Downconverters for Mode S

As part of the development of a series of articles on getting going on AO-40 I obtained various units for evaluation and trial. Prices keep dropping and conversion gets easier. A big plus is the improved performance, particularly related to easy of modification. For simplicity and as an IC-PCR1000 receiver was already available, all the downconverters were left on their original IF frequencies of 122 MHz out - equaling 2400 MHz.

Two Drake 2880 units were imported direct from Japan. These required extensive modification to obtain satisfactory performance. There are many reports of these units being used with minimal change for reception of AO-40. While there are many reports of these being used with minimum changes, with simple helix antenna full modification is advised.

The last circulated modification brought the front-end noise figure down to <2 db. Even before the final modifications were completed a unit was used (with a “paper” 11 turn helix) to receive the middle beacon on AO-40 at a good range.

These units are no longer readily available but if you can get hold of one they do work very well.

Next to available was a TranSystem AIDC 3733. Modifications where less extensive but still included some intricate work around the second filter to install a Toko filter for image rejection and pass-band retuning.

In addition it was necessary to obtain a special “N” connector to replace the built-in dipole used in the original application. This was additional expense. In comparison with the Drake there was a marked improvement, compact unit and an all round more robust converter. Modification sheets suggest a noise figure of about 2.5 db. Some users are reporting noise figures of <1 db by “tuning” the input impedance.

These units are available from Bob Seydler, K5GNA via Ebay at:-
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1241393182

Shortly after, two more units appeared, this time on Mark Fossum’s web-site:-http://www.markfossum.com/productindex.html/

These were the NorSat 2500 and the California Amplifier 31732. Both offered improved performance and greatly simplified conversion. Again the units were left on their original frequencies.

The NorSat 2500 has a larger PC board layout that makes conversion easy. Modification is by retuning using PTFE tape across the main filter. The PTFE tape used for this is supplied with each unit.

The NorSat 2500 Downconverter has many nice characteristics; small size, low power requirements. Output IF level is lower than the other units, in fact at first it didn’t appear to be working, but the performance indicates good conversion gain and low noise figure, probably around 3db.

The California Amplifier 31732 downconverter is an excellent unit. Again, conversion consists of retuning using the supplied PTFE tape. Performance is excellent and the noise figure is reported at <2db. Comparison checks verified the performance. Conversion gain is good and IF output is strong. While it is more expensive than the Norsat it is better value particularly if used without a low noise pre-amplifier. It has a larger PC board layout that simplifies conversion of the unit. Modification involves application of a piece of PTFE tape to tune the unit. It is included at no extra charge with your California Amplifier order.

Details of the conversion of these converters can be found at Ward, WC0Y’s web site: http://communities.msn.com/wc0y

For Drake conversions, the best information is available via: http://www.g0mrf.freeserve.co.uk/

Comparison.

Modification:

Drake: Extensive and intricate surgery including the replacement of the front end device and insertion of a band-pass filter after the first stage. The unit tested was modified to the “<2dB” standard as published. This includes alterations to the IF, the input device (MGA86576), Toko inter-stage filter, PTC for stability control. (Requires intricate work – not for the faint hearted.)

Transystem AIDC 3733: Inter-stage Toko filter fitted, dipole system replaced with “N” connector, internal coaxial Trap disconnected. (While modification is simpler than the Drake, it still requires great care.)

California Amplifier: (CalAmp) PTFE tape added and trap removed by cutting PC board track. (Easy to alter, requires only screw driver, “Stanley” type knife and a pair of scissors.)

NorSat: PTFE tape added. (Requires only the screw driver and the scissors to modify –quick, easy and almost fool proof.)

Testing.

Exhaustive tests and checks were performed and re-performed to confirm results. While the results are reported in specific terms, this was a comparison check made without the use of an advanced laboratory RF cage or other extra expensive equipment.

Steps where taken to ensure the results are a true indication of the performance of the down-converters.

Input was via a 50 attenuation pad to ensure source impedance of 50 ohms.

A low-noise amplifier was used between the units under test and the analyser to ensure the analyser front end did not measurably add to the results.

  Signal -105 dBm signal at 50 ohms.

  Output measured using a HewlettPackard 141T Spectrum Analyser.

  Bandwidth 3kHz.

  SignalGen >> 15dB pad >> DownConv >> 15dB LNA >> Spectrum Analyser

The figures obtained were then inserted in the AO40 link program "ao40v2.0.xls" written by Gene Marcus. Various factors where zeroed to ensure the results were true. http://home.HiWAAY.net/~mmarcus/download/ao40v2.0.xls

RESULTS:

Input signal -105 dBm @ 50 ohms.

D/Conv            S/N(dB)         Floor(dBm)      NF(dB)       deg.K      Gain(dB)

AIDC 3733         35                  -140                  2.6              238                32

CalAmp               37                  -142                  1.8              149                33

NorSat                36                  -141                  2.2              191                25

Drake                 35                  -140                  2.6              238                34

(NOTE: Gain figures quoted measured without 15dB LNA.)

Many reports give the Norsat a noise figure of <4 dB. Without the 15dB LNA similar results were obtained leading to suspicion that other tests were influenced by the lower output into the measuring system. Practical tests verify the excellent performance of the NorSat unit.

Conclusion:

Readers should draw there own conclusion as to the best unit to obtain. All give excellent performance especially when preceded by a suitable 13 cm LNA.

The lower conversion gain of the NorSat masks an excellent performance. While in-use tests have not been concluded, there is reason to expect that a masthead LNA/NorSat combination would be excellent value for money.

Watch Break In for a series of satellite articles especially about getting on to AO-40.

Last updated August 27, 2001