Interfacing the FLEX-1500 to a Transverter

Content provided by: Greg Jurrens, K5GJ

The FlexRadio Systems FLEX-1500™ QRP Software Defined Radio is ideal for interfacing to VHF transverters. Papers have been presented on the subject (i) that covers various implementations in great detail. This article focuses on the basic HOWTO to get your FLEX-1500 on the air with your favorite transverter. For this example, I will use a DownEast Microwave(ii) 1296-28 1296MHz to 28Mhz IF transverter. A typical configuration is pictured below.

All transverter connections to the FLEX-1500 will be through the Rear panel of the radio. Please reference the pictures below for connector location.


FLEX-1500 Front Panel View

FLEX-1500 Rear Panel View


STEP 1: Build an interface cable

The interface between the FLEX-1500 and the transverter can be as simple as a PTT Out signal. For the DEMI, the cable will be as pictured below.

For a complete description of the FLEXWIRE v2 interface on the FLEX-1500, please review the KB article FlexWire v2 Connector Pinout for the FLEX-1500.

NTOE: PLEASE pay careful attention to the pinout of the DB-9 connector. If you connect the wrong pins to ground you can damage the FLEX-1500. The good news for this application… Pin 3 is the TOP-MIDDLE pin and both Pin1 and Pin5 are ground.

STEP 2: Connect the IF (FLEX-1500) to the Transverter
The FLEX-1500 will support both a “COMMON IF” or “SPLIT IF” interface for transverters. This will vary based upon your design. Here are 2 examples: (Courtesy DownEast Microwave)


Common IF Transverter I/O

Separate RX and TX IF I/O


For the Common IF configuration, simply connect the FLEX-1500 XVTX/C BNC to the COMMON IF BNC using a BNC male to BNC male coax jumpers.

For the Split IF configuration, connect the FLEX-1500 XVTX/C to the Demi TXIF and the FLEX-1500 XVRX to the Demi RXIF connectors using BNC male to BNC male coax jumpers.

STEP 3: Configure PowerSDR™
The FLEX-1500 will operate as a 28MHz IF frequency to the transverter. Usually, the operator must know (and remember!) his LO frequency to calculate the true RF frequency. Often, LO frequencies are chosen so that the IF lands on 28.1MHz for the desired frequency band. In my DEMI example, 1296MHz is mixed with a 1268MHz LO to yield 28MHz. Thus 1296.1MHz appears as 28.1MHz to the IF radio. PowerSDR can help keep up with this and even make the readout read your RF frequency. See below:

PowerSDR in 1296MHz transverter mode

The trick to setting up PowerSDR is in the XVTR form. Click on the “XVTRs” tab at the top of the main PowerSDR screen. Then set up the form as shown below.

PowerSDR XVTR form

You will need to set the following:

  1. BUTTON TEXT – Name the button something short and easy: “23cm” or “1296” works.
  2. Calculate and enter the LO Offset in MHz. In my example 1296 – 28 = 1268.0MHz.
  3. Enter the BEGIN FREQ of your Transverter’s band. It’s 1240MHz in my example.
  4. Enter the END FREQ of your Transverter’s band. 1300MHz for the 1296 band.
  5. Check the RX ONLY box if your transverter doesn’t transmit.
  6. Set the POWER to the maximum power level. The FLEX-1500 puts out around +3dBm at the “100” setting. This will vary by transverter. WARNING: Be sure you know what you are doing here. You can ‘toast’ your transverter mixer if you put too much power into it. (NOTE: PowerSDR V2.0.16 currently does NOT support high power TXIF through the FLEX-1500 ANT port. We are evaluating a change for an upcoming release. )
  7. Check the “use XVTR Pwr for TUNE” box if you want to have the TUNE function use the Power level you set in step 6.

Next, we need to set up the proper ANTENNA port configuration. From the main PowerSDR screen, click on ANTENNA. Configure the form as shown below depending on your IF configuration.

Antenna Config: Common IF

Antenna Config: Split IF

Notice for this example, I have checked the “PTT OUT Enable” box and entered a delay of 100ms. This will vary by transverter, external amplifier, pre-amp, and sequencer configuration. As my kids say “YMMV!” Your Mileage May Vary!

Start PowerSDR normally an configure the XVTR and ANTENNA forms as described previously. On the PowerSDR main screen, select the “VHF+” band button. You will then see your newly created “1296” button as shown below:

PowerSDR VHF+ Band Selector

PowerSDR Transverter Bands

At this point, you should be ready to get on the air with your VHF tranverter! Generate an appropriate test signal into your transverter’s RF port and you should see the signal appear on the Panadapter. PowerSDR will perform just like normal including the great filtering and Panadapter display. If you don’t see a signal, check the XVTR form to make sure you entered the proper LO offset. Check the ANTENNA form to make sure you configured for your IF configuration.
To test the TRANSMIT, start by running the DRIVE level down to zero. Also, Click the SETUP then TRANSMIT tabs to set the TUNE power level to a low value like 10. See below.



Press the MOX button. The PTT light on the transveter should light up. If not, troubleshoot your cable, then check the ANTENNA form for proper PTT out settings. Once you have good keying, connect an antenna or load to your transverter’s RF output. This would be a great time to use or borrow a wattmeter or power meter to properly set your drive level. On the DEMI transverters there is a TX LEVEL Pot that adjusts the amount of drive that gets to the mixer. Follow your transveter’s instructions. (WARNING: KNOW how much power your transverter can handle. The XVTX port on the FLEX-1500 will generate a maximum of around +3dBm. It’s a good idea to set your transverter up so it can handle the maximum power the FLEX-1500 XVTX port can deliver) Press the “TUN” button. This will key the FLEX-1500 and generate an RF signal with a CW tone. Slowly increase the DRIVE and verify you are getting the proper transverter output.

As a final test of your FLEX-1500 transverter interface, point your antennas at EM10 and call K5GJ/r. I can really use the extra Grid multipliers! HI HI!

(1) 2010 Central States VHF Conference Proceedings – “FLEX-1500 Software Defined Radio Use in Weak Signal Operations”, Steve Hicks – N5AC; Pg. 198-204.
(2) DownEast Microwave –
(3) FlexRadio Systems



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