2005-11-09

SSB Electronics LT23 S

Translated by G. J. Coyne and G. Mussler.

23 cm Linear-Transverter LT 23 S

The LT 23 S is a new 23 cm device, a Linear-Transverter, employing leading-edge technology "achieving a specification never achieved before" in a compact design.

Key technical features

Robust Parallel Output stage with “Valvo” Transistors producing 10 W UHF output power!
Low noise GaAs FET input amplifier, overall noise figure NF = 1,8 dB!
Two separate, switchable crystal-controlled oscillators two IF ranges.
Built-in power meter monitoring RF output.
Built-in IF relay - no transverter output required.
Built-in adjustable input attenuator – works with any input power in the range100 mW to 10 W.
Manual or PTT-line transmit-receive switching.
Antenna change-over relays and external pre-amplifier unit supported by control outputs.
Robust, plastic-laminated steel sheet case.

Technical Data

Transmit

Input frequency	Oscillator F2	144 -146	MHz
Output Frequency		1296 -1298	MHz
Input Power	internal preset	0,1 - 10	W
Output Power		10	W

Receive

Input frequency		1296 -1298	MHz
Intermediate Frequency	Oscillator F2	144 -146	MHz
Noise Figure		Typical 1,8	dB
Gain		Typical 18	dB

General

Power Supply		14,5	V
Transmit current drain		Approx. 2.5	A
Receive current drain		Approx. 0,2	A
Standard connector		BNC
Dimensions	Width x Depth x Height	300 x 22 x 90	mm
Weight		2,5	kg

Front Panel Connectors

‘ON’ switch	This controls the DC power to the unit. The LED is lit for normal operation.
‘TX’ switch	Allows manual switch-over to transmit. The LED is lit on transmit.
‘F1 - F2’ switch	Selects of one of two crystal oscillators. The frequency 2 (F2) oscillator has a crystal fitted for the default IF range. The switch allows the default IF range, as quoted in the technical description, to be overridden.
‘Power Output’	Reads UHF power output directly in Watt.

Rear Panel Connectors

‘1296 MHz out’	Provides a maximum 10 W UHF output on transmit.
‘144 MHz in/out’	Connection to 2 m transceiver. Do not exceed the maximum drive power! Always keep to the minimum 2 m drive necessary.
‘+/- 14.5V’	A heavy-duty cable should be used for connection to the power supply. A supply voltage of 12 V is not enough for the LT 23 S to produce an output of 10 W; this voltage would produce a maximum output power to approx. 7,5 W. The supply voltage (14,5 V) should be measured directly at the connection to the LT 23 S when transmitting with full power.
'PTT'	‘Cinch’ socket for connecting the PTT line from the transceiver. Grounding this terminal switches the LT 23 S to transmit.
'+ Relay'	On receive this terminal shows a voltage of approx. 14 V (measured with respect to earth potential). This voltage is used to power the antenna relay. In addition, it can be used to power an external GaAs FET pre-amplifier.

Adjusting the Input Attenuation

The LT 23 S incorporates an adjustable attenuator allowing for a 2 m input drive up to a maximum of 10 W. To adjust to the available drive, proceed as follows:

Remove upper cover of the LT 23 S by (removing the four screws) and locate the control for attenuation. This control is labelled "P in" and is next to a pair of power resistors.
Turn the control fully anti-clockwise, i.e. towards ground potential. Connect up the 2 m transceiver as normal. Connect the output "1296 MHz out" to a well-matched dummy-load. If you have to connect a 23 cm antenna as the load instead, use a high-loss cable (e.g. RG 58/U) in order to improve the match.
Now switch the transceiver to transmit (FM or CW) and carefully turn the attenuator control clockwise until the power meter of the LT 23 S shows 10 W. Don’t increase the drive power further as this will only reduce the signal quality. Due to the inertia of the meter, the needle will usually show peaks of only 3 W on SSB, however the actual modulation peaks of the LT 23 S will reach 10 W.

Using the second crystal oscillator

The second crystal oscillator is brought into use by fitting an appropriate crystal in the base "Q2" and by setting the front-panel switch to "F1" . Carefully adjust the ferrite core of the neighbouring inductor (marked blue/brown) to achieve oscillation. A maximum IF shift of +/- 2 MHz is possible.

For example:

To prevent strong stations breaking through on the 2 m IF, the LT 23 S can be set up to use an IF of 146 to148 MHz. (Many 2 m transceivers can cover this range!).

The required crystal frequency is calculated as follows:

(fin – IF)/12

(1296 – 144)/12 = 95,8333 MHz

The required crystal frequency is: 95,8333 MHz

Base: HC-25/U

Resonance: series mode resonance

Operation with Antenna Change-Over Relays

The LT 23 S does not contain its own antenna change-over relay. A built-in antenna change-over relay would only make it harder to connect a linear amplifier or a masthead (GaAs FET) pre-amplifier. Instead, each of these options may be supported with a pair of coaxial relays for each amplifier.

It is good practice to install a high quality coaxial relay near the antenna (say in the roof space) and to use a pair of coaxial cables. The receive cable is connected to the ‘1296 MHz in’ socket. The transmit cable is connected to the ‘1296 MHz out’ socket.

One side of the coaxial relay solenoid is connected to ground. (The ground return is via the outer of the coaxial cable). The other side of the solenoid is connected to the ‘+ Relay’ terminal with a single conductor wire.

On receive, the coaxial relay is energised, connecting the antenna to the input of the LT 23 S. On transmit, the coaxial relay is not energised connecting the antenna to the output of the LT 23 S.

Warning

Switching the coaxial relay induces a high voltage across the solenoid. Make sure a diode (e.g. 1N4001) is connected across the solenoid. The cathode (marked with a ring) is connected to the positive end of the solenoid.

Operation with an extra GaAs FET preamplifier

A GaAs FET pre-amplifier is easily supported with a coaxial relay and two coaxial cables: the pre-amplifier input socket is connected to the receive [????] output of the coaxial relay via a short piece of coaxial cable. The output cable to the transverter is connected to the output of the preamplifier.

The pre-amplifier shares its power source with the associated coaxial relay solenoid of the coaxial relay. The earth return is via the outer of the coaxial cable.

We recommend the DX1296 as the GaAs FET preamplifier.

Electronic protection circuit

When the LT 23 S is switched to receive, built-in protective circuitry ensures that up to 10 W of 2 m RF may be tolerated by the receiver input.

This guards against the possibility that RF is applied to the receive circuitry, perhaps due to a break in the P.T.T. line, or as a result of forgetting to manually switch to transmit mode.