es available for understanding and make it possible to change the configuration at any
time to suit your requirements.

Circuit Description: (All functions mentioned may or may not be enabled in your transverter)


The DEM TC circuit board is mounted in the front of the transverter. It includes the DC
power switch and both the Power On and the Transmit On LEDs. The TC contains a 1.5 amp, 9
Volt regulator that supplies the transverter with all of the regulated DC voltage it requires and
allows some head room for other external circuits. The TC controls all of the IF switching functions
utilizing a high isolation RF relay that is rated through 1 GHz. It allows the use of any frequency of
IF up to 1300 MHz. On transmit it will allow the use of up to a maximum of 10 watts of drive
depending on configuration. The TC also incorporates adjustable attenuators for both transmit and
receive and has a provision for adding additional receive IF gain. The TC allows either a PTT High
or Low for keying the transmit and receive circuits. Another feature included on the TC circuit
board is a 24 Volt relay driver designed to operate most SMA relays used for Transmit and
Receive switching of the RF frequencies. This circuit may not work with certain types of relays but
will operate with any SMA relay supplied by DEMI. Other options include different auxiliary DC
switching schemes, keying the transverter through the IF coax, and supplying PTT keying signals
to external equipment. Connection and use of each circuit is explained below. Refer to schematic
and component list for all component designations.

RXIF Gain and Adjustment:



The installation of the receive IF gain stage is the most asked about device in the
transverter. If the transverter is to be installed at the antenna with a long run of coax for the IF line,
you may wish to install IC1. Understand that installing this gain stage will not improve the system
noise figure and in most cases it will slightly degrade it. If you wish to improve the system noise
figure, you should add a LNA at the RF frequency. Then depending on the gain of the additional
RF LNA, you may not need the RXIF gain stage even if the transverter mast mounted. If you wish
to install IC1, refer to the component placement to determine where it is installed along with its bias
resistor R9. Any low frequency MMIC may be used but select it for the correct amount of gain
required. Also consider the noise figure unless you have a RF LNA in the system. Select the value
of R9 for the MMIC to be used for 13.8VDC operation. To bypass the gain stage, install a leaded
120pF capacitor in place of IC1. Keep the leads short.

The RXIF gain can be adjusted with R7. There should be approximately 20 dB of range.
With your system completely connected, R7 may be adjusted to an acceptable level determined by
your ear. For best results, keep the gain to a minimum. Only a slight noise increase should be
noticed by cycling the power switch of the transverter. If your S meter of your IF radio is at half
scale, you have lost half of the dynamic range of your system. For the same reasoning, adding the
gain stage and then adjusting the RXIF to maximum will degrade the dynamic range further.

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TXIF Drive Level and Adjustment.


The TXIF drive level adjustment (R2) has approximately 20 dB of range. Depending on the
configuration ordered, this adjustment range may not be acceptable. If so the fixed attenuator may
need to be installed, removed, or adjusted. If you wish to use less than 50 milliwatts of IF drive
power, install a short jumper wire in the R4 position and remove the large 50 ohm termination that
may be installed on the front panel. If you wish to use up to 2 watts, install R4 as shown with the
large 50 ohm termination. If you plan to use 1 - 10 watts of IF drive, install a 1pF capacitor in the
R4 position with the 50 ohm termination. If you need to experiment with other drive levels, any
combination of variable capacitor or resistor may be used in the R4 position. You may also adjust
the values of R1 and R3 as needed. Input power to the TXIF adjustment is dependent on the
attenuation installed on the C3 side of the K1 relay. The 50 ohm load resistor is designed to
handle 35 watts with proper heat sinking. When mounted to the front panel of the transverter, it
will not tolerate more than 10-12 watts reliably for a long period of time. You may experiment with
external heat sinking if you desire. External attenuation may also be used in the transceive path.
Remember that the added attenuation is also on the receive signal but may be overcome by the
additional gain of the RXIF gain stage. The level then may be adjusted with the RXIF adjustment.

