MA168B Diversity Switch

Under construction.

I'm sorry but as the majority of my equipment is packed away at present I cant get to my MA168 to take photographs of it. As soon as I get things set up again I will include pictures.


The MA168 Diversity switch is intended to provide diversity reception for receivers such as the RA17 and RA117. The unit is fed with the 100 khz I.F outputs from two receivers and it will then switch between the stronger of the two inputs.Switching time is around 10 microseconds. Its main use was for RTTY reception but it also works with SSB signals. Audio outputs are by way of 600 ohm lines although there is a headphone socket on the front panel.There are two panel meters that show the relative signal levels of each channel and the unit can be set to output either "Receiver A" , "Receiver B" or "Diversity". An indicator shows which channel has switched and a BFO is provided for CW use. The unit will accept MCW or CW signals of telegraph speeds of up to 1000 Baud

If the unit is used with the RA17 a small modification is required. Capacitor C143A must be removed from the base of PL11 and Pin 5 of V12, the connection of PL11 is then taken directly to Pin 5 V12. Valve V1 from the designated "B" receiver must be removed.

Use with the RA117 is relatively straight forward, crystal XL1 has to be removed from receiver "B" and the interconnections on the rear panels have to be re configured. I don't have room for a full description here but I can Email details to those who require them.

Front Panel.

A description of the front panel is as follows:- on the far left is the Signal Selector Switch, which can be set for CW or MCW. To the immediate right is the Channel Indicator which shows which of the two channels has been selected.Next we have the Signal Level Meter for channel "A" and below this the Phones socket and audio Gain Control. In the center is the selector switch for Receiver "A" , Receiver "B" or diversity output. To the right of this is the Signal Level Meter for channel "B" and below this the Line Level Control and the Set Level switch.Next we have the BFO control and then the Mains On/Off switch

Rear Panel.

On the rear panel from left to right we have:- Mains Input Connector,AC and DC Fuses. Balanced or Unbalanced Switch, Audio Output Connection Block, two inputs for 2nd VFO Coupling, Receiver "B" I.F input, Diversity I.F Output and Receiver "A" I.F output.

Brief Description.

Mode F1,F4,F6,A1,A2,A3,A3A
Switching levels Does not switch below 10mv, 10-30 mv=6db, above 30 mv=3db
Switching Time Less than 10 microseconds
Switching Time Constant 0.2 to 40 ms, variable
Meter Indication I.F Level for each channel, Audio output level
BFO +- 8.5 Khz , Stability less than 10 hz per hour drift
AF Output Two independent amplifiers each delivering 1.75 mw into 600 ohm
Power requirements 100-120 and 200-250V AC 45-65 Watts

What is "Diversity Reception" ?

In the context here , diversity reception is used to help counter fading signals over HF links. The basic idea being to select the better signal .We have all experienced fading on the SW bands so imagine what this is like when you have an important teleprinter link and the signal drops right in the middle of a transmission. By connecting , in the case of the MA168, two receivers to the diversity switch, the stronger of two signals can be selected to hopefully keep the output to the teleprinter steady.

Obviously diversity reception can be used with any radio system that suffers from fading signal paths.

Space diversity is where two physically separate antennas are used .

Frequency diversity uses two different receive frequencies.

Polarization Diversity and Time Diversity are also techniques that can be used used.

Obviously with the MA168 we are really only concerned with two channels but in other systems two or more frequencies may be used with their associated antennas. Diversity techniques are also used in Mobile Communications and Wireless Microphone Systems amongst others.


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