-817. The open collector transistor could probably handle this current and voltage but the traces from that small accessory connector to the main board might not. A keyline interface will be required.

For the purposes of a comprehensive specifications list our observations are;

PTTe = 13.8V

PTTi = 62mA

A keyline buffer circuit is relatively simple. Here is one design.

Keyline Schematic


The theory is straightforward. Voltage is applied to the base of the 2N2222 via a 10K resistor which turns the 2N2222 on. This applies the forward voltage drop of the 1N4002 + Vcesat to appear at the gate of the IRF610. Since this is less than the switching threshold the MOSFET is off, and the Keyline is not grounded. When the PTT input is grounded the 2N2222 is turned off. This allows the 2X10K voltage divider to apply 6.9V to the gate of the IRF610 turning it on. The Keyline is then connected to the transverter. PTTe should be around 1.5V and PTTi about 2 milliamps. I used a small piece of perfboard about 1.5X2″ to build this board. It took about 20 minutes from parts scrounging to testing.

We are almost completed with the peripheral and interface electronics. The only thing left is a T/R switch. You could use a small SMA relay but these are generally expensive. I opted for one of Down East Microwave inc. low power T/R relay kits. (DEM UTR-12X) They aren’t much as kits go since they consist of a relay, a diode, and a printed circuit board. It will take about 5 minutes to assemble.

Here is a picture of the transverter board suite

Let’s summarize and think about what to do next. We have a working base transverter board, a PTT buffer board, and a T/R relay. It makes sense to start thinking about the actual enclosure we will use. I’m not sure whether one more board will be required but the assembly could be ready to go on the air reasonably soon as is. The one more board that I am considering is an amplifier to take this transverter from about +20dBM to +37dBm. Life really is too short for QRP.

I decided a power amplifier is a separate project and will go with the barefoot transverter for the moment. Perhaps I have been working with DEMI transverters too long but a Hammond type box seemed like the right way to go to package a transverter. A 4.75X4.75X2.25″ box was chosen and equipped with standoffs, RF, PTT, IF, and power ports. The main transverter board was mounted on four metal standoffs as was the T/R relay assembly and the keyline buffer. It took about an hour to wire the transverter and go to the test bench. The wiring will be dressed properly after final testing. Here is a picture of the wired transverter.

A BNC (f) was used for IF, RCA phono jack for PTT, an SMA (f) for RF, and a twisted pair for power input. A fuse should be included in the twisted pair to protect the unit in the event of a short or other high current fault. The boards are mounted at different levels and orientations within the box as they fit and functioned best. Shielded cable was used to connect the TX and RX ports on the transverter board to the T/R relay board and from the T/R relay board to the RF I/O SMA(f) receptacle.

The updated specifications list:

RF RX sensitivity = <-133dBm

IF input maximum = 0.5W

RF TX output power = 0.09W (19.45dBm)

DC input = 13.8V

RX current = 120mA

TX current = 240mA

PTTe = 1.19V

PTTi = 1.34mA

Note that this is significantly improved from the previous PTTe/i of 13.8V and 62mA due to the PTT buffer circuitry. This is easily keyed by the FT-817 with no worries about damage due to over current on the PTT line.

On the air testing:

Testing during the January VHF contest proved to be interesting. I asked Don (N0YE) to come up to 222.1 and he was S9 from Boulder for the first contact with the new transverter. We had a nice QSO and he gave me a Q5 S0 report and about 700Hz low. Next my goal was a DX contact. Could I find Dave (W6OAL) on a contest weekend? The short answer is yes. Don and Dave QSYed to 222.1 and we had a nice conversation. There was some broad band noise that interfered with my low power transmissions at Daves place. However, it is time to declare victory and put this project in the win column.

Here is a picture of the transverter in operation

Thanks to all Dave (W6OAL) and Don (N0YE) for their review and input on all parts of the project. Bruce (W7BAS) for his guidance and for having the will and ability to provide a 222 transverter kit.

A bit more power would be a nice addition to this project.