Telemetry Reception with the PK-232

by Howard Sodja, W6SHP
w6shp@amsat.org

The following article was originally published in The AMSAT Journal, Vol.13 #4, Sept 1990 and QEX number 107, January 1991. The following version has been updated August 1995.


The PK-232 can copy satellite telemetry from AO-13 and UO-11. A minor hardware modification is needed to copy UO-11. Satellite related bulletins as well as satellite status reports are sent in plain text for part of each telemetry frame daily on AO-13 and most days on UO-11.

Reception of PSK or FSK telemetry sent by the Microsats and FO-20 requires an added external PSK or FSK modem and telemetry software (Note 1). Reasonably priced modems are available from advertisers in AMSAT publications. TAPR sells a modem disconnect board that plugs into a 40 pin IC socket inside the PK-232 and offers an easy way to interface any PSK or FSK modem to the PK-232 (Note 2). The external modem connector ports on the PK-232 do not have the clock signal so using it requires making modifications to the PK-232 circuit board. Installing the modem disconnect board is easier and leaves you with the PK-232's two external modem disconnect ports free so you will now have three external modem ports. As I had four modems I needed to add switches to select between my PSK and FSK modems on my TAPR disconnect board port.

The following settings will copy AO-13 SSB Baudot RTTY telemetry which is sent on 145.812 MHz on the hour +15 and +45 minutes when operating mode B and on 435.651 Mhz on the hour +00, +15, +30 and +45 minutes when in mode JL (Note 3):

     BA
     RXR OFF (mode B), or RXR ON (mode JL)
     WI OFF
     RB 50

Frequencies are plus or minus Doppler shift. An oscilloscope connected to the PK-232 will simplify keeping up with the doppler shifted tones on AO-13 as a shift in the cross pattern is seen more easily than a shift of the PK-232's LEDs. Tones do not change with Doppler on UO-11 as it transmits FM AFSK.

RXR does not operate above 300 baud because of a PK-232 hardware limitation. To copy UO-11's inverted shift an unused inverter in the PK-232 (U15 pins 1 and 2) must be wired to a switch in series with the output to JP4 as follows (Note 4):

  1. Solder a jumper wire between U15's pins 1 and 6.
  2. Cut the circuit board trace connecting U15's pin 6 to the inside pin of JP4. This trace is easiest to cut where it comes out from under R20 on the top (component side) of the PC board. Be sure to double check that this is the correct trace with an ohm meter. Cut with care (with a sharp X-acto knife) so adjacent traces are not touched.
  3. Mount a SPDT switch on the front panel where it will not interfere with other components (like near the upper left corner by the AEA logo).
  4. Connect the center of this switch to the inner pin of JP4 or the trace that goes to it (which was cut to disconnect it from U15 pin 6).
  5. Connect the switch contact which will be "RXR OFF" to U15 pin 6.
  6. Connect the switch contact which will be "RXR ON" to U15 pin 2.

As you have the PK-232 apart, this is a good time to sand off all the paint where the two cabinet shells mate and screws fasten to improve RFI shielding. Star washers under the cabinet screws will also improve grounding. This reduced PK-232 RFI into my receiver. Toroids on all cables connected to your receiver may also help reduce RFI. Those square "snap-together" ones are easy to use. I needed one on my 12 volt line.

With your RXR switch in the "reverse" position, the following software settings will now copy UO-11 FM AFSK ASCII Telemetry on 145.825 Mhz (if not heard try 435.025 MHz):

     AS
     WI ON
     AB 1200

Be sure to return your RXR switch to "normal" when you want to return to normal operation, as this switch is on line for all modes when the PK-232's internal modem is used.

If you have an HF RTTY modem you can use this with the PK-232 to improve copy of AO-13, (as well as greatly improve terrestrial HF operation of the PK-232) as the lack of individual tone filters and 200 Hz shift of the PK-232 results in less than optimum decoding. I found that with my HF modem's 170 Hz shift (which is AO-13's RTTY tone shift) and separate tone filters I can copy AO-13 RTTY telemetry from horizon to horizon error free, while the PK-232's internal modem will return occasional errors until the signal reaches a high enough strength to overcome the PK-232's modem's bandpass (not separate tone) filtering and 200 Hz shift limitations. Also on HF RTTY I can copy weak signals with a greatly reduced error rate.

All you need do is wire in 3 double throw switches with the center pins of JP4, JP5 and JP6 connected to the center lugs of your double throw switches. Then wire the inside and outside pins of JP4, JP5 and JP6 to these switches so that the JP center pins are switched to the inside pins for the internal PK-232 modem or to the outside pins for your external modem. These switches can be mounted on the front panel where space permits (like the upper right corner under "model PK-232"). To ease future servicing I recommend not soldering directly to the pins on JP4, JP5 and JP6. Check your catalogs (or local electronic supply store) for DIP connectors that are .100 center/.040 square hole size and wire your switches to them. Be sure to mark each connector so you'll always reconnect them to the correct pins in the future. A look at the PK-232 schematic will clearly show you what you are doing. A triple pole double throw switch would be nice if you can find one. I made do by ganging a SPDT and DPDT together.

Earlier PK-232's connected the middle pins of JP4, 5, 6 to the inner pins with a circuit trace on the bottom of the PC board. These three traces must be cut. Check beneath the JP pins to see if your model has these traces. Current models use three removable jumpers that will not be needed with the switches.

The above information is based on my own experimentation and several conversations with the helpful technical staff at Advanced Electronics Applications (AEA).


Article by Howard Sodja, W6SHP. HTML conversion by and feedback to KB5MU.

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