[John KC0G]

I shall try to fill in a bit of the background to Synchrodyne's queries about the Marconi H2001 Hydrus receiver.. I do not have the main reference, ie
.... P.L. Painter, Hydrus - a new H.F. Receiving Equipment, Point-to-Point Telecommunications, Vol. 12, no. 2, April 1968, pp 82-96
but I do have
....J.V. Beard, A Survey of Communication Receivers, Point-to-Point Communication, Vol. 14, no. 3, August 1970, pp 110-120
(covers H2102 MST, H2002 MST, H2001 Hydrus, H2900 series, EC958 series, H2310 Argo, N2020, N2023)
which helps to place it in context.

The H2102 (MST) receiver was primarily designed for point-to-point communications, including ISB as an option. It used the H1500 frequency synthesizer with 100Hz steps, and could be used with extended or remote control. It took about half of a full height rack. The H2002 (MST) receiver was a double diversity version of the H2102 for FSK operation. It took up about 3/4 of a full height rack.

The H2001 Hydrus receiver was designed to offer similar performance to the MST receivers, but without the remote or extended operation. Also to keep the cost down, the frequency synthesis system operated with 100kHz steps, and used a high-stability L-C oscillator for interpolation. It consisted of four 3U modules, and sat in a rack about 24" high. It could receive ISB, FSK, and voice-frequency tone (VFT) traffic. The article does not list it as being suitable for Piccolo service, or for use on ship-to-shore (naval) circuits. The Royal Navy had specific requirements for equipment to be operable by unskilled personnel, which effectively meant that equipment had to be fully synthesized.

The H2900 series which came later, was shown as being usable for most services including ISB and Piccolo (FSK required adaptors). Pat Hawker, G3VA, (Radio Communication, RSGB, Feb 1970, pp92-93) noted that the frequency synthesizer tuned in 1Hz steps, Interesting the radio was not shown as being appropriate for ship-to-shore (mercantile circuits). Presumably it was too expensive.

The Eddystone EC958 is described. It could not receive any of ISB, FSK, VFT traffic, or Piccolo. It was not suitable for ship-to-shore (naval) circuits. Again I think that it required a skilled operator. The H2310 Argo receiver was an enhanced version of the EC958, with a frequency counter which could then be used to lock the VFO. It is shown as being able to receive VFT traffic. It is also shown as being suitable for ship-to-shore (naval) circuits. Presumably it could be operated more easily.

The NC2020 was designed to meet "stringent naval specifications". It could be directly tuned by setting decade switches, Alll local-oscillator frequencies were generated by a frequency synthesizer. The N2023 was a version which was designed for dual-path diversity reception.

Pat Hawker, G3VA, (Radio Communication, RSGB, May 1968, pp 300-301) briefly mentioned some of the Hydrus circuit features, and noted the extensive use of FET's.

A block diagram of the Hydrus frequency synthesis system is shown in
....V.F. Kroupa, Frequency Synthesis - Theory Design and Applications, Charles Griffin (also published by John Wiley), 1973, pp 253-254, 262 and 158, ie Fig 8-14 and associated text.
Kroupa references [5.12]:
....The Marconi Company, Hydrus HF Receiver, Telecommunications, Vol. 2, July 1968, pp 40-41
There are two-drift-cancelling loops. The auxiliary one mixes the output of the interpolation VFO with one of 10 harmonics from a 100KHz crystal to give an interpolation range of 3 to 2 Mhz. This is then mixed into another drift-cancelling loop where the local oscillator was "tuned" in 1MHz steps. The first two IF frequencies were 39.5 to 40.5Mhz and 5MHz.

I cannot answer the question about how the carrier was extracted for ISB reception on the Hydrus. Somebody needs to turn up a copy of the article by Painter.

73 John KC0G