Understanding Frequency Relationships on HF/SSB APRS

Introduction   |  Conventional 300-Baud FSK  |  PSK63
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FM, SSB and TNC Tones

Perhaps the most confusing issue for newcomers to HF APRS is the difference between the FM modulation mode they know on VHF, and the FSK or SSB modulation modes used on HF. There is considerable mis-understanding about the relationship between the audio frequencies produced by packet TNCs or "sound card" data programs on computers, and the radio frequencies that result from these tones.

On VHF-FM, 1200-baud packet data transmission is done by rapidly shifting an audio tone between two frequencies traditionally referred to as the "MARK" and "SPACE" frequencies. These tones are inserted into an FM radio's mic jack or equivalent. On 1200baud VHF packet, these two tones are 1000 Hz apart and standardized on 1200 and 2200 Hz.
 

On 300 baud HF, the actual RF carrier of the transmitter (essentially a continuous key-down CW transmission) is shifted between two frequencies 200 Hz apart at the 300 baud data rate in a process known as FSK (frequency shift keying).  On thirty meters APRS, these two frequencies, a.k.a "mark" and "space" are 10.149 200 Mhz and 10.149 400 MHz. The transients created by rapidly shifting from one frequency to the other create sidebands that spread the two discrete frequencies into a band of frequencies about 400 Hz wide. 

This is a 300-baud packet burst viewed on the waterfall display (i.e. audio frequency spectrum analyzer) of the MixW program.

The traditional ham convention is to specify the actual RF frequencies of the tones. The commercial/military/regulatory convention is to specify the single frequency midway between the two RF frequencies, along with the shift. In this format, the 30M APRS channel would be quoted as:

   "10.149 300 MHz with +/- 100 Hz shift"   or   "200 Hz Shift Centered on 10.149 300 MHz" .

These two frequencies can be produced directly by some transceivers that have a dedicated FSK (frequency shift keying) mode that responds to RS-232 or TTL-level serial data provided to a dedicated port on the radio. 

However, it is far more common to create these RF frequencies by feeding alternating audio tones, 200 Hz apart, into the mic jack of an SSB transceiver. SSB radio transmission can be through of as a frequency translator that shifts audio frequencies to radio frequencies (RF) while retaining their relative spacing.  At the receiver, the radio frequencies are shifted back down to audio. If the transmitter and receiver are on exactly the same radio frequency, the recovered audio frequencies will be the same as the originals.   If the transmitter and receiver are not on the exact same radio frequency, the recovered audio frequencies will be different from the originals.  For example, if the receiver is on a frequency 100 Hz different from the transmitter, all recovered audio tones will be 100 Hz higher or lower, depending on the direction of the error. 

The effective bandwidth of the individual tuned mark and space audio tone filters in TNCs such as KAMS on FSK, are extremely narrow (only about 75 Hz).  The DSP-synthesized audio filters in PSK receive software are often even narrower. Because the audio frequencies recovered in the receiver are directly proportional to radio frequency errors,  you must be able to accurately set the radio frequency to within 20 Hz or so, and keep it there indefinitely.  Old mechanical-dial analog-VFO radios such as Kenwood TS-820s, Yaesu FT-101s, Heathkit SB-101s, etc are hopeless for this kind of operation. They simply can't be set precisely enough in the first place and thermally-induced frequency drift will guarantee that they will quickly "wander" off frequency.  The frequency calibration/resolution issue is especially critical if you are going to transmit "in the blind" without a signal to tune in on receive first! (I.e. typical transmit-only mobile trackers that don't receive.) Ideally you want a modern synthesized rig with an accurately calibrated TCXO high-stability master oscillator.

On SSB equipment: 

• The indicated "dial frequency" on SSB is the suppressed carrier frequency.
   
• The suppressed carrier frequency is NOT transmitted.
   
• Frequencies are transmitted that are offset above the carrier freq on USB (or below the carrier on LSB) by the exact value of the AUDIO tones fed into the radio mic jack from the TNC, soundcard, modem, etc.
   
• Feeding a single audio tone into the mic jack of an SSB rig causes the transmitter to produce a single RF frequency. Feeding a different frequency audio tone into the same rig causes a different single RF frequency to be produced.  With NO tone into the mic jack, a keyed-up SSB transmitter produces NO RF output at all.
   
• On voice, the complex mixture of multiple simultaneous audio frequencies in the voice causes a corresponding group  of simultaneous RF frequencies to be generated; i.e. a "sideband" rather than a single RF frequency.

The resulting RF signal frequencies are the sum (USB) or difference (LSB) of the the RF "dial" frequency and the modulating audio tone frequencie(s).  

The actual transmitted RF frequencies are the indicated suppressed carrier frequency (i.e. "dial frequency") plus the audio tone frequencies (USB)  -or- the suppressed carrier frequency minus the audio tone frequencies (LSB).  The actual dial frequency you want WILL DEPEND ON THE SIDEBAND YOU CHOOSE (either one will work) --AND-- ON THE PARTICULAR AUDIO TONE FREQUENCIES your TNC or other device produces.

It is highly recommended to use USB rather than the more common LSB because:

• USB is the universal standard for HF/SSB operation with commercial/marine/aviation/military radios on all frequencies. Only ham equipment normally offers LSB operation.
   
• Audio spectrum and "waterfall" displays read "right-side up"; i.e. a higher RF frequency shows as a higher audio frequency. (On LSB, the relationship between RF frequency and audio frequency is inverted -- can be very confusing when tuning.)
   
• You avoid the need to "hack" the radio to allow "out-of-band" transmit (i.e. set the VFO to an indicated frequency above 10.150 000 MHz) as is required on LSB. 
   

