FT-817 Zero Beat

Ever wondered what happens with the FT-817 transceiver when changing modes between CW and SSB, and why does the displayed frequency change?  Answers may be found on this page!

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FT-817 Zero Beat, and switching between CW and SSB modes

This information is adapted from a posting titled "Re: Zero Beat confusion" which I emailed to the FT-817 group on Saturday 09 October 2010 .  The original question asked about the best way to "zero beat" another station when operating CW with the FT-817.  (This can be achieved by pressing and holding the HOME button on the front panel of the FT-817 - this causes the sidetone to be heard in the speaker (or headphones) at the same time as the received signal - while adjusting the tuning knob until the sidetone has the same note as the received CW signal.  When doing this, listen for the beat note between the sidetone and the received signal - this will reduce in frequency as the tuned frequency approaches the frequency of the received CW signal).

Subsequent discussion extended the original question to related issues; such as the actual transmit frequency when operating in CW or CW-R mode; the difference between these two modes; and why does the displayed frequency change when switching between CW, CWR, USB and LSB modes.  My posting attempted to provide some answers to these latter questions.


To illustrate what the FT-817 does when switching between CW, CWR, USB and LSB modes, I used the FT-817 to listen to the VK2WI CW practice beacon transmitted from Dural (near Sydney, NSW) on 3699 kHz.  My FT-817 is configured for a 800 Hz CW pitch, and is not fitted with a narrow filter for CW reception.  For this experiment, the clarifier and IF shift were off.

(Note that you can conduct the same experiment with your own FT-817 even if you're not within hearing distance of the VK2WI CW practice beacon.  All you need is a medium to strong CW signal which is transmitting continuously.)

The discussion concludes with a comparison between SSB and CW modes.

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CW Mode

Firstly, I selected CW mode, and tuned the FT-817 to 3699 kHz. The front panel display indicates this as "3.699.00". Pressing and holding the HOME button confirmed that the received signal had the same pitch as the sidetone, therefore the displayed frequency was that of the received CW signal.  The diagram below illustrates the radio frequency spectrum (as "seen" at the RF front end of the FT-817), and a representation of the FT-817 front panel display.

RF spectrum - FT-817 in CW mode

The "Radio Frequency Spectrum" section of the diagram above shows the passband of the IF filter, but referenced to the frequencies "seen" at the receiver RF front end.  The passband is shown as being 3 kHz wide, which is an approximation of the published IF filter bandwidth.  (IF filter bandwidths are often specified at the -6dB and -60dB points of the filter response; however, when used in practice, the filter passband sounds as if it is wider than the -6dB bandwidth but narrower than the -60dB bandwidth - this is because the human ear can hear signals below the -6dB level.)  The CW signal from VK2WI beacon is shown as falling within this passband, and this is suggested by the "S+" and appropriate signal strength bars shown on the FT-817 display.  The FT-817 internally tunes the Beat Frequency Oscillator (BFO) so that it is below the received signal frequency by the same amount as the CW pitch, ie 800 Hz.  This is achieved by tuning the FT-817 so that the display shows the transmitted frequency of the desired station - in this case, 3699 kHz for VK2WI's CW beacon signal.  If I was to transmit with the FT-817 in this configuration (which is not recommended, as my transmission would interfere with the beacon transmission), then the CW transmission would occur on 3699 kHz.

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CWR Mode

Next, I switched to CWR mode. The FT-817 displayed "3.699.00", and the received signal had the same pitch.  The diagram below illustrates what happened.

RF spectrum - FT-817 in CW-R mode

This diagram is very similar to that shown for the CW mode.  The CW signal from VK2WI beacon is shown as falling within the filter passband, and this is indicated by the "S+" and appropriate signal strength bars shown on the FT-817 display.  However, this time the BFO is tuned so that it is above the received signal frequency by the same amount as the CW pitch, ie 800 Hz.  Note that no retuning of the FT-817 was required.  Again; if I was to transmit with the radio in this configuration (which is not recommended for the reason given above), then the CW transmission would occur on 3699 kHz.

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USB Mode

Without touching the tuning knob, I switched to USB mode. The FT-817 now displayed "3.698.20", and the received signal pitch had not changed. The difference between the frequency displayed for CW
and CWR, and the frequency displayed for USB, is 800 Hz - the sidetone pitch for my FT-817. The diagram below illustrates what happened.

RF spectrum - FT-817 in USB mode

The diagram above is similar to that shown for the CW and CWR modes.  The CW signal from VK2WI beacon is shown falling within the filter passband, and this is indicated by the "S+" and appropriate signal strength bars shown on the FT-817 display.  However, this time the BFO is tuned so that it is below the received signal frequency by the same amount as the CW pitch, ie 800 Hz.  The BFO being tuned to a frequency just below the passband is expected for USB mode - it is equivalent to the suppressed carrier associated with what amateur radio operators called "SSB".  (The mode is more correctly known as "Single Sideband - Suppressed Carrier", or SSB-SC.)  If I was to transmit with the radio in this configuration (which is not recommended for the reason given above), then my SSB signal would fall within the band between 3698.2 kHz and 3701.2 kHz, and interfere with the VK2WI CW beacon signal.  Also, the upper 1.2 kHz of my USB signal would fall outside the 80m band allocated for use by amateur radio operators in Australia, so it would not be a legal transmission!

