Trango VTX2500 2.4 GHz Video Transmitter Experiments

Overview

This is quick hack to allow a Trango Systems VTX2500 EaglePLUS 2.4 GHz video transmitter to transmit on slightly out-of-band frequencies.

The stock Trango VTX2500 is a high-quality analog video transmitter (FM with NTSC/PAL video input) operating in the standard 2.4 GHz license-free Part 15 band.  The video signal can be received on a matching Trango Systems VRX2550 receiver, or with a similar wireless receiver covering the same frequency range.

The Trango VTX2500 has a PIC16C73 controlling a National LMX2326 PLL synthesizer to determine the center transmit frequency for each of the four channels.  It's possible to reprogram the LMX2326 for your own transmit frequency or to even replace the voltage tune line with something manual.

Trango Systems VTX2500 EaglePLUS


Channel     Transmit Frequency (GHz)     VCO Vt (Volts)    RF Output Power (dBm)
1           2.413                        2.530             7.8
2           2.432                        2.846             7.6
3           2.451 (microwave ovens)      3.167             7.5
4           2.470                        3.479             7.5

You can manually "tune" the Trango VTX2500's center transmit frequency by replacing the LMX2326 synthesizer control line going to the Z-Comm V800ME10 VCO with a DC control voltage from an external potentiometer.

By manually sweeping this voltage from 0 to around 5 volts, the Z-Comm V800ME10 will cover approximately 2.22 - 2.56 GHz.  There is no bandpass filtering on the RF output, so the signal will be stable over that entire frequency range.  The RF output power will be around +7 dBm (5 mW) over that same frequency range.

A DPDT relay will used to "swap" the video input going to the VCO from the PLL network.  This will require a few modifications and delicate trace cuts to the VTX2500's stock PC board.

A 50 kohm 10-turn potentiometer will be used for the manual tuning control.  A "turns count" knob should be added to the potentiometer to get a general idea of the center transmit frequency.  Here is a general mapping of the turns count to transmit frequency.  Don't exceed 5 volts on the VCO Vt line.  One complete turn of the potentiometer equals a 100 count:


Turns Count     Transmit Frequency (GHz)     VCO Vt (Volts)    RF Output Power (dBm)
0               2.234                        0.04              7.8
100             2.278                        0.51              7.7
200             2.316                        1.03              7.7
300             2.349                        1.52              7.7
400             2.383                        2.03              7.8
500             2.414                        2.54              7.6
600             2.446                        3.04              7.5
700             2.475                        3.53              7.3
800             2.505                        4.06              7.4
900             2.535                        4.53              7.2
1000            2.565                        5.04              7.2

Pictures & Construction Notes

Overview of a stock Trango Systems VTX2500 EaglePLUS video transmitter.

The VTX2500 board can be powered from +6 to +12 VDC.  The current draw is around 150 mA.

The BNC jack is for the Video Input.

The white RCA jack is for Left Audio Input.  This connection also provides the audio input when operating in monaural (mono) mode.

The red RCA jack is for Right Audio Input.

The Toggle/Standby push button selects the transmitter's channel.  The four green LEDs indicate which channel (1-4) the transmitter is tuned to.  In "standby" mode, the transmitter is shut down.  This can be for security or power consumption reasons.

The screw terminals are for the DC power and the alarm outputs.  The alarm connections will not be used in this application.

The exposed pad near the "G" in GND silkscreen can be used as a tap for the regulated +5 VDC power.

Overview of the Trango Systems VTX2500 EaglePLUS video transmitter circuit board.

The antenna connection is via the right-angle reverse-polarity SMA jack on the right.

The silver square item is the Z-Comm V800ME10 VCO.

The two variable inductors are part of the 6.0/6.5 MHz audio subcarrier generator circuits.

The 28-pin device is the PIC16C73 which loads the PLL and the controls other circuits.

Overview of the reverse-polarity SMA antenna jack.

You can, if you're very careful, unsolder this connector and replace it with a standard right-angle SMA connector.

This view also shows the VCO voltage tune line on a stock Trango Systems VTX2500 EaglePLUS video transmitter circuit board.

This line controls the tuning voltage to the Z-Comm V800ME10 VCO.  Switching in a potentiometer to vary this tuning voltage from approximately 0 - 5 volts allows you to manually tune the transmitter over a (slightly) wider frequency range.

Overview of the National LMX2326 PLL synthesizer and Z-Comm V800ME10 VCO.

This circuit sets the center transmit frequency.

The capacitors are part of the loop filter to "lock" the PLL and couple the video signal into the VCO's voltage tune line.  The large brown capacitor is 10 µF and is on the video input.

The smaller silver square device is a 12 MHz clock oscillator and generates the reference frequency for the LMX2326.

Pin 12 on the PIC16C73 connects the the LMX2326's Data line.

Pin 11 on the PIC16C73 connects the the LMX2326's Clock line.

Pin 6 on the PIC16C73 connects the the LMX2326's Load Enable line.

Overview of the four channel indicating LEDs.

These will be removed and panel-mounted for convenience.

Solder-side view of the LEDs.  The have tiny 220 ohm resistors for current-limiting, but you may want to remove these so you have a bigger pad to solder to.

New 220 ohm resistors will added externally.

Spectrum analyzer view of a video modulated RF signal at 2.47 GHz (channel 4).

The screen is 5 MHz per division (horizontal) and 10 dB per division (vertical).

Scrap off the solder resist with an X-ACTO knife and cut the traces as shown.

A DPDT relay will be used to switch between PLL and manual tuning for the V800ME10 VCO.

Drill a small hole in the PC board and route four #30 wires as shown.

The letters correspond to th relay connection points on the schematic.

Overhead view of the solder connections.

Completed internal overview.

Try to keep the manual tuning wires as short as possible.  The multi-turn potentiometer is optional, but is handy for better frequency resolution when in manual tuning mode.

The stock audio RCA jacks and video BNC jack where removed.  New jacks were panel-mounted to the side of the case.

The white relay is used to switch between PLL and manual tuning.  The use of a relay is to avoid additional capacitance which could prevent the PLL from locking.

The 220 ohm resistors are for the new panel-mounted channel indicating LEDs.

Alternate overview.

The stock Toggle/Standby push button was removed and a new button added to the side of the case.  Connect the button back to the original solder pads.

Overview of the panel-mounted LEDs.

The four green LEDs indicate the transmit channel.

Overview of the panel-mounted controls.

The banana jacks provide the DC power.

The RCA jacks are for the audio and video inputs.  These inputs should be at the standard "line levels."  The impedance of the audio inputs is 600 ohms (unbalanced) and the video input is 75 ohms (unbalanced).

The manual tune 50 kohm potentiometer is on the left.  This is an (optional) 10-turn potentiometer with a turns counter.

The red switch is for DC power.

The white switch enables the manual tune relay.

The black push button is in parallel with the stock Toggle/Standby switch on the transmitter board.