Frequencies Used By Different Devices

The differing AUDIO frequencies are really not a problem on SSB, and are easily accommodated. Unlike FM, the audio frequency heard at the receiving end is affected by the exact RF frequencies the transmitter and/or receiver are set to.

The audio frequency heard in the receiver will change exactly Hz for Hz with changes of the radio frequency tuning dial. You cause the the audio tone frequencies heard in the receiver to be the ones required by your device or program, by tuning the receiver to a slightly higher or lower RF frequency.

[This cuts both ways. If the transmitter is off frequency, the audio tones recovered at a correctly-tuned receiver will be correspondingly off-frequency.]

No matter how you fiddle and diddle around with audio tone frequencies and dial settings,
     THE ACTUAL RESULTING *RADIO FREQUENCIES*  MUST ALWAYS BE THE SAME.

Again, on 30M APRS FSK, the two actual transmitted RF frequencies (no matter how you get there) must be:

   10.149 200 MHz
   10.149 400 MHz

Note that unlike classic RTTY with it's fixed sense of mark and space frequencies that require both parties to be using the same sideband, packet data is unaffected by sideband choice. Packet uses NRZI coding -- you can use either sideband and not worry about "being on the wrong sideband", or having to "invert" the sense of the received data.

Conventional 300-Baud FSK Tone Pairs

To produce the correct RF frequencies with a KAM, TNC2, TinyTrak III (300 Baud HF mode) or paid version of AGWpe whose default audio tones are 1600/1800 Hz, you must set your radio to:

10.151.00 LSB
10.151.000 - 1.800 = 10.149.200
10.151.000 - 1.600 = 10.149.400

Or

10.147.60 USB   [Recommended Choice!]
10.147.600 + 1.600 = 10.149.200
10.147.600 + 1.800 = 10.149.400

To produce the correct RF frequencies with a PK232 TNC whose default audio tones are 2110/2310 you must set your radio to:

10.151.51 LSB
10.151.510 - 2.310 = 10.149.200
10.151.510 - 2.110 = 10.149.400

Or

10.147.09 USB     [Recommended Choice!]
10.147.090 + 2.110 = 10.149.200
10.147.090 + 2.310 = 10.149.400

To produce the correct RF frequencies with a TigerTrak whose 300 Baud/narrow shift audio tones are 1100/1300 (weird pairing -- but actually very nice because the tone pairs are in the dead center of the typical SSB transceiver's filter bandpass and suffer the absolutely least amount of phase and group delay distortion) you must set your radio to:

10.150.50 LSB
10.150.500 - 1.300 = 10.149.200
10.150.500 - 1.100 = 10.149.400

Or

10.148.10 USB
10.148.100 + 1.100 = 10.149.200
10.148.100 + 1.300 = 10.149.400

To produce the correct RF frequencies with the free version of the AGW Packet Engine softmodem, whose default audio tones on 300 baud HF are 2100/2300 you must set your radio to:

10.151.50 LSB
10.151.500 - 2.300 = 10.149.200
10.151.500 - 2.100 = 10.149.400

Or

10.147.00 USB
10.147.100 + 2.100 = 10.149.200
10.147.100 + 2.300 = 10.149.400

Note that technically, the LSB modes have the suppressed carrier outside the ham band.  The suppressed carrier frequency IS NOT TRANSMITTED (assuming your rig is properly aligned with good carrier supression).  The resulting modulation products from the audio tones will be inside the ham band, assuming you don't overdrive the transmitter modulator and produce intermodulation distortion.

HOWEVER, most current ham gear will not transmit when the indicated carrier frequency is moved outside the ham bands.  You will have to hack (modify) the radio for the so-called general-coverage transmit capability.  This is usually a trivial exercise, done by clipping a single diode on the radio's controller board or slashing a PC board trace with an X-Acto knife, and then resetting the radio's CPU.   See

<http://mods.dk> for info on the general-coverage mod for virtually any HF/SSB transceiver.

Some HF radios with special "DATA" or "FSK" modes offset the indicated dial frequency to correct for the difference between the suppressed carrier freq and the actual mark frequency, typically assuming the lower tone is 2125 Hz (or sometimes 1800 Hz). This will force you to compute offsets different from what I have listed for LSB/USB. Or even (if you are lucky) just set the radio to an indicated 10.149.200 MHz.

PSK63 APRS Considerations

The same issues apply to the single tone of PSK63 APRS generated by the APRS Messenger program.  The single actual transmitted RF frequency for 30-meter PSK63 APRS operation is

10.149 700 MHz

(This radio frequency is 300 Hz higher than the higher of the
 two FSK frequencies for conventional AX.25 APRS packet.)

Set the PSK63 audio tone in APRS Messenger to 2100 Hz, and the transceiver's RF ("dial frequency") to 10.147 600 MHz USB.

It is highly recommended to use USB rather than the more common LSB because:

• USB is the universal standard for HF/SSB operation with commercial/marine/aviation/military radios on all frequencies. Only ham equipment normally offers LSB operation.
   
• Audio spectrum and "waterfall" displays read "right-side up"; i.e. a higher RF frequency shows as a higher audio frequency. (On LSB, the relationship between RF frequency and audio frequency is inverted -- can be very confusing when tuning.)
   
• You avoid the need to "hack" the radio to allow "out-of-band" transmit (i.e. set the VFO to an indicated frequency above 10.150 000 MHz) as is required on LSB. 
   

The audio tones used by particular devices, the desire to use narrow-band CW or audio filters, passband tuning limitations, or the needs of simultaneous FSK/PSK63 operation may require you to use other tone frequencies and/or LSB mode operation. 

The combinations of audio tone frequencies and indicated RF "dial" frequencies required to place the actual transmitted signal on the correct RF frequency (10.149 700 MHz) are summarized in this table: