The Watkins Johnson DMS-105a-2 is a VLF to MF receiver. While technically classified as a demodulator, It performs well as a standalone receiver when connected to a good antenna. (pictures are to come once they are formatted correctly)
The DMS-105a-2 receives the frequency range from 1 to 1600 Kilocycles in 2 bands of 1 to 600 Kilocycles, and 540 to 1600 Kilocycles. It is a heterodyne receiver, with a high IF of 2 Megacycles. Receive modes are AM, FM, CW, LSB, and USB. and it has bandwidths of 150 cycles through 16 Kilocycles in the dual sideband modes, and bandwidths of 2.5 through 8 Kilocycles in the single sideband modes.
The DMS-105a-2 was originally designed to demodulate subcarriers from frequency division multiplexed microwave systems. The presence of a single sideband bandwidth of 4 Kilocycles corroborates this, as that is the channel width of a single voice channel. The US department of defense and CIA were most likely the original customers this unit was built for, and the goverment cost in the early 70's was between $12000 and $16000.
The Front End of the WJ DMS-105a-2 is comprised of two impedance matching transformers and filters. the lower band uses a seven pole low pass filter, and the upper band uses a 14 pole bandpass filter. These preselectors are not tuneable, and pass the entire selected band.
Relay switched signal transformers precede the Front End Filter. They allow an input impedance of 50 or 1000 ohms in either band. The voltage sensitivity is subsequently decreased in the 1000 ohm setting, as no amplification precedes the filters.
The receiver has a switchable attenuator which follows the filter network. The attenuator is variable in 10 db steps from 0 to 50 db. The dynamic range of the receiver is quite high even without the input attenuator, and will detect a minimum signal of about -130dbm (about 2.2uV into 50 ohms or 10uV into 1000 ohms). The sensitivity listed in the manual is 30uV, However this represents the full scale value.
downconversion and filtering
The Mixer used in the WJ-DMS-105a is a custom WJ part, but it is a diode ring double balanced mixer. Its input is buffered by a push pull emitter follower to lower the noise figure and provide drive current and isolation. There are some filtering elements ahead of the buffer to block the Local Oscillator. The receivers AM,BFO,and FM modes share filters, while the LSB and USB have their own. This is due to the asymmetrical nature of SSB filters. All bandwidths are crystal filters,and each filter has its own buffer for input and output, which is powered only when that filter is selected. This means that if a filter stage fails, any other bandwidth can still be used. All of the symmetrical filters are centered on the 2 Megacycle IF, and the SSB filter band edges fall upon the IF.
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