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LSB Mode

Then, I switched to LSB mode. The '817 displayed "3.698.20", and the VK2WI signal could not be heard. This is because the signal is at 3.699.00, and the FT-817 is receiving signals falling in the 3 kHz wide passband between 3.698.20 and 3.695.20.  The diagram below illustrates what happened.

RF spectrum - FT-817 in LSB mode

The diagram is similar to that shown above for the CW, CWR and USB modes.  However, the CW signal from VK2WI beacon is shown as falling outside the filter passband, and this is indicated by the "S1" and appropriate signal strength bar shown on the FT-817 display.  (S1 was the level of some in-band noise and other weak signals at the time.)  The BFO is tuned for receiving LSB signals in the band between 3698.2 kHz and 3695.2 kHz.  The BFO being tuned to a frequency just above the passband is expected for LSB mode - it is equivalent to the suppressed carrier associated with LSB.  If I was to transmit with the radio in this configuration, then my LSB signal would not interfere with the VK2WI CW beacon at all.

Retuning the FT-817 (still in LSB mode) to 3699.8 kHz brought the VK2WI CW signal back to zero beat, ie 800 Hz pitch, as shown in the diagram below.

RF spectrum - FT-817 in LSB mode after retuning

This diagram is similar to that shown above for LSB.  In this case, the CW signal from VK2WI beacon is shown falling within the filter passband, and this is indicated by the "S+" and appropriate signal strength bars shown on the FT-817 display.  However, this time the BFO is tuned so that it is above the received signal frequency by the same amount as the CW pitch, ie 800 Hz.  Note that the '817 had to be retuned to a frequency 800 Hz above VK2WI CW beacon transmit frequency in order to obtain the 800 Hz pitch.  If I was to transmit with the radio in this configuration (which is not recommended for the reason given above), then my LSB signal would fall within the band between 3699.8 kHz and 3696.8 kHz - thus causing interference to the beacon transmission.

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Comparisons between SSB (USB and LSB) and CW (CW and CWR) modes

Given the discussion above regarding CW, CWR, USB and LSB modes; it is interesting to compare the CW and USB modes, and the CWR and LSB modes.  It is also interesting to compare LSB and USB modes.

USB & CW modes

RF spectrum - FT-817 in CW & USB modes
The diagram above shows the radio frequencies at the RF front end of the FT-817's receiver, and the representation of the FT-817's front panel display, for USB and CW modes.  Note that the IF filter passband, when referenced to the receiver RF front end, covers the same frequency band regardless of the mode.  Also note that the BFO is at the same frequency, regardless of the mode.  In USB and CW modes, the FT-817 implements a USB receiver.  This can be verified by selecting either mode and listening to a CW signal - increasing the received frequency decreases the pitch of the received CW signal; and conversely decreasing the received frequency will increase the pitch of the received CW signal.  That is exactly what is expected of a USB receiver.

The frequency displayed on the front panel depends on the mode.  This is because in CW mode, the displayed frequency is the frequency on which the CW transmission will occur; whereas in USB mode, the displayed frequency is the frequency of the suppressed carrier.  (The carrier is suppressed in the balanced modulator.)

LSB and CWR modes

RF spectrum - FT-817 in CWR & LSB modes

The diagram above shows the radio frequencies at the RF front end of the FT-817's receiver, and the representation of the FT-817's front panel display, for LSB and CWR modes.  Note that the IF filter passband, when referenced to the receiver RF front end, covers the same frequency band regardless of the mode.  Also note that the BFO is at the same frequency, regardless of the mode.  In LSB and CWR modes, the FT-817 implements a LSB receiver.  Again; this can be verified by selecting either mode and listening to a CW signal - increasing the received frequency increases the pitch of the received CW signal; and conversely decreasing the received frequency decreases the pitch of the received CW signal. That is exactly what is expected of a LSB receiver.

Again, the frequency displayed on the front panel depends on the mode.  This is because in CW mode, the displayed frequency is the frequency on which the CW transmission will occur; whereas in LSB mode, the displayed frequency is the frequency of the suppressed carrier.  (Per the USB case, the carrier is suppressed in the balanced modulator.)

USB and LSB modes

 RF spectrum - FT-817 receiving DSB & CW signals

The diagram above shows the radio frequencies at the RF front end of the FT-817's receiver, and the representation of the FT-817's front panel display, for USB and LSB modes.  Note that the IF filter passband, when referenced to the receiver RF front end, effectively moves from one side of the BFO (or suppressed carrier frequency) to the other.  For a symmetrical filter passband, this can be thought of as the filter passband being "mirrored" around the BFO frequency.  Also note that the BFO is at the same frequency, regardless of the mode.  In USB mode, the receiver is listening to a 3 kHz segment of the band extending from the suppressed carrier frequency up to a frequency 3 kHz greater than that suppressed carrier frequency; and when in LSB mode, the receiver is listening to an equivalent band extending from the suppressed carrier frequency down to a frequency 3 kHz less than that suppressed carrier frequency.   This is well illustrated when listening to a DSB signal - when the suppressed carrier frequency in either LSB or USB mode is correctly tuned to the DSB suppressed carrier frequency, then switching between LSB and USB will not cause any difference - the received signal is still intelligible (ignoring any effects of QRM, etc, on one sideband and not the other).  In the diagram above, the indicated signal strength in USB mode is higher due to the CW carrier and DSB upper sideband being received, whereas in LSB mode only the DSB lower sideband is received.

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This page was created by Mike Dower VK2IG: 24 Dec 2010; last updated: 26 Mar 2011.  Material may be copied for personal or non-profit use only.