Directly Tune FT 991 A IF Frequency - rfrht/FT991A-PAT GitHub Wiki

FT-991/A directly tuning IF

Tuning beyond SDR 2.4 MHz bandwidth

Let the scenario. Your radio is tuned in 146.410 MHz. And you are interested in monitoring a signal in 144.300 MHz. Can I monitor this signal that far?

Yes, it is possible. But takes some effort.

The radio's Intermediate Frequency outputs the 146.410 MHz signal in 69.450 MHz. Since the RTL-SDR maximum optimum bandwidth is 2.4 MHz, it has 1.2 MHz in either direction, below and above (center: 146.410; minimum: 145.210; maximum: 147.610).

The catch here is that the IF signal is inverted. Meaning, if you tune the SDR a MHz above, it will be a MHz below the real frequency. Example: Radio tuned in: 146.410 MHz IF frequency: 69.450 MHz

If I tune in HDSDR the IF frequency to 68.450 MHz (one MHz below), then HDSDR will monitor 147.410 MHz.

The reverse is also true: If you tune the IF to 70.450, you are actually tuning to 145.410 MHz.

So going back to the example, how do I tune in the SDR 144.300 while the radio is monitoring 146.410?

  1. First of all, you have to disable the CAT to Radio control (Options - Cat to Radio - Uncheck 'Sync Rig1')
  2. Disable the HDSDR tune in the IF frequency (Options - RF Front-End Configuration - SDR Hardware connected to antenna)
  3. Calculate the difference from the actually tuned in the radio frequency to the desired frequency: 146410-144300 = 2090
  4. Since we want to monitor a frequency BELOW the actually tuned frequency, you have to ADD it to the actual FI value: 69450+2090=71540
  5. Tune HDSDR to 71540000. That is the 144.300 MHz signal when listening to 146.410 MHz.

Directly tuning SSB signals

FT-991/A shifts the signal in +1500 Hz (Hertz! Not megahertz!) in LSB mode and -1500 Hz in USB mode. Be sure to correct your frequency with the proper values in order to be able to tune it correctly.

Example:

Radio: Tuned in 7.093 IF center: 69.450, outputting 7.093 Want to monitor: 7.115 Mode: LSB

Calculate: 7.115 - 7.093 = 22 KHz diff. Real direction: Above IF calculation direction: Below, subtract IF result: 69.428 LSB offset: +1500 Final IF tune: 69.429.500 Hz