[TangerineSDR] Frequencies for FT8 & WSPR

[Engelke, Bill]

Hello All:

A week ago in the TangerineSDR telco, we touched on an issue regarding frequencies for FT8 and WSPR. I'm interested to know what folks think about what should be done here.

The question is this:  when setting the Data Engine (i.e., the radio part) to a frequency for FT8 or WSPR reception, we have used the term "center frequency," thus suggesting we might set the DE to the center of the respective sub-band (see table below). Perhaps that is compatible with IQ sampling, where we recognize the frequency of the signal by the phase difference between I and Q; so signals below the center would have a negative phase shift ("negative frequency"), signals above a positive shift.

I have a question:


  *   The decoders for both WSPR and FT8 assume the receiver is set to a dial frequency (per the WSPR and FT8 band plans) and receiver is in USB. This produces a baseband signal (that can be collected by a sound card) in the audio range. I am fuzzy on the correct way to convert this signal - do we set the DE's frequency to the dial frequency and assume that the decoder will ignore everything in the negative frequency area, or do we set the DE's frequency to the center of the sub-band?
  *   Just as a note, Scotty and Martin, who are planning to do the FPGA programming, voiced some concerns about whether the DE could actually be set to the required frequency (be it "dial frequency" or "center of the sub-band"), due to open questions about the clock's output (?) - I am fuzzy about the details here, but as recall, the way we left it was that Scotty & Martin are going to have a look at the Pavel Demin FPGA code to see how he accomplished it (which may answer the question above). What I do know is that the Raspberry Pi does not have the capacity to accept a wide-band data feed( (i.e., encompassing all FT8 & WSPR frequencies) and doing the filtering to slice out the sub-bands for the decoder; that would take a high-end desktop.

If anyone has any thoughts, please post. Below is the table of frequencies:

Frequencies for FT8

(4000 samples/sec)


Dial Freq. in MHz
in Hz
lowest signal
center
top of band
1.84
1840000
1840200
1841500
1843000
3.573
3573000
3573200
3574500
3576000
7.074
7074000
7074200
7075500
7077000
10.136
10136000
10136200
10137500
10139000
10.074
10074000
10074200
10075500
10077000
18.1
18100000
18100200
18101500
18103000
21.074
21074000
21074200
21075500
21077000
24.915
24915000
24915200
24916500
24918000
28.074
28074000
28074200
28075500
28077000
50.313
50313000
50313200
50314500
50316000
50.323
50323000
50323200
50324500
50326000





Frequencies for WSPR

(375 samples/sec)


Dial Freq. in MHz
in Hz

center

0.136
136000

137500

0.474
474000

475500

1.8366
1836600

1838100

3.5686
3568600

3570100

7.0386
7038600

7040100

10.1387
10138700

10140200

14.0956
14095600

14097100

18.1046
18104600

18106100

21.0946
21094600

21096100

24.9246
24924600

24926100

28.1246
28124600

28126100

50.239
50239000

50240500



-73- Bill , AB4EJ
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