OwenDuffy.net 


Making sense of the frequency readout on transceivers

The convention in ham radio is that an A1 CW and voice SSB, AM, or FM channel is designated by its carrier or virtual carrier frequency (as appropriate). I will deal with FSK later because it is so different.

Radios that observe this convention are easier to use.

Modern radios that derive ALL oscillators from a single Master Oscillator usually transmit CW, AM and FM carrier on the indicated frequency, and the virtual carrier for USB or LSB voice is displayed. A small exception, many radios will use a separate crystal oscillator for FM generation due to issues in trying to create FM (PM) from a DDS output, and provided it is adjusted properly, the previous statement applies, but it is not ‘locked’ to the MO. Another exception is that some radios that at first appear to generate all oscillators from a Master Oscillator do you use the Master Oscillator to derive their DSP clocks, and there is potential for a fixed frequency error due to the DSP clock source.

On modes other than CW, there is no magic.

In SSB mode, the receiver passband is usually offset from the virtual carrier frequency by say 300 to 2700Hz.

On CW, you want to listen to a beat note on the incoming signal, a beat created by mixing with a virtual local oscillator, and lets say your radio is configured for a 800Hz beat note and you tune for that beat note, you want the receiver passband offset so that it is centred around 800Hz offset from the virtual local oscillator, eg 550 to 1050Hz offset for a 500Hz wide filter. You want the display to show the frequency of the incoming signal that causes an 800Hz beat note. When you close the key, you want to send a carrier on EXACTLY the same frequency as that which causes an 800Hz beat note in your receiver.

The CW beat note is a configurable option on many modern radios, it is easy to do when all oscillators are derived from a single MO.

In use, you configure for your preferred beat note. When you tune a signal in for that beat note, their carrier frequency is displayed, and if you transmit, you will transmit carrier on that frequency.

In radios that support configurable CW beat note, they should also use that frequency for the side tone oscillator. If you have a radio that allows you to hear sidetone without transmitting (eg by turning the VOX off), you can beat your sidetone with the incoming signal… when they zero beat with each other, you are said to have ‘netted’ to them, you have adjusted your transmitter to transmit on their frequency.

If you perform the netting procedure above on an A1 CW beacon, and your radio is calibrated, the displayed frequency is the carrier frequency.
What happens when you listen to CW in USB or LSB mode. If you tune for a beat note of xHz, the received signal carrier frequency will be xHz higher or lower respectively than displayed.

Older radios try to a greater or lesser extent to match this operating convenience. Their achievement of the goal is dependent on correct alignment.

Now, I said FSK is different.

FSK telegraph channels are usually designated by the MARK frequency which by standard (ITU F.246-3) is the higher radio frequency. (Note that the ACMA register designates the channel by the "emission centre frequency", a point midway between the allocated upper and lower frequency limits of the channel).

FSK may be generated at baseband (AFSK) using telephone line modem chips, the standards for which set the MARK frequency low, so an inverting mix (LSB) must be performed to translate AFSK to radio frequencies.

Morse key down condition is MARK (carried over from key down causing a mark on the original paper tapes used).

Ham equipment for 170Hz shift FSK RTTY often generates an audio baseband signal (AFSK) and feeds it to a LSB transmitter. The audio tones used are often 2125 and 2295 for a 170Hz shift, 2125 is the MARK frequency (see above). (FSK mode will normally be a inverting mix like LSB, no matter what band is selected.)

So most modern transceivers in FSK mode when tuned to the MARK frequency of an incoming unmodulated FSK signal will deliver a 2125Hz tone to the speaker and display the MARK frequency.

Beacon’s are not usually particularly accurate in frequency, and FSK beacons are not necessarily conventional shift (170Hz or 850Hz) or conventional polarity (MARK high). The introduction of GPS disciplined beacons should serve as an impetus for standardisation of beacons.

A rational setup of beacons is:

With improved frequency accuracy, beacons that carry a 12WPM 170Hz shift ID at 10 minute interval and QRSS 5Hz shift ID in between are feasible. Feasible, but unlikely as beacons are controlled by "beacon police" and innovation does not seem a high priority.

An example of the confusion that exists, VK2RSY is licenced for 850HF1A on an emission centre frequency of 144.42MHz. The VKLOGGER beacon database shows observation the beacon as a 'CW' beacon as opposed to an 'FSK' beacon, which is inconsistent with the licence (which is for FSK mode). The 850H in the licenced emission designator is the necessary bandwidth, 850Hz, but widely and wrongly interpreted in the ham community as the shift or twice the peak deviation, 850Hz necessary bandwidth will sustain a maximum of 690Hz shift at 20WPM.

Some FSK beacons are reported to run unmodulated on one frequency, then to key ID to the other frequency. This is effectively 'KEY UP' between ID. The sensible thing to do is to compose the ID message with a space at start and finish, and to run 'KEY DOWN' between ID messages. In the latter scheme, listening to the MARK frequency  with a narrow bandwidth receiver will sound just like listening to an A1A Morse signal with 'KEY DOWN' between ID messages.

Some beacons send a lengthy informational message, which presumes that listeners hear it well enough to decode medium speed Morse code... an unwarranted assumption and compromising their main purpose if their purpose is mainly for revealing weak signal propagation possibilities.

Some 'spotters' in the community will log a beacon solely on the frequency observed, ie without hearing ID. This logging without adequate identification makes a champion spotter, but of course trashes the value of spot databases as a scientific information resource.

The one things that is consistent with beacons is that they are inconsistent!

Links

Changes

Version Date Description
1.01 03/04/2008 Initial.
1.02    
1.03    
1.04    
1.05    
1.06    
1.07    
1.08    

 


© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.