Split Transmit and Receive or Common IF Configuration:

If you wish to use your transverter with separate TX and RX ports, first remove C3 and C5.
Then attach coax from the IF connectors to the corresponding TXIF and RXIF connections at the
C3 and C5 locations. If you are converting a split IF to a common IF, C3 and C5 may or may not
be already installed. Select the coax you wish to keep as the common and connect it to the
common input of K1.

PTT-H and PTT-L


The TC has the option of either using a PTT-H or PTT-L keying circuit. The PTT-H requires
+1.5 to +18VDC to activate it and will sink up to 2 mA. If using this circuit, be certain that the sink
current will not exceed your transceivers rating. The PTT-L circuit requires a connection to ground
to be activated. It is connected to the K1 relay and will source up to 25 mA to ground when keyed.
If this exceeds your transceivers rating you may modify the TC as shown in our design note DN0??
Found in our library on our web site or call for a copy.

PTT Keying Through the IF Coax:


Some transceivers such as certain models of the Yeasu FT290 supply a positive voltage on
its RF output connector during transmit or receive. This voltage may be used for keying the
transverter. After verifying or modifying your transceiver for this function, the transverter may have
the option installed. Select a choke of 1.0
µh or larger and install it in the L1 position of the TC.
Then connect a short wire jumper from the DC side of L1 to the PTT-H connection. When the
transceiver is keyed, the voltage in the coax will key the PTT-H of the transverter. This is the most
fool proof connection of the PTT line that can be made with any transceiver and is highly
recommended by DEMI.

If the PTT-L connection is connected to the PTT connector, it will not affect the operation of
the system. Just do not key both lines at the same time! If you will never use it, it may be
disconnected and the external connector may be used for any other auxiliary connections.
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+DC Switching Functions:

Relay K2 controls all of the +DC switching functions in the transverter. One side of the relay
switches the raw +DC supply voltage to the transverter and the other side switches the regulated
+9 volts. There are extra connection holes on the PCB if you require any additional switched
voltage. Be sure not to exceed the 1.5 amp limit on the 9 volt regulated supply. The transverters
current drain is listed on its configuration page and allow for some overhead when the oscillator is
not warmed up.
The switched voltages may be used for external LNAs, switching circuits for power
amplifiers or relays. You may need to add an extra hole for a connector if the AUX connector is
used. Be sure to fuse any external connections. The relays contacts (K2) are rated for 3 amps.

24 Volt Relay Driver:




The TC is designed with a 24 Volt relay driver. A brief explanation of the circuit is as follows.
When the TC is in the receive mode, a 330
µF capacitor is charged to the raw input voltage of the
transverter (12-14 VDC). When the PTT circuit is activated, K3 switches this charged capacitor in
series with the TXON voltage and outputs a brief spike of 24-28 VDC at the +R connection on the
TC board. Depending on the current drain of the connected circuit (or relay), the charged capacitor
bleeds off down to the raw input voltage of the transverter. The spike is enough to energize most
SMA relays and the raw transverter voltage is enough to keep the relay energized. This circuit will
not work with all relays but will operate with all relays supplied by DEMI in the optional WTR kit. If
you wish to use a relay with a higher current drain, the 330
µF capacitor may be increased. Just be
sure that the relay being used will stay energized with +12 VDC. This circuit will not operate
latching relays unless additional circuitry is installed.

PLEASE NOTE! Even though this connection will bleed down to the raw voltage of the
transverter, the circuit should never be used on any electronic circuit that will not tolerate a
+28VDC input! It will damage most +12VDC circuits. It will also not operate any circuit requiring
voltage greater than the transverters supply voltage other than a relay.
If a switched +VDC is required, move the wire from the +R connection (if installed) to either
the +VTX, +RX, +9, +13RX, +13TX, or +13.8SW. Refer to the schematic for their functions.


RF Sensing Transverter Keying


The TC is not designed to be a RF sensed switch. It has 3 relays and it may be connected
to external mechanical relays. If any RF sensing scheme is added to the circuit, it